From 97ec40544b21caa857f1106c03c8fd0bd99113b2 Mon Sep 17 00:00:00 2001 From: Galoz <40101938+galoz11@users.noreply.github.com> Date: Sun, 29 Jun 2025 20:18:01 +0300 Subject: [PATCH] add rm --- Bash Cheat Sheet.md | 912 ++++++++++++++++++++++++++++++++++++ Command Line.md | 603 ++++++++++++++++++++++++ Linux Keyboard Shortcuts.md | 54 +++ Linux Special Characters.md | 73 +++ 4 files changed, 1642 insertions(+) create mode 100644 Bash Cheat Sheet.md create mode 100644 Command Line.md create mode 100644 Linux Keyboard Shortcuts.md create mode 100644 Linux Special Characters.md diff --git a/Bash Cheat Sheet.md b/Bash Cheat Sheet.md new file mode 100644 index 0000000..27181e4 --- /dev/null +++ b/Bash Cheat Sheet.md @@ -0,0 +1,912 @@ +--- +title: Bash Cheat Sheet +created: '2025-06-29T16:47:55.778Z' +modified: '2025-06-29T16:57:39.219Z' +--- + +# Bash Cheat Sheet +### A cheat sheet for bash commands. + + + +>**Note** 💡
+ +> - A single hyphen `-` is used to represent a short option, such as `-a`.
+> - A double hyphen `--` is used to denote a long option, such as `--all`.
+> - Example command : `ls -a|--all` (List all directories). Can use either use the short or long options. + +> - Both short and long options were incorporated for educational purposes, allowing users to choose the preferred format.
+> - It's important to note that not all commands have both short and long options. Some commands may only have short options, while others may only have long options. + +### Contents +- [Command Information](#command-information) +- [Command History](#command-history) +- [Navigating Directories](#navigating-directories) +- [Creating Directories](#creating-directories) +- [Moving Directories](#moving-directories) +- [Deleting Directories](#deleting-directories) +- [Creating Files](#creating-files) +- [Standard Output, Error and Input](#standard-output-standard-error-and-standard-input) +- [Moving Files](#moving-files) +- [Deleting Files](#deleting-files) +- [Reading Files](#reading-files) +- [Sorting Files](#sorting-files) +- [File Permissions](#file-permissions) +- [Finding Files](#finding-files) +- [Find in Files](#find-in-files) +- [Replace in Files](#replace-in-files) +- [File Editor](#file-editor) +- [Symbolic Links](#symbolic-links) +- [Compressing Files](#compressing-files) +- [Decompressing Files](#decompressing-files) +- [Packages](#packages) +- [Disk Usage](#disk-usage) +- [Memory Usage](#memory-usage) +- [Shutdown and Reboot](#shutdown-and-reboot) +- [Identifying Processes](#identifying-processes) +- [Process Priority](#process-priority) +- [Killing Processes](#killing-processes) +- [Date & Time](#date--time) +- [Scheduled Tasks](#scheduled-tasks) +- [User Mangement](#user-management) +- [HTTP Requests](#http-requests) +- [Network Troubleshooting](#network-troubleshooting) +- [DNS](#dns) +- [Hardware](#hardware) +- [System Information](#system-information) +- [Terminal Multiplexers](#terminal-multiplexers) +- [Secure Shell Protocol (SSH)](#secure-shell-protocol-ssh) +- [Secure Copy](#secure-copy) +- [Bash Profile](#bash-profile) +- [Bash Script](#bash-script) +# + +## Command Information + +```bash +man chmod # Display page manual of a command +man -f|--whatis chmod # Display short description about a command +man -k|--apropos permission # Display all related commands from a specific keyword + +chmod --help # Display usage options of a command +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Command History + +```bash +history # View all previous commands +history | grep foo # View the commands using a specific word +history | grep -E|--extended-regexp -i|--ignore-case 'foo1|foo2|foo3' # View the commands using more than 1 specific word(case sensitive) +history | head -n|--lines 3 # View the first 3 executed commands +history 3 # View the last 3 executed commands +history -d 99 # Clear a command from a specific line +history -c # Clears all history commands +!! # Run the last command executed + +touch foo.sh # <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<┐ +chmod +x !$ # !$ is the last argument of the last command i.e. foo.sh <<<<┘ +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Navigating Directories + +```bash +pwd # Print current directory path +ls # List directories +ls -a|--all # List directories including hidden +ls -l # List directories in long form +ls -l -h|--human-readable # List directories in long form with human readable sizes +ls -t # List directories by modification time, newest first +stat foo.txt # List size, created and modified timestamps for a file +stat foo # List size, created and modified timestamps for a directory +tree # List directory and file tree +tree -a # List directory and file tree including hidden +tree -d # List directory tree + +cd foo # Go to foo sub-directory +cd # Go to home directory +cd ~ # Go to home directory +cd - # Go to the previously chosen directory +pushd foo # Go to foo sub-directory and add previous directory to stack +popd # Go back to directory in stack saved by `pushd` +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Creating Directories + +```bash +mkdir foo # Create a directory +mkdir foo bar # Create multiple directories +mkdir -p|--parents foo/bar # Create nested directory +mkdir -p|--parents {foo,bar}/baz # Create multiple nested directories + +mktemp -d|--directory # Create a temporary directory +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Moving Directories + +```bash +cp -R|--recursive foo bar # Copy directory +mv foo bar # Move directory + +rsync -z|--compress -v|--verbose /foo /bar # Copy directory, overwrites destination +rsync --ignore-existing -a|--archive -z|--compress -v|--verbose /foo /bar # Copy directory, without overwriting destination +rsync -avz /foo username@hostname:/bar # Copy local directory to remote directory +rsync -avz username@hostname:/foo /bar # Copy remote directory to local directory +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Deleting Directories + +```bash +rmdir foo # Delete non-empty directory +rm -r|--recursive foo # Delete directory including contents +rm -r|--recursive -f|--force foo # Delete directory including contents, ignore nonexistent files and never prompt +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Creating Files + +```bash +touch foo.txt # Create file or update existing files modified timestamp +touch foo.txt bar.txt # Create multiple files +touch {foo,bar}.txt # Create multiple files +touch test{1..3} # Create test1, test2 and test3 files +touch test{a..c} # Create testa, testb and testc files + +mktemp # Create a temporary file +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Standard Output, Standard Error and Standard Input + +```bash +echo "foo" > bar.txt # Overwrite file with content +echo "foo" >> bar.txt # Append to file with content + +ls exists 1> stdout.txt # Redirect the standard output to a file +ls noexist 2> stderror.txt # Redirect the standard error output to a file +ls > out.txt 2>&1 # Redirect standard output and error to a file +ls > /dev/null # Discard standard output and error + +read foo # Read from standard input and write to the variable foo +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Moving Files + +```bash +cp foo.txt bar.txt # Copy file +mv foo.txt bar.txt # Move file + +rsync -z|--compress -v|--verbose /foo.txt /bar # Copy file quickly if not changed +rsync -z|--compress -v|--verbose /foo.txt /bar.txt # Copy and rename file quickly if not changed +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Deleting Files + +```bash +rm foo.txt # Delete file +rm -f|--force foo.txt # Delete file, ignore nonexistent files and never prompt +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Reading Files + +```bash +cat foo.txt # Print all contents +less foo.txt # Print some contents at a time (g - go to top of file, SHIFT+g, go to bottom of file, /foo to search for 'foo') +head foo.txt # Print top 10 lines of file +tail foo.txt # Print bottom 10 lines of file +tail -f|--follow foo.txt # Print bottom 10 lines of file updating with new data +open foo.txt # Open file in the default editor +wc foo.txt # List number of lines words and characters in the file +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Sorting Files + +```bash +sort foo.txt # Sort file (ascending order) +sort -r|--reverse foo.txt # Sort file (descending order) +sort -n|--numeric-sort foo.txt # Sort numbers instead of strings +sort -t|--field-separator: -k 3n /foo/foo.txt # Sort by the third column of a file + +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## File Permissions + +| # | Permission | rwx | Binary | +| - | - | - | - | +| 7 | read, write and execute | rwx | 111 | +| 6 | read and write | rw- | 110 | +| 5 | read and execute | r-x | 101 | +| 4 | read only | r-- | 100 | +| 3 | write and execute | -wx | 011 | +| 2 | write only | -w- | 010 | +| 1 | execute only | --x | 001 | +| 0 | none | --- | 000 | + +For a directory, execute means you can enter a directory. + +| User | Group | Others | Description | +| - | - | - | - | +| 6 | 4 | 4 | User can read and write, everyone else can read (Default file permissions) | +| 7 | 5 | 5 | User can read, write and execute, everyone else can read and execute (Default directory permissions) | + +- u - User +- g - Group +- o - Others +- a - All of the above + +```bash +ls -l /foo.sh # List file permissions +chmod +100 foo.sh # Add 1 to the user permission +chmod -100 foo.sh # Subtract 1 from the user permission +chmod u+x foo.sh # Give the user execute permission +chmod g+x foo.sh # Give the group execute permission +chmod u-x,g-x foo.sh # Take away the user and group execute permission +chmod u+x,g+x,o+x foo.sh # Give everybody execute permission +chmod a+x foo.sh # Give everybody execute permission +chmod +x foo.sh # Give everybody execute permission +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Finding Files + +Find binary files for a command. + +```bash +type -a wget # Display all locations of executable +which -a wget # Display all locations of executables +whereis wget # Find the binary, source, and manual page files +``` + +`locate` uses an index and is fast. + +```bash +updatedb # Update the index + +locate foo.txt # Find a file +locate --ignore-case # Find a file and ignore case +locate f*.txt # Find a text file starting with 'f' +``` + +`find` doesn't use an index and is slow. + +```bash +find /path -name foo.txt # Find a file +find /path -iname foo.txt # Find a file with case insensitive search +find /path -name "*.txt" # Find all text files +find /path -name foo.txt -delete # Find a file and delete it +find /path -name "*.png" -exec pngquant {} # Find all .png files and execute pngquant on it +find /path -type f -name foo.txt # Find a file +find /path -type d -name foo # Find a directory +find /path -type l -name foo.txt # Find a symbolic link +find /path -type f -mtime +30 # Find files that haven't been modified in 30 days +find /path -type f -mtime +30 -delete # Delete files that haven't been modified in 30 days +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Find in Files + +```bash +grep 'foo' /bar.txt # Search for 'foo' in file 'bar.txt' +grep 'foo' /bar -r|--recursive # Search for 'foo' in directory 'bar' +grep 'foo' /bar -R|--dereference-recursive # Search for 'foo' in directory 'bar' and follow symbolic links +grep 'foo' /bar -l|--files-with-matches # Show only files that match +grep 'foo' /bar -L|--files-without-match # Show only files that don't match +grep 'Foo' /bar -i|--ignore-case # Case insensitive search +grep 'foo' /bar -x|--line-regexp # Match the entire line +grep 'foo' /bar -C|--context 1 # Add N line of context above and below each search result +grep 'foo' /bar -v|--invert-match # Show only lines that don't match +grep 'foo' /bar -c|--count # Count the number lines that match +grep 'foo' /bar -n|--line-number # Add line numbers +grep 'foo' /bar --colour # Add colour to output +grep 'foo\|bar' /baz -R # Search for 'foo' or 'bar' in directory 'baz' +grep --extended-regexp|-E 'foo|bar' /baz -R # Use regular expressions +grep -E 'foo|bar' /baz -R # Use regular expressions +``` + +### Replace in Files + +```bash +sed 's/fox/bear/g' foo.txt # Replace fox with bear in foo.txt and output to console +sed 's/fox/bear/gi' foo.txt # Replace fox (case insensitive) with bear in foo.txt and output to console +sed 's/red fox/blue bear/g' foo.txt # Replace red with blue and fox with bear in foo.txt and output to console +sed 's/fox/bear/g' foo.txt > bar.txt # Replace fox with bear in foo.txt and save in bar.txt +sed -i|--in-place 's/fox/bear/g' foo.txt # Replace fox with bear and overwrite foo.txt +sed -i|--in-place '/red fox/i\blue bear' foo.txt # Insert blue bear before red fox and overwrite foo.txt +sed -i|--in-place '/red fox/a\blue bear' foo.txt # Insert blue bear after red fox and overwrite foo.txt +sed -i|--in-place '10s/find/replace/' foo.txt # Replace the 10th line of the file +sed -i|--in-place '10,20s/find/replace/' foo.txt # Replace in the file 10-20 lines +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +### File Editor + + ```bash +nano # Open a new file in nano +nano foo.txt # Open a specific file +nano -m|--mouse foo.txt # Enable the use of the mouse +nano -l|--linenumbers foo.txt # Show line numbers in front of the text +nano +line,10 foo.txt # Open file positioning the cursor at the specified line and column +nano -B|--backup foo.txt # Create a backup file (`foo~`) when saving edits +``` + +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Symbolic Links + +```bash +ln -s|--symbolic foo bar # Create a link 'bar' to the 'foo' folder +ln -s|--symbolic -f|--force foo bar # Overwrite an existing symbolic link 'bar' +ls -l # Show where symbolic links are pointing +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Compressing Files + +### zip + +Compresses one or more files into *.zip files. + +```bash +zip foo.zip /bar.txt # Compress bar.txt into foo.zip +zip foo.zip /bar.txt /baz.txt # Compress bar.txt and baz.txt into foo.zip +zip foo.zip /{bar,baz}.txt # Compress bar.txt and baz.txt into foo.zip +zip -r|--recurse-paths foo.zip /bar # Compress directory bar into foo.zip +``` + +### gzip + +Compresses a single file into *.gz files. + +```bash +gzip /bar.txt foo.gz # Compress bar.txt into foo.gz and then delete bar.txt +gzip -k|--keep /bar.txt foo.gz # Compress bar.txt into foo.gz +``` + +### tar -c + +Compresses (optionally) and combines one or more files into a single *.tar, *.tar.gz, *.tpz or *.tgz file. + +```bash +tar -c|--create -z|--gzip -f|--file=foo.tgz /bar.txt /baz.txt # Compress bar.txt and baz.txt into foo.tgz +tar -c|--create -z|--gzip -f|--file=foo.tgz /{bar,baz}.txt # Compress bar.txt and baz.txt into foo.tgz +tar -c|--create -z|--gzip -f|--file=foo.tgz /bar # Compress directory bar into foo.tgz +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Decompressing Files + +### unzip + +```bash +unzip foo.zip # Unzip foo.zip into current directory +``` + +### gunzip + +```bash +gunzip foo.gz # Unzip foo.gz into current directory and delete foo.gz +gunzip -k|--keep foo.gz # Unzip foo.gz into current directory +``` + +### tar -x + +```bash +tar -x|--extract -z|--gzip -f|--file=foo.tar.gz # Un-compress foo.tar.gz into current directory +tar -x|--extract -f|--file=foo.tar # Un-combine foo.tar into current directory +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Packages + +```bash +apt update # Refreshes repository index +apt search wget # Search for a package +apt show wget # List information about the wget package +apt list --all-versions wget # List all versions of the package +apt install wget # Install the latest version of the wget package +apt install wget=1.2.3 # Install a specific version of the wget package +apt remove wget # Removes the wget package +apt upgrade # Upgrades all upgradable packages +apt clean # Clears out the local repository of downloaded package files + +dpkg -i|--install package_name.deb # Install deb file +dpkg -P package_name.deb # Remove a deb file installion (including configuration files) +rpm -i|--install package_name.rpm # Install a rpm file installion +rpm -e package_name.rpm # Remove rpm file installion (including dependencies) +``` + +### Install package source code + +```bash +tar zxvf sourcecode.tar.gz +cd sourcecode +./configure +make +make install +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Disk Usage + +```bash +df # List disks, size, used and available space +df -h|--human-readable # List disks, size, used and available space in a human readable format + +du # List current directory, subdirectories and file sizes +du /foo/bar # List specified directory, subdirectories and file sizes +du -h|--human-readable # List current directory, subdirectories and file sizes in a human readable format +du -d|--max-depth # List current directory, subdirectories and file sizes within the max depth +du -d 0 # List current directory size +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Memory Usage + +```bash +free # Show memory usage +free -h|--human # Show human readable memory usage +free -h|--human --si # Show human readable memory usage in power of 1000 instead of 1024 +free -s|--seconds 5 # Show memory usage and update continuously every five seconds +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Shutdown and Reboot + +```bash +shutdown # Shutdown in 1 minute +shutdown now # Immediately shut down +shutdown +5 # Shutdown in 5 minutes + +shutdown -r|--reboot # Reboot in 1 minute +shutdown -r|--reboot now # Immediately reboot +shutdown -r|--reboot +5 # Reboot in 5 minutes +shutdown -c # Cancel a shutdown or reboot + +reboot # Reboot now +reboot -f # Force a reboot +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Identifying Processes + +```bash +top # List all processes interactively +htop # List all processes interactively +ps ax # List all processes +pidof foo # Return the PID of all foo processes + +CTRL+Z # Suspend a process running in the foreground +bg # Resume a suspended process and run in the background +fg # Bring the last background process to the foreground +fg 1 # Bring the background process with the PID to the foreground + +sleep 30 & # Sleep for 30 seconds and move the process into the background +jobs # List all background jobs +jobs -p # List all background jobs with their PID + +lsof # List all open files and the process using them +lsof -itcp:4000 # Return the process listening on port 4000 +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Process Priority + +Process priorities go from -20 (highest) to 19 (lowest). + +```bash +nice -n -20 foo # Change process priority by name +renice 20 PID # Change process priority by PID +ps -o ni PID # Return the process priority of PID +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Killing Processes + +```bash +CTRL+C # Kill a process running in the foreground +kill PID # Shut down process by PID gracefully. Sends TERM signal. +kill -9 PID # Force shut down of process by PID. Sends SIGKILL signal. +pkill foo # Shut down process by name gracefully. Sends TERM signal. +pkill -9 foo # force shut down process by name. Sends SIGKILL signal. +killall foo # Kill all process with the specified name gracefully. +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Date & Time + +```bash +date # Print the date and time +date --iso-8601 # Print the ISO8601 date +date --iso-8601=ns # Print the ISO8601 date and time + +date -s "02 DEC 2020 12:02:02" # Manually change date and time +dpkg-reconfigure tzdata # Change date/timezone + +uptime # Print how long the system has been running +time tree # Print amount of time to tree takes to execute +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Scheduled Tasks + +```pre + * * * * * +Minute, Hour, Day of month, Month, Day of the week +``` + +```bash +crontab -l # List cron tab +crontab -e # Edit cron tab in a file editor +crontab /path/crontab # Load cron tab from a file +crontab -l > /path/crontab # Save cron tab to a file + +* * * * * foo # Run foo every minute +*/15 * * * * foo # Run foo every 15 minutes +0 * * * * foo # Run foo every hour +15 6 * * * foo # Run foo daily at 6:15 AM +44 4 * * 5 foo # Run foo every Friday at 4:44 AM +0 0 1 * * foo # Run foo at midnight on the first of the month +0 0 1 1 * foo # Run foo at midnight on the first of the year + +at -l # List scheduled tasks +at -c 1 # Show task with ID 1 +at -r 1 # Remove task with ID 1 +at now + 2 minutes # Create a task in a file editor to execute in 2 minutes +at 12:34 PM next month # Create a task in a file editor to execute at 12:34 PM next month +at tomorrow # Create a task in a file editor to execute tomorrow +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## User Management + +```bash +sudo su # Switch to root user +sudo foo # Execute commands(has permission denied) as the root user +sudo nano /foo/foo.txt # Open directories and files(is not writable) as the root user +su username # Switch to a different user + +passwd # To change the password of a user +adduser username # To add a new user +userdel username # To remove user +userdel -r|--remove username # To remove user with home directory and mail spool +usermod -a|--append -G|--groups GROUPNAME USERNAME # To add a user to a group +deluser USER GROUPNAME # To remove a user from a group + +last # Display information of all the users logged in +last username # Display information of a particular user +w # Display who is online +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## HTTP Requests + +```bash +curl https://example.com # Return response body +curl -i|--include https://example.com # Include status code and HTTP headers +curl -L|--location https://example.com # Follow redirects +curl -O|--remote-name foo.txt https://example.com # Output to a text file +curl -H|--header "User-Agent: Foo" https://example.com # Add a HTTP header +curl -X|--request POST -H "Content-Type: application/json" -d|--data '{"foo":"bar"}' https://example.com # POST JSON +curl -X POST -H --data-urlencode foo="bar" http://example.com # POST URL Form Encoded + +wget https://example.com/file.txt # Download a file to the current directory +wget -O|--output-document foo.txt https://example.com/file.txt # Output to a file with the specified name +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Network Troubleshooting + +```bash +ifconfig # Display all network card and interface information +ifconfig -a # Display information of all network cards (including those that are not started at boot) +ifconfig eth0 # Display specific device information +ifconfig eth0 up # Turn on the network card +ifconfig eth0 down # Turn off the network card +ifconfig eth0 192.168.120.56 # Configure IP address for network card + +curl ifconfig.me # Obtain external IP address + +ping example.com # Send multiple ping requests using the ICMP protocol +ping -c 10 -i 5 example.com # Make 10 attempts, 5 seconds apart + +ip addr # List IP addresses on the system +ip route show # Show IP addresses to router + +netstat -i|--interfaces # List all network interfaces and in/out usage +netstat -l|--listening # List all open ports + +traceroute example.com # List all servers the network traffic goes through + +mtr -w|--report-wide example.com # Continually list all servers the network traffic goes through +mtr -r|--report -w|--report-wide -c|--report-cycles 100 example.com # Output a report that lists network traffic 100 times + +nmap 0.0.0.0 # Scan for the 1000 most common open ports on localhost +nmap 0.0.0.0 -p1-65535 # Scan for open ports on localhost between 1 and 65535 +nmap 192.168.4.3 # Scan for the 1000 most common open ports on a remote IP address +nmap -sP 192.168.1.1/24 # Discover all machines on the network by ping'ing them +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## DNS + +```bash +dig example.com # Show query information of domain A records +dig -4 example.com # Show IPv4 A records +dig -6 example.com # Show IPv6 AAA records +dig example.com @nameserver # Show query of a specific nameserver +dig example.com -p 123 # Show query of a specific port number + +cat /etc/resolv.conf # Nameservers file +cat /etc/systemd/resolved.conf # DNS resolver config file +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Hardware + +```bash +lsusb # List USB devices +lspci # List PCI hardware +lshw # List all hardware +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## System Information + +```bash +uname -s # Print kernel name +uname -r # Print kernel release +uname -m # Print Architecture +uname -o # Print Operating System +uname -a # Print all Systen info + +lsb_release -a # Print distribution-specific information +dpkg --print-architecture # Print-architecture by name + +cat /proc/cpuinfo # Show cpu info +cat /proc/meminfo # Show memory info + +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Terminal Multiplexers + +Start multiple terminal sessions. Active sessions persist reboots. `tmux` is more modern than `screen`. + +```bash +tmux # Start a new session (CTRL-b + d to detach) +tmux ls # List all sessions +tmux attach -t 0 # Reattach to a session + +screen # Start a new session (CTRL-a + d to detach) +screen -S foo # Start a new named session +screen -ls # List all sessions +screen -R 31166 # Reattach to a session + +exit # Exit a session +reset # Reset the terminal(when binary and the terminal state is messed up) +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Secure Shell Protocol (SSH) + +```bash +ssh hostname # Connect to hostname using your current user name over the default SSH port 22 +ssh -i foo.pem hostname # Connect to hostname using the identity file +ssh user@hostname # Connect to hostname using the user over the default SSH port 22 +ssh user@hostname -p 8765 # Connect to hostname using the user over a custom port +ssh ssh://user@hostname:8765 # Connect to hostname using the user over a custom port +``` + +Set default user and port in `~/.ssh/config`, so you can just enter the name next time: + +```bash +$ cat ~/.ssh/config +Host name + User foo + Hostname 127.0.0.1 + Port 8765 +$ ssh name +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Secure Copy + +```bash +scp foo.txt ubuntu@hostname:/home/ubuntu # Copy foo.txt into the specified remote directory +scp ubuntu@hostname:/home/ubuntu/foo.txt /C:\Users\Admin # Copy foo.txt from the specified remote directory +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Bash Profile + +- bash - `.bashrc` +- zsh - `.zshrc` + +```bash +# Always run ls after cd +function cd { + builtin cd "$@" && ls +} + +# Prompt user before overwriting any files +alias cp='cp --interactive' +alias mv='mv --interactive' +alias rm='rm --interactive' + +# Always show disk usage in a human readable format +alias df='df -h' +alias du='du -h' +``` +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) + +## Bash Script + +### Variables + +```bash +#!/bin/bash + +foo=123 # Initialize variable foo with 123 +declare -i foo=123 # Initialize an integer foo with 123 +declare -r foo=123 # Initialize readonly variable foo with 123 +echo $foo # Print variable foo +echo ${foo}_'bar' # Print variable foo followed by _bar +echo ${foo:-'default'} # Print variable foo if it exists otherwise print default + +export foo # Make foo available to child processes +unset foo # Make foo unavailable to child processes and current session +``` + +### Environment Variables + +```bash +#!/bin/bash + +set # List all environment variables +echo $PATH # Print PATH environment variable +export FOO=Bar # Set an environment variable +``` + +### Functions + +```bash +#!/bin/bash + +greet() { + local world="World" + echo "$1 $world" + return "$1 $world" +} +greet "Hello" +greeting=$(greet "Hello") +``` + +### Exit Codes + +```bash +#!/bin/bash + +exit 0 # Exit the script successfully +exit 1 # Exit the script unsuccessfully +echo $? # Print the last exit code +``` + +### Conditional Statements + +#### Boolean Operators + +- `$foo` - Is true +- `!$foo` - Is false + +#### Numeric Operators + +- `-eq` - Equals +- `-ne` - Not equals +- `-gt` - Greater than +- `-ge` - Greater than or equal to +- `-lt` - Less than +- `-le` - Less than or equal to +- `-e` foo.txt - Check file exists +- `-z` foo - Check if variable exists + +#### String Operators + +- `=` - Equals +- `==` - Equals +- `-z` - Is null +- `-n` - Is not null +- `<` - Is less than in ASCII alphabetical order +- `>` - Is greater than in ASCII alphabetical order + +#### If Statements + +```bash +#!/bin/bash + +[[ +if [[$foo = 'bar']]; then + echo 'one' +elif [[$foo = 'bar']] || [[$foo = 'baz']]; then + echo 'two' +elif [[$foo = 'ban']] && [[$USER = 'bat']]; then + echo 'three' +else + echo 'four' +fi +]] +``` + +#### Inline If Statements + +```bash +#!/bin/bash + +[[ $USER = 'rehan' ]] && echo 'yes' || echo 'no' +``` + +#### While Loops + +```bash +#!/bin/bash + +[ +declare -i counter +counter=10 +while [$counter -gt 2]; do + echo The counter is $counter + counter=counter-1 +done +] +``` + +#### For Loops + +```bash +#!/bin/bash + +for i in {0..10..2} + do + echo "Index: $i" + done + +for filename in file1 file2 file3 + do + echo "Content: " >> $filename + done + +for filename in *; + do + echo "Content: " >> $filename + done +``` + +#### Case Statements + +```bash +#!/bin/bash + +echo "What's the weather like tomorrow?" +read weather + +case $weather in + sunny | warm ) echo "Nice weather: " $weather + ;; + cloudy | cool ) echo "Not bad weather: " $weather + ;; + rainy | cold ) echo "Terrible weather: " $weather + ;; + * ) echo "Don't understand" + ;; +esac +``` + +[⬆ ʀᴇᴛᴜʀɴ ᴛᴏ ᴄᴏɴᴛᴇɴᴛꜱ](#contents) diff --git a/Command Line.md b/Command Line.md new file mode 100644 index 0000000..2358074 --- /dev/null +++ b/Command Line.md @@ -0,0 +1,603 @@ +--- +title: Command Line +created: '2025-06-29T17:03:50.849Z' +modified: '2025-06-29T17:13:03.188Z' +--- + +# Command Line + + +- [Meta](#meta) +- [Basics](#basics) +- [Everyday use](#everyday-use) +- [Processing files and data](#processing-files-and-data) +- [System debugging](#system-debugging) +- [One-liners](#one-liners) +- [Obscure but useful](#obscure-but-useful) +- [macOS only](#macos-only) +- [Windows only](#windows-only) +- [More resources](#more-resources) + + +## Meta + +Scope: + +- This guide is for both beginners and experienced users. The goals are *breadth* (everything important), *specificity* (give concrete examples of the most common case), and *brevity* (avoid things that aren't essential or digressions you can easily look up elsewhere). Every tip is essential in some situation or significantly saves time over alternatives. +- This is written for Linux, with the exception of the "[macOS only](#macos-only)" and "[Windows only](#windows-only)" sections. Many of the other items apply or can be installed on other Unices or macOS (or even Cygwin). +- The focus is on interactive Bash, though many tips apply to other shells and to general Bash scripting. +- It includes both "standard" Unix commands as well as ones that require special package installs -- so long as they are important enough to merit inclusion. + +Notes: + +- To keep this to one page, content is implicitly included by reference. You're smart enough to look up more detail elsewhere once you know the idea or command to Google. Use `apt`, `yum`, `dnf`, `pacman`, `pip` or `brew` (as appropriate) to install new programs. +- Use [Explainshell](http://explainshell.com/) to get a helpful breakdown of what commands, options, pipes etc. do. + + +## Basics + +- Learn basic Bash. Actually, type `man bash` and at least skim the whole thing; it's pretty easy to follow and not that long. Alternate shells can be nice, but Bash is powerful and always available (learning *only* zsh, fish, etc., while tempting on your own laptop, restricts you in many situations, such as using existing servers). + +- Learn at least one text-based editor well. The `nano` editor is one of the simplest for basic editing (opening, editing, saving, searching). However, for the power user in a text terminal, there is no substitute for Vim (`vi`), the hard-to-learn but venerable, fast, and full-featured editor. Many people also use the classic Emacs, particularly for larger editing tasks. (Of course, any modern software developer working on an extensive project is unlikely to use only a pure text-based editor and should also be familiar with modern graphical IDEs and tools.) + +- Finding documentation: + - Know how to read official documentation with `man` (for the inquisitive, `man man` lists the section numbers, e.g. 1 is "regular" commands, 5 is files/conventions, and 8 are for administration). Find man pages with `apropos`. + - Know that some commands are not executables, but Bash builtins, and that you can get help on them with `help` and `help -d`. You can find out whether a command is an executable, shell builtin or an alias by using `type command`. + - `curl cheat.sh/command` will give a brief "cheat sheet" with common examples of how to use a shell command. + +- Learn about redirection of output and input using `>` and `<` and pipes using `|`. Know `>` overwrites the output file and `>>` appends. Learn about stdout and stderr. + +- Learn about file glob expansion with `*` (and perhaps `?` and `[`...`]`) and quoting and the difference between double `"` and single `'` quotes. (See more on variable expansion below.) + +- Be familiar with Bash job management: `&`, **ctrl-z**, **ctrl-c**, `jobs`, `fg`, `bg`, `kill`, etc. + +- Know `ssh`, and the basics of passwordless authentication, via `ssh-agent`, `ssh-add`, etc. + +- Basic file management: `ls` and `ls -l` (in particular, learn what every column in `ls -l` means), `less`, `head`, `tail` and `tail -f` (or even better, `less +F`), `ln` and `ln -s` (learn the differences and advantages of hard versus soft links), `chown`, `chmod`, `du` (for a quick summary of disk usage: `du -hs *`). For filesystem management, `df`, `mount`, `fdisk`, `mkfs`, `lsblk`. Learn what an inode is (`ls -i` or `df -i`). + +- Basic network management: `ip` or `ifconfig`, `dig`, `traceroute`, `route`. + +- Learn and use a version control management system, such as `git`. + +- Know regular expressions well, and the various flags to `grep`/`egrep`. The `-i`, `-o`, `-v`, `-A`, `-B`, and `-C` options are worth knowing. + +- Learn to use `apt-get`, `yum`, `dnf` or `pacman` (depending on distro) to find and install packages. And make sure you have `pip` to install Python-based command-line tools (a few below are easiest to install via `pip`). + + +## Everyday use + +- In Bash, use **Tab** to complete arguments or list all available commands and **ctrl-r** to search through command history (after pressing, type to search, press **ctrl-r** repeatedly to cycle through more matches, press **Enter** to execute the found command, or hit the right arrow to put the result in the current line to allow editing). + +- In Bash, use **ctrl-w** to delete the last word, and **ctrl-u** to delete the content from current cursor back to the start of the line. Use **alt-b** and **alt-f** to move by word, **ctrl-a** to move cursor to beginning of line, **ctrl-e** to move cursor to end of line, **ctrl-k** to kill to the end of the line, **ctrl-l** to clear the screen. See `man readline` for all the default keybindings in Bash. There are a lot. For example **alt-.** cycles through previous arguments, and **alt-*** expands a glob. + + +- Alternatively, if you love vi-style key-bindings, use `set -o vi` (and `set -o emacs` to put it back). + +- For editing long commands, after setting your editor (for example `export EDITOR=vim`), **ctrl-x** **ctrl-e** will open the current command in an editor for multi-line editing. Or in vi style, **escape-v**. + +- To see recent commands, use `history`. Follow with `!n` (where `n` is the command number) to execute again. There are also many abbreviations you can use, the most useful probably being `!$` for last argument and `!!` for last command (see "HISTORY EXPANSION" in the man page). However, these are often easily replaced with **ctrl-r** and **alt-.**. + +- Go to your home directory with `cd`. Access files relative to your home directory with the `~` prefix (e.g. `~/.bashrc`). In `sh` scripts refer to the home directory as `$HOME`. + +- To go back to the previous working directory: `cd -`. + +- If you are halfway through typing a command but change your mind, hit **alt-#** to add a `#` at the beginning and enter it as a comment (or use **ctrl-a**, **#**, **enter**). You can then return to it later via command history. + +- Use `xargs` (or `parallel`). It's very powerful. Note you can control how many items execute per line (`-L`) as well as parallelism (`-P`). If you're not sure if it'll do the right thing, use `xargs echo` first. Also, `-I{}` is handy. Examples: +```bash + find . -name '*.py' | xargs grep some_function + cat hosts | xargs -I{} ssh root@{} hostname +``` + +- `pstree -p` is a helpful display of the process tree. + +- Use `pgrep` and `pkill` to find or signal processes by name (`-f` is helpful). + +- Know the various signals you can send processes. For example, to suspend a process, use `kill -STOP [pid]`. For the full list, see `man 7 signal` + +- Use `nohup` or `disown` if you want a background process to keep running forever. + +- Check what processes are listening via `netstat -lntp` or `ss -plat` (for TCP; add `-u` for UDP) or `lsof -iTCP -sTCP:LISTEN -P -n` (which also works on macOS). + +- See also `lsof` and `fuser` for open sockets and files. + +- See `uptime` or `w` to know how long the system has been running. + +- Use `alias` to create shortcuts for commonly used commands. For example, `alias ll='ls -latr'` creates a new alias `ll`. + +- Save aliases, shell settings, and functions you commonly use in `~/.bashrc`, and [arrange for login shells to source it](http://superuser.com/a/183980/7106). This will make your setup available in all your shell sessions. + +- Put the settings of environment variables as well as commands that should be executed when you login in `~/.bash_profile`. Separate configuration will be needed for shells you launch from graphical environment logins and `cron` jobs. + +- Synchronize your configuration files (e.g. `.bashrc` and `.bash_profile`) among various computers with Git. + +- Understand that care is needed when variables and filenames include whitespace. Surround your Bash variables with quotes, e.g. `"$FOO"`. Prefer the `-0` or `-print0` options to enable null characters to delimit filenames, e.g. `locate -0 pattern | xargs -0 ls -al` or `find / -print0 -type d | xargs -0 ls -al`. To iterate on filenames containing whitespace in a for loop, set your IFS to be a newline only using `IFS=$'\n'`. + +- In Bash scripts, use `set -x` (or the variant `set -v`, which logs raw input, including unexpanded variables and comments) for debugging output. Use strict modes unless you have a good reason not to: Use `set -e` to abort on errors (nonzero exit code). Use `set -u` to detect unset variable usages. Consider `set -o pipefail` too, to abort on errors within pipes (though read up on it more if you do, as this topic is a bit subtle). For more involved scripts, also use `trap` on EXIT or ERR. A useful habit is to start a script like this, which will make it detect and abort on common errors and print a message: +```bash + set -euo pipefail + trap "echo 'error: Script failed: see failed command above'" ERR +``` + +- In Bash scripts, subshells (written with parentheses) are convenient ways to group commands. A common example is to temporarily move to a different working directory, e.g. +```bash + # do something in current dir + (cd /some/other/dir && other-command) + # continue in original dir +``` + +- In Bash, note there are lots of kinds of variable expansion. Checking a variable exists: `${name:?error message}`. For example, if a Bash script requires a single argument, just write `input_file=${1:?usage: $0 input_file}`. Using a default value if a variable is empty: `${name:-default}`. If you want to have an additional (optional) parameter added to the previous example, you can use something like `output_file=${2:-logfile}`. If `$2` is omitted and thus empty, `output_file` will be set to `logfile`. Arithmetic expansion: `i=$(( (i + 1) % 5 ))`. Sequences: `{1..10}`. Trimming of strings: `${var%suffix}` and `${var#prefix}`. For example if `var=foo.pdf`, then `echo ${var%.pdf}.txt` prints `foo.txt`. + +- Brace expansion using `{`...`}` can reduce having to re-type similar text and automate combinations of items. This is helpful in examples like `mv foo.{txt,pdf} some-dir` (which moves both files), `cp somefile{,.bak}` (which expands to `cp somefile somefile.bak`) or `mkdir -p test-{a,b,c}/subtest-{1,2,3}` (which expands all possible combinations and creates a directory tree). Brace expansion is performed before any other expansion. + +- The order of expansions is: brace expansion; tilde expansion, parameter and variable expansion, arithmetic expansion, and command substitution (done in a left-to-right fashion); word splitting; and filename expansion. (For example, a range like `{1..20}` cannot be expressed with variables using `{$a..$b}`. Use `seq` or a `for` loop instead, e.g., `seq $a $b` or `for((i=a; i<=b; i++)); do ... ; done`.) + +- The output of a command can be treated like a file via `<(some command)` (known as process substitution). For example, compare local `/etc/hosts` with a remote one: +```sh + diff /etc/hosts <(ssh somehost cat /etc/hosts) +``` + +- When writing scripts you may want to put all of your code in curly braces. If the closing brace is missing, your script will be prevented from executing due to a syntax error. This makes sense when your script is going to be downloaded from the web, since it prevents partially downloaded scripts from executing: +```bash +{ + # Your code here +} +``` + +- A "here document" allows [redirection of multiple lines of input](https://www.tldp.org/LDP/abs/html/here-docs.html) as if from a file: +``` +cat <logfile 2>&1` or `some-command &>logfile`. Often, to ensure a command does not leave an open file handle to standard input, tying it to the terminal you are in, it is also good practice to add `>> 2+3 +5 +``` + + +## Processing files and data + +- To locate a file by name in the current directory, `find . -iname '*something*'` (or similar). To find a file anywhere by name, use `locate something` (but bear in mind `updatedb` may not have indexed recently created files). + +- For general searching through source or data files, there are several options more advanced or faster than `grep -r`, including (in rough order from older to newer) [`ack`](https://github.com/beyondgrep/ack2), [`ag`](https://github.com/ggreer/the_silver_searcher) ("the silver searcher"), and [`rg`](https://github.com/BurntSushi/ripgrep) (ripgrep). + +- To convert HTML to text: `lynx -dump -stdin` + +- For Markdown, HTML, and all kinds of document conversion, try [`pandoc`](http://pandoc.org/). For example, to convert a Markdown document to Word format: `pandoc README.md --from markdown --to docx -o temp.docx` + +- If you must handle XML, `xmlstarlet` is old but good. + +- For JSON, use [`jq`](http://stedolan.github.io/jq/). For interactive use, also see [`jid`](https://github.com/simeji/jid) and [`jiq`](https://github.com/fiatjaf/jiq). + +- For YAML, use [`shyaml`](https://github.com/0k/shyaml). + +- For Excel or CSV files, [csvkit](https://github.com/onyxfish/csvkit) provides `in2csv`, `csvcut`, `csvjoin`, `csvgrep`, etc. + +- For Amazon S3, [`s3cmd`](https://github.com/s3tools/s3cmd) is convenient and [`s4cmd`](https://github.com/bloomreach/s4cmd) is faster. Amazon's [`aws`](https://github.com/aws/aws-cli) and the improved [`saws`](https://github.com/donnemartin/saws) are essential for other AWS-related tasks. + +- Know about `sort` and `uniq`, including uniq's `-u` and `-d` options -- see one-liners below. See also `comm`. + +- Know about `cut`, `paste`, and `join` to manipulate text files. Many people use `cut` but forget about `join`. + +- Know about `wc` to count newlines (`-l`), characters (`-m`), words (`-w`) and bytes (`-c`). + +- Know about `tee` to copy from stdin to a file and also to stdout, as in `ls -al | tee file.txt`. + +- For more complex calculations, including grouping, reversing fields, and statistical calculations, consider [`datamash`](https://www.gnu.org/software/datamash/). + +- Know that locale affects a lot of command line tools in subtle ways, including sorting order (collation) and performance. Most Linux installations will set `LANG` or other locale variables to a local setting like US English. But be aware sorting will change if you change locale. And know i18n routines can make sort or other commands run *many times* slower. In some situations (such as the set operations or uniqueness operations below) you can safely ignore slow i18n routines entirely and use traditional byte-based sort order, using `export LC_ALL=C`. + +- You can set a specific command's environment by prefixing its invocation with the environment variable settings, as in `TZ=Pacific/Fiji date`. + +- Know basic `awk` and `sed` for simple data munging. See [One-liners](#one-liners) for examples. + +- To replace all occurrences of a string in place, in one or more files: +```sh + perl -pi.bak -e 's/old-string/new-string/g' my-files-*.txt +``` + +- To rename multiple files and/or search and replace within files, try [`repren`](https://github.com/jlevy/repren). (In some cases the `rename` command also allows multiple renames, but be careful as its functionality is not the same on all Linux distributions.) +```sh + # Full rename of filenames, directories, and contents foo -> bar: + repren --full --preserve-case --from foo --to bar . + # Recover backup files whatever.bak -> whatever: + repren --renames --from '(.*)\.bak' --to '\1' *.bak + # Same as above, using rename, if available: + rename 's/\.bak$//' *.bak +``` + +- As the man page says, `rsync` really is a fast and extraordinarily versatile file copying tool. It's known for synchronizing between machines but is equally useful locally. When security restrictions allow, using `rsync` instead of `scp` allows recovery of a transfer without restarting from scratch. It also is among the [fastest ways](https://web.archive.org/web/20130929001850/http://linuxnote.net/jianingy/en/linux/a-fast-way-to-remove-huge-number-of-files.html) to delete large numbers of files: +```sh +mkdir empty && rsync -r --delete empty/ some-dir && rmdir some-dir +``` + +- For monitoring progress when processing files, use [`pv`](http://www.ivarch.com/programs/pv.shtml), [`pycp`](https://github.com/dmerejkowsky/pycp), [`pmonitor`](https://github.com/dspinellis/pmonitor), [`progress`](https://github.com/Xfennec/progress), `rsync --progress`, or, for block-level copying, `dd status=progress`. + +- Use `shuf` to shuffle or select random lines from a file. + +- Know `sort`'s options. For numbers, use `-n`, or `-h` for handling human-readable numbers (e.g. from `du -h`). Know how keys work (`-t` and `-k`). In particular, watch out that you need to write `-k1,1` to sort by only the first field; `-k1` means sort according to the whole line. Stable sort (`sort -s`) can be useful. For example, to sort first by field 2, then secondarily by field 1, you can use `sort -k1,1 | sort -s -k2,2`. + +- If you ever need to write a tab literal in a command line in Bash (e.g. for the -t argument to sort), press **ctrl-v** **[Tab]** or write `$'\t'` (the latter is better as you can copy/paste it). + +- The standard tools for patching source code are `diff` and `patch`. See also `diffstat` for summary statistics of a diff and `sdiff` for a side-by-side diff. Note `diff -r` works for entire directories. Use `diff -r tree1 tree2 | diffstat` for a summary of changes. Use `vimdiff` to compare and edit files. + +- For binary files, use `hd`, `hexdump` or `xxd` for simple hex dumps and `bvi`, `hexedit` or `biew` for binary editing. + +- Also for binary files, `strings` (plus `grep`, etc.) lets you find bits of text. + +- For binary diffs (delta compression), use `xdelta3`. + +- To convert text encodings, try `iconv`. Or `uconv` for more advanced use; it supports some advanced Unicode things. For example: +```sh + # Displays hex codes or actual names of characters (useful for debugging): + uconv -f utf-8 -t utf-8 -x '::Any-Hex;' < input.txt + uconv -f utf-8 -t utf-8 -x '::Any-Name;' < input.txt + # Lowercase and removes all accents (by expanding and dropping them): + uconv -f utf-8 -t utf-8 -x '::Any-Lower; ::Any-NFD; [:Nonspacing Mark:] >; ::Any-NFC;' < input.txt > output.txt +``` + +- To split files into pieces, see `split` (to split by size) and `csplit` (to split by a pattern). + +- Date and time: To get the current date and time in the helpful [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601) format, use `date -u +"%Y-%m-%dT%H:%M:%SZ"` (other options [are](https://stackoverflow.com/questions/7216358/date-command-on-os-x-doesnt-have-iso-8601-i-option) [problematic](https://unix.stackexchange.com/questions/164826/date-command-iso-8601-option)). To manipulate date and time expressions, use `dateadd`, `datediff`, `strptime` etc. from [`dateutils`](http://www.fresse.org/dateutils/). + +- Use `zless`, `zmore`, `zcat`, and `zgrep` to operate on compressed files. + +- File attributes are settable via `chattr` and offer a lower-level alternative to file permissions. For example, to protect against accidental file deletion the immutable flag: `sudo chattr +i /critical/directory/or/file` + +- Use `getfacl` and `setfacl` to save and restore file permissions. For example: +```sh + getfacl -R /some/path > permissions.txt + setfacl --restore=permissions.txt +``` + +- To create empty files quickly, use `truncate` (creates [sparse file](https://en.wikipedia.org/wiki/Sparse_file)), `fallocate` (ext4, xfs, btrfs and ocfs2 filesystems), `xfs_mkfile` (almost any filesystems, comes in xfsprogs package), `mkfile` (for Unix-like systems like Solaris, Mac OS). + +## System debugging + +- For web debugging, `curl` and `curl -I` are handy, or their `wget` equivalents, or the more modern [`httpie`](https://github.com/jkbrzt/httpie). + +- To know current cpu/disk status, the classic tools are `top` (or the better `htop`), `iostat`, and `iotop`. Use `iostat -mxz 15` for basic CPU and detailed per-partition disk stats and performance insight. + +- For network connection details, use `netstat` and `ss`. + +- For a quick overview of what's happening on a system, `dstat` is especially useful. For broadest overview with details, use [`glances`](https://github.com/nicolargo/glances). + +- To know memory status, run and understand the output of `free` and `vmstat`. In particular, be aware the "cached" value is memory held by the Linux kernel as file cache, so effectively counts toward the "free" value. + +- Java system debugging is a different kettle of fish, but a simple trick on Oracle's and some other JVMs is that you can run `kill -3 ` and a full stack trace and heap summary (including generational garbage collection details, which can be highly informative) will be dumped to stderr/logs. The JDK's `jps`, `jstat`, `jstack`, `jmap` are useful. [SJK tools](https://github.com/aragozin/jvm-tools) are more advanced. + +- Use [`mtr`](http://www.bitwizard.nl/mtr/) as a better traceroute, to identify network issues. + +- For looking at why a disk is full, [`ncdu`](https://dev.yorhel.nl/ncdu) saves time over the usual commands like `du -sh *`. + +- To find which socket or process is using bandwidth, try [`iftop`](http://www.ex-parrot.com/~pdw/iftop/) or [`nethogs`](https://github.com/raboof/nethogs). + +- The `ab` tool (comes with Apache) is helpful for quick-and-dirty checking of web server performance. For more complex load testing, try `siege`. + +- For more serious network debugging, [`wireshark`](https://wireshark.org/), [`tshark`](https://www.wireshark.org/docs/wsug_html_chunked/AppToolstshark.html), or [`ngrep`](http://ngrep.sourceforge.net/). + +- Know about `strace` and `ltrace`. These can be helpful if a program is failing, hanging, or crashing, and you don't know why, or if you want to get a general idea of performance. Note the profiling option (`-c`), and the ability to attach to a running process (`-p`). Use trace child option (`-f`) to avoid missing important calls. + +- Know about `ldd` to check shared libraries etc — but [never run it on untrusted files](http://www.catonmat.net/blog/ldd-arbitrary-code-execution/). + +- Know how to connect to a running process with `gdb` and get its stack traces. + +- Use `/proc`. It's amazingly helpful sometimes when debugging live problems. Examples: `/proc/cpuinfo`, `/proc/meminfo`, `/proc/cmdline`, `/proc/xxx/cwd`, `/proc/xxx/exe`, `/proc/xxx/fd/`, `/proc/xxx/smaps` (where `xxx` is the process id or pid). + +- When debugging why something went wrong in the past, [`sar`](http://sebastien.godard.pagesperso-orange.fr/) can be very helpful. It shows historic statistics on CPU, memory, network, etc. + +- For deeper systems and performance analyses, look at `stap` ([SystemTap](https://sourceware.org/systemtap/wiki)), [`perf`](https://en.wikipedia.org/wiki/Perf_%28Linux%29), and [`sysdig`](https://github.com/draios/sysdig). + +- Check what OS you're on with `uname` or `uname -a` (general Unix/kernel info) or `lsb_release -a` (Linux distro info). + +- Use `dmesg` whenever something's acting really funny (it could be hardware or driver issues). + +- If you delete a file and it doesn't free up expected disk space as reported by `du`, check whether the file is in use by a process: +`lsof | grep deleted | grep "filename-of-my-big-file"` + + +## One-liners + +A few examples of piecing together commands: + +- It is remarkably helpful sometimes that you can do set intersection, union, and difference of text files via `sort`/`uniq`. Suppose `a` and `b` are text files that are already uniqued. This is fast, and works on files of arbitrary size, up to many gigabytes. (Sort is not limited by memory, though you may need to use the `-T` option if `/tmp` is on a small root partition.) See also the note about `LC_ALL` above and `sort`'s `-u` option (left out for clarity below). +```sh + sort a b | uniq > c # c is a union b + sort a b | uniq -d > c # c is a intersect b + sort a b b | uniq -u > c # c is set difference a - b +``` + +- Pretty-print two JSON files, normalizing their syntax, then coloring and paginating the result: +``` + diff <(jq --sort-keys . < file1.json) <(jq --sort-keys . < file2.json) | colordiff | less -R +``` + +- Use `grep . *` to quickly examine the contents of all files in a directory (so each line is paired with the filename), or `head -100 *` (so each file has a heading). This can be useful for directories filled with config settings like those in `/sys`, `/proc`, `/etc`. + + +- Summing all numbers in the third column of a text file (this is probably 3X faster and 3X less code than equivalent Python): +```sh + awk '{ x += $3 } END { print x }' myfile +``` + +- To see sizes/dates on a tree of files, this is like a recursive `ls -l` but is easier to read than `ls -lR`: +```sh + find . -type f -ls +``` + +- Say you have a text file, like a web server log, and a certain value that appears on some lines, such as an `acct_id` parameter that is present in the URL. If you want a tally of how many requests for each `acct_id`: +```sh + egrep -o 'acct_id=[0-9]+' access.log | cut -d= -f2 | sort | uniq -c | sort -rn +``` + +- To continuously monitor changes, use `watch`, e.g. check changes to files in a directory with `watch -d -n 2 'ls -rtlh | tail'` or to network settings while troubleshooting your wifi settings with `watch -d -n 2 ifconfig`. + +- Run this function to get a random tip from this document (parses Markdown and extracts an item): +```sh + function taocl() { + curl -s https://raw.githubusercontent.com/jlevy/the-art-of-command-line/master/README.md | + sed '/cowsay[.]png/d' | + pandoc -f markdown -t html | + xmlstarlet fo --html --dropdtd | + xmlstarlet sel -t -v "(html/body/ul/li[count(p)>0])[$RANDOM mod last()+1]" | + xmlstarlet unesc | fmt -80 | iconv -t US + } +``` + + +## Obscure but useful + +- `expr`: perform arithmetic or boolean operations or evaluate regular expressions + +- `m4`: simple macro processor + +- `yes`: print a string a lot + +- `cal`: nice calendar + +- `env`: run a command (useful in scripts) + +- `printenv`: print out environment variables (useful in debugging and scripts) + +- `look`: find English words (or lines in a file) beginning with a string + +- `cut`, `paste` and `join`: data manipulation + +- `fmt`: format text paragraphs + +- `pr`: format text into pages/columns + +- `fold`: wrap lines of text + +- `column`: format text fields into aligned, fixed-width columns or tables + +- `expand` and `unexpand`: convert between tabs and spaces + +- `nl`: add line numbers + +- `seq`: print numbers + +- `bc`: calculator + +- `factor`: factor integers + +- [`gpg`](https://gnupg.org/): encrypt and sign files + +- `toe`: table of terminfo entries + +- `nc`: network debugging and data transfer + +- `socat`: socket relay and tcp port forwarder (similar to `netcat`) + +- [`slurm`](https://github.com/mattthias/slurm): network traffic visualization + +- `dd`: moving data between files or devices + +- `file`: identify type of a file + +- `tree`: display directories and subdirectories as a nesting tree; like `ls` but recursive + +- `stat`: file info + +- `time`: execute and time a command + +- `timeout`: execute a command for specified amount of time and stop the process when the specified amount of time completes. + +- `lockfile`: create semaphore file that can only be removed by `rm -f` + +- `logrotate`: rotate, compress and mail logs. + +- `watch`: run a command repeatedly, showing results and/or highlighting changes + +- [`when-changed`](https://github.com/joh/when-changed): runs any command you specify whenever it sees file changed. See `inotifywait` and `entr` as well. + +- `tac`: print files in reverse + +- `comm`: compare sorted files line by line + +- `strings`: extract text from binary files + +- `tr`: character translation or manipulation + +- `iconv` or `uconv`: conversion for text encodings + +- `split` and `csplit`: splitting files + +- `sponge`: read all input before writing it, useful for reading from then writing to the same file, e.g., `grep -v something some-file | sponge some-file` + +- `units`: unit conversions and calculations; converts furlongs per fortnight to twips per blink (see also `/usr/share/units/definitions.units`) + +- `apg`: generates random passwords + +- `xz`: high-ratio file compression + +- `ldd`: dynamic library info + +- `nm`: symbols from object files + +- `ab` or [`wrk`](https://github.com/wg/wrk): benchmarking web servers + +- `strace`: system call debugging + +- [`mtr`](http://www.bitwizard.nl/mtr/): better traceroute for network debugging + +- `cssh`: visual concurrent shell + +- `rsync`: sync files and folders over SSH or in local file system + +- [`wireshark`](https://wireshark.org/) and [`tshark`](https://www.wireshark.org/docs/wsug_html_chunked/AppToolstshark.html): packet capture and network debugging + +- [`ngrep`](http://ngrep.sourceforge.net/): grep for the network layer + +- `host` and `dig`: DNS lookups + +- `lsof`: process file descriptor and socket info + +- `dstat`: useful system stats + +- [`glances`](https://github.com/nicolargo/glances): high level, multi-subsystem overview + +- `iostat`: Disk usage stats + +- `mpstat`: CPU usage stats + +- `vmstat`: Memory usage stats + +- `htop`: improved version of top + +- `last`: login history + +- `w`: who's logged on + +- `id`: user/group identity info + +- [`sar`](http://sebastien.godard.pagesperso-orange.fr/): historic system stats + +- [`iftop`](http://www.ex-parrot.com/~pdw/iftop/) or [`nethogs`](https://github.com/raboof/nethogs): network utilization by socket or process + +- `ss`: socket statistics + +- `dmesg`: boot and system error messages + +- `sysctl`: view and configure Linux kernel parameters at run time + +- `hdparm`: SATA/ATA disk manipulation/performance + +- `lsblk`: list block devices: a tree view of your disks and disk partitions + +- `lshw`, `lscpu`, `lspci`, `lsusb`, `dmidecode`: hardware information, including CPU, BIOS, RAID, graphics, devices, etc. + +- `lsmod` and `modinfo`: List and show details of kernel modules. + +- `fortune`, `ddate`, and `sl`: um, well, it depends on whether you consider steam locomotives and Zippy quotations "useful" + + +## macOS only + +These are items relevant *only* on macOS. + +- Package management with `brew` (Homebrew) and/or `port` (MacPorts). These can be used to install on macOS many of the above commands. + +- Copy output of any command to a desktop app with `pbcopy` and paste input from one with `pbpaste`. + +- To enable the Option key in macOS Terminal as an alt key (such as used in the commands above like **alt-b**, **alt-f**, etc.), open Preferences -> Profiles -> Keyboard and select "Use Option as Meta key". + +- To open a file with a desktop app, use `open` or `open -a /Applications/Whatever.app`. + +- Spotlight: Search files with `mdfind` and list metadata (such as photo EXIF info) with `mdls`. + +- Be aware macOS is based on BSD Unix, and many commands (for example `ps`, `ls`, `tail`, `awk`, `sed`) have many subtle variations from Linux, which is largely influenced by System V-style Unix and GNU tools. You can often tell the difference by noting a man page has the heading "BSD General Commands Manual." In some cases GNU versions can be installed, too (such as `gawk` and `gsed` for GNU awk and sed). If writing cross-platform Bash scripts, avoid such commands (for example, consider Python or `perl`) or test carefully. + +- To get macOS release information, use `sw_vers`. + +## Windows only + +These items are relevant *only* on Windows. + +### Ways to obtain Unix tools under Windows + +- Access the power of the Unix shell under Microsoft Windows by installing [Cygwin](https://cygwin.com/). Most of the things described in this document will work out of the box. + +- On Windows 10, you can use [Windows Subsystem for Linux (WSL)](https://msdn.microsoft.com/commandline/wsl/about), which provides a familiar Bash environment with Unix command line utilities. + +- If you mainly want to use GNU developer tools (such as GCC) on Windows, consider [MinGW](http://www.mingw.org/) and its [MSYS](http://www.mingw.org/wiki/msys) package, which provides utilities such as bash, gawk, make and grep. MSYS doesn't have all the features compared to Cygwin. MinGW is particularly useful for creating native Windows ports of Unix tools. + +- Another option to get Unix look and feel under Windows is [Cash](https://github.com/dthree/cash). Note that only very few Unix commands and command-line options are available in this environment. + +### Useful Windows command-line tools + +- You can perform and script most Windows system administration tasks from the command line by learning and using `wmic`. + +- Native command-line Windows networking tools you may find useful include `ping`, `ipconfig`, `tracert`, and `netstat`. + +- You can perform [many useful Windows tasks](http://www.thewindowsclub.com/rundll32-shortcut-commands-windows) by invoking the `Rundll32` command. + +### Cygwin tips and tricks + +- Install additional Unix programs with the Cygwin's package manager. + +- Use `mintty` as your command-line window. + +- Access the Windows clipboard through `/dev/clipboard`. + +- Run `cygstart` to open an arbitrary file through its registered application. + +- Access the Windows registry with `regtool`. + +- Note that a `C:\` Windows drive path becomes `/cygdrive/c` under Cygwin, and that Cygwin's `/` appears under `C:\cygwin` on Windows. Convert between Cygwin and Windows-style file paths with `cygpath`. This is most useful in scripts that invoke Windows programs. + +## More resources + +- [awesome-shell](https://github.com/alebcay/awesome-shell): A curated list of shell tools and resources. +- [awesome-osx-command-line](https://github.com/herrbischoff/awesome-osx-command-line): A more in-depth guide for the macOS command line. +- [Strict mode](http://redsymbol.net/articles/unofficial-bash-strict-mode/) for writing better shell scripts. +- [shellcheck](https://github.com/koalaman/shellcheck): A shell script static analysis tool. Essentially, lint for bash/sh/zsh. +- [Filenames and Pathnames in Shell](http://www.dwheeler.com/essays/filenames-in-shell.html): The sadly complex minutiae on how to handle filenames correctly in shell scripts. +- [Data Science at the Command Line](http://datascienceatthecommandline.com/#tools): More commands and tools helpful for doing data science, from the book of the same name + + diff --git a/Linux Keyboard Shortcuts.md b/Linux Keyboard Shortcuts.md new file mode 100644 index 0000000..857a3a8 --- /dev/null +++ b/Linux Keyboard Shortcuts.md @@ -0,0 +1,54 @@ +--- +title: Linux Keyboard Shortcuts +created: '2025-06-29T16:45:05.400Z' +modified: '2025-06-29T16:45:10.372Z' +--- + + + + +## Linux Keyboard Shortcuts + +| **Key** | **Function** | +| :--: | :-- | +| **Cursor Movement** | +| Ctrl+B | Move cursor backward one character position | +| Ctrl+F | Move cursor forward one character position | +| Alt+F | Move cursor one word forward in line | +| Alt+B | Move cursor one word backward in line | +| Ctrl+A | Move cursor to beginning of text in line | +| Ctrl+E | Move cursor to end of text in line | +| **Text Editing** | +| Ctrl+D | Erase character forward from cursor left to right | +| Ctrl+H | Erase character backward from cursor right to left | +| Alt+D | Erase the next word from cursor left to right | +| Ctrl+W | Erase backwards to first non-alphanumeric character or empty space | +| Ctrl+U | Erase a line of input from cursor right to left | +| Ctrl+K | Erase a line of input from cursor left to right | +| Alt+R | Revert any changes to a previously executed command that's being edited | +| Alt+. | Fetch and paste the last word at end of a command from previous commands | +| Ctrl+T | Reverse the position of the character the cursor is on with the previous character | +| Alt+T | Reverse the position of the word the cursor is in with the previous word | +| Alt+U | Capitalize every character from cursor left to right to the end of one word | +| Alt+L | Uncapitalize every character from cursor left to right to the end of one word | +| Ctrl+Y | Paste (yank) text cut by Ctrl+U, Ctrl+K, or Ctrl+W | +| Ctrl+_ | Undo last change | +| **Command History** | +| Ctrl+P | Fetch previous command from history list | +| Ctrl+N | Fetch next command from history list | +| Ctrl+R | Reverse search commands from history list | +| **Process Control** | +| Ctrl+C | Break/Terminate a foreground job | +| Ctrl+Z | Suspend a foreground job | +| Ctrl+S | Suspend output to screen | +| Ctrl+Q | Resume output to screen | +| **Terminal Control** | +| Ctrl+L | Clear screen contents (command-line needs to be empty) | +| Ctrl+D | Log out from a shell (similar to exit) or send EOF | +| Ctrl+O | Accept the current line for execution and fetch the next command | +| Ctrl+X (twice) | Toggle cursor between current position and beginning of the line | +| **Text Editing (Terminal)** | +| Ctrl+Shift+C | Copy | +| Ctrl+Shift+V | Paste | +| Shift+Insert | Paste (alternative) | + diff --git a/Linux Special Characters.md b/Linux Special Characters.md new file mode 100644 index 0000000..6cebd5d --- /dev/null +++ b/Linux Special Characters.md @@ -0,0 +1,73 @@ +--- +title: Linux Special Characters +created: '2025-06-29T16:35:54.601Z' +modified: '2025-06-29T16:50:23.796Z' +--- + +## Linux Special Characters +> **Note** 💡
+> This symbol "└─>" represents an alternative meaning of some characters. + +| Char.     | Description | +| --- | --- | +| ~ | Home directory [tilde]. The path to a user's home directory location. | +| - | Last directory [hyphen]. Go to the previously chosen directory. | +| └─> | Option flag for a command or filter. | +| └─> | Arithmetic operator. Minus of arithmetic operations. | +| / | Root directory [forward slash]. The path to root directory location. | +| └─> | Filename path separator. | +| \ | Escape [backslash]. A quoting mechanism for single characters. It preserves the literal value of the next character that follows, with the exception of newline. | +| └─> | Arithmetic operator. Divider of arithmetic operations. | +| \| | Pipe. This is a method of chaining commands together. Connects the output (stdout) of command1 to the input (stdin) of command2. Each command reads the previous command’s output. | +| \|& | This operator connects the output (stdout) and error (stderr) of command1 to the input (stdin) of command2. | +| \|\| | The OR operator is used to chain commands. It will execute the first command then stop if successful, if not, it will proceed pass failed commands until one is successful and stop. | +| && | The AND operator is used to chain commands. It will execute commands only if the first command is successful and proceed until one fails. | +| \; | Command separator [semicolon]. Used to separate multiple commands and output all successful and failed ones. | +| & | Run job in background [and]. A command followed by an & will run in the background. | +| >, >>, < | Redirect a command's standard output (stdout) or input (stdin) into a file. | +| &>, >& | Redirects a command's both standard output (stdout) and error (stderr) into a file. | +| <&- | Close standard input (stdin) to prevent showing from a file. | +| >&- | Close standard output (stdout) to prevent showing from a file. | +| >| | Force redirection (even if the noclobber option is set). This will forcibly overwrite an existing file. | +| " | Partial quoting [double quotes]. Protects the text inside them from being split into multiple words or arguments, yet allow substitutions to occur, meaning most other special characters is usually prevented. | +| . | Source command [period]. To evaluate commands in the current execution context. This is a bash builtin. | +| └─> | "As a component of a filename. When working with filenames, a leading dot is the prefix of a "hidden" file, a file that an ls will not normally show. | +| └─> | Character match. When matching characters, as part of a regular expression, a "dot" matches a single character. | +| ' | Full quoting [single quotes]. Protects the text inside them so that it has a literal meaning. This is a stronger form of quoting than double quotes. | +| ` | Command substitution [backquotes]. Assign the output of a shell command to a variable. | +| # | Comment [number sign]. Lines in files beginning with a # (with the exception of #!) are comments and will not be executed. | +| ! | Reverse (or negate) [exclamation]. The ! operator inverts the exit status of the command to which it is applied. It also inverts the meaning of a test operator. | +| * | Wild card [asterisk]. The * character serves as a "wild card" for filename expansion in globbing. By itself, it matches every filename in a given directory. | +| └─> | Arithmetic operator. Multiplier of arithmetic operations. | +| ? | Wild card [question mark]. The ? character serves as a single-character "wild card" for filename expansion in globbing, as well as representing one character in an extended regular expression. | +| └─> | Test operator. Within certain expressions, the ? indicates a test for a condition. | +| { } | Inline group [curly brackets]. Commands inside the curly braces are treated as if they were one command. | +| └─> | Placeholder for output text. | +| ( ) | Subshell group [parentheses]. Commands within are executed in a subshell (a new process) Used much like a sandbox, if a command causes side effects (like changing variables), it will have no effect on the current shell. | +| [ ] | Test expression [brackets]. It is part of the shell builtin test. | +| └─> | Array element. Brackets set off the numbering of each element. | +| └─> | Range of characters. As part of a regular expression, brackets delineate a range of characters to match. | +| \[\[ \]\] | Test/Evaluate [double brackets] a condition expression to determine whether true or false. It is more flexible than the single-bracket [ ] test. | +| (( )) | Arithmetic expression [double parentheses]. Characters such as +, -, *, and / are mathematical operators used for calculations. | +| ~+ | Current working directory. | +| ~- | Previous working directory. | +| : | Null command [colon]. This is the shell equivalent of a "NOP" (no op, a do-nothing operation). It may be considered a synonym for the shell builtin true. | +| ;; | Terminator [double semicolon]. Only used in case constructs to indicate the end of an alternative. | +| " " | Whitespace. This is a tab, newline, vertical tab, form feed, carriage return, or space. Bash uses whitespace to determine where words begin and end. | +| ,, , | Lowercase conversion in parameter substitution. | +| ^, ^^ | Uppercase conversion in parameter substitution. | +| $ | Variable substitution. A $ prefixing a variable name indicates the value the variable holds | +| └─> | End-of-line. In a regular expression, a "$" addresses the end of a line of text. | +| $* | All the arguments are seen as a single word. | +| !! | The previous command. | +| !$ | The last argument to the previous command. | +| !* | All the arguments from the previous command. | +| $? | The exit status of the last command executed. | +| $# | The number of arguments supplied to a script. | +| $$ | The process number of the current shell. For shell scripts, this is the process ID under which they are executing. | +| $! | The process number of the last background command. | +| $_ | Special variable set to final argument of previous command executed. | +| $- | Expands to the current option flags as specified upon invocation, by the set builtin command, or those set by the shell itself (such as the -i option). | +| $0 | Used to reference the name of the current shell or current shell script. | +| $n | These variables correspond to the arguments with which a script was invoked. Here n is a positive decimal number corresponding to the position of an argument (the first argument is $1, the second argument is $2, and so on). | +