BASH Cheat Sheet - Practically Networked

This BASH cheat sheet has been designed as a quick reference guide for both beginners and advanced users. The BASH (Bourne Again SHell) environment is powerful and versatile, serving as the default shell for the majority of Linux distributions.

It allows users to interact with their operating system, automate tasks, and even write scripts to enhance functionality.

When a user types a command in BASH, the shell interprets that command and communicates with the operating system to execute it. It is essentially a bridge between the user and the operating system. BASH also features programming capabilities, allowing users to write scripts – sequences of commands – to automate repetitive tasks.

It offers variables, conditional statements, loops, and functions, making it a powerful tool for managing and interacting with a Linux system. Its flexibility and scriptability make it an essential tool for system administrators and programmers alike.

This cheat sheet comprises a range of essential commands and functionalities, including file and directory manipulation, process control, permissions management, and more. Whether you’re troubleshooting a problem, managing your system, or learning the basics, this cheat sheet is a handy resource to make your journey smoother and more efficient.

Bash Scripting Basics

Syntax	Explanation
`#!/bin/bash`	Instructs the operating system to interpret the file using the given path
`bash file.sh`	Triggers the execution of the script in the command line
`./file.sh`	Executes the script, given that it has execute permissions
`#`	Denotes commentary within the script for user readability
`&&`	Represents a logical AND operation
`\|\|`	Stands for a logical OR operation
`$#`	Returns the count of arguments supplied to the script
`$0`	Displays the name of the shell or shell script
`$1, $2`	Acts as an input parameter which can be provided when launching the script
`exit [0-255]`	Terminates the script, returning a value between 0 and 255
`$`	Prefixed to parameters and variables for referencing
`()`	Creates a subshell to run commands
`$()`	Captures the output from commands
`(())`	Allows for arithmetic operations
`$(( ))`	Captures the result of arithmetic computations
`[]`	Useful for file name expansion and string manipulation
`<( )`	Enables process substitution, akin to a pipe
`{ }`	Expands sequences
`${ }`	Aids in variable interpolation and string manipulation
`\|`	Connects multiple commands to run in conjunction
`>`	Redirects output to a specified file
`>>`	Appends output to a given file
`;`	Serves as a separator for multiple commands
`<`	Fetches input from a specified file
`~`	Shortcut to the home directory
`~/.bashrc`	File accessed by each non-login shell
`/etc/profile`	Auto-executed at user login

File Test Operators

Operators	Explanation
`-e`	Evaluates if a file or directory exists
`-f`	Confirms if a provided file is a regular file
`-d`	Assesses if the input file is actually a directory
`-b`	Tests if the input file is a block device
`-s`	Validates if the file has a size greater than zero
`-L`	Examines if the file is a symbolic link
`-S`	Checks whether the file is a socket
`-r`	Tests for read permissions on the file
`-w`	Determines if the file holds write permissions
`-x`	Examines if the file has execute permissions
`-g`	Applies the group id on file or directory
`-u`	Assigns the user id on file or directory
`-k`	Sets the sticky bit on a file or directory
`-O`	Verifies if the user is the owner of the file
`f1 -nt f2`	Assesses if file f1 is newer than file f2
`f1 -ot f2`	Evaluates if file f1 is older than file f2

Loops and Conditions

Loops	Explanation
`if then fi`	Tests a condition, proceeding if true
`if then else fi`	Tests a condition, with an alternate path if false
`if then elif else fi`	Tests multiple conditions, with a fallback if all are false
`for do done`	Iterates over a list of items
`while do done`	Repeats a set of commands while a certain condition is true
`until do done`	Executes commands until a specific condition becomes false
`sleep time`	Pauses the script for a specified duration
`break`	Exits the current loop, but allows the script to continue
`continue`	Skips the current iteration and moves to the next loop iteration

Comparison Operators

Integer Operators

Operators	Explanation
`-eq`	Denotes equality
`-ne`	Denotes inequality
`-gt`	Indicates a greater numerical value
`-ge`	Indicates a numerical value that is greater or equal
`-lt`	Indicates a lesser numerical value
`-le`	Indicates a numerical value that is less or equal

String Operators

Operators	Explanation
`=`	Specifies that two strings are identical
`==`	Specifies that two strings are identical
`!=`	Specifies that two strings are different
`<`	Indicates that one string is less than another
`<=`	Indicates that a string is less than or identical to another
`>`	Indicates that one string is greater than another
`>=`	Indicates that a string is greater than or identical to another
`-z`	Tests if the string is null or empty
`-n`	Tests if the string is non-null or not empty

Regular Expressions

Expressions	Explanation
`.`	Corresponds to any solitary character
`?`	The item before is optional and can be matched at most once
`*`	Allows the item before to be matched zero times or more
`+`	Allows the item before to be matched at least once
`{N}`	Ensures the item before is matched exactly N times
`{N,}`	Ensures the item before is matched at least N times
`^`	Matches an empty string that starts a line
`$`	Matches an empty string that ends a line
`[a-d]`	Matches any single character between a and d inclusively

Bash Arrays and Functions

Array	Explanation
`array=("elements of array")`	Establishes an array containing string elements
`${array[0]}`	Retrieves the first element in the array
`${array[*]}`	Retrieves all the elements contained in the array
`${array[-1]}`	Retrieves the final element in the array
`${array[@]}`	Expands and presents all elements of the array
`shift`	Moves argument from the second position `$2` to the first `$1`
`function() { content-of-function }`	Outlines the structure for defining a function
`alias`	Shows all the aliases set in the current session
`alias alias='any command'`	Defines a new alias for a particular command

Shell Builtins

Builtins	Explanation
`.`	Executes commands from a specified file in the active shell
`alias`	Establishes an alias for a given command
`bg`	Executes a job in the background
`bind`	Assigns a keyboard sequence to a command
`break`	Exits the current loop in a script
`cd`	Changes the current directory
`command`	Runs a specific command, bypassing shell function lookup
`continue`	Moves to the next iteration of the current loop in a script
`declare`	Declares a variable
`dirs`	Displays the stack of remembered directories
`disown`	Removes a job from the shell’s job table
`enable`	Enables or disables a built-in shell command
`exec`	Replaces the shell with the specified command
`exit`	Terminates the shell with a defined exit status
`export`	Makes a variable accessible to sub-processes
`fc`	Selects a range of commands from the history list
`fg`	Executes a job in the foreground
`hash`	Records the full path of the specified command for future use
`help`	Displays the help manual
`history`	Shows the command history
`jobs`	Lists all running jobs
`logout`	Exits the current shell session
`pwd`	Shows the current directory path
`read`	Reads a line from standard input and assigns it to a variable
`popd`	Removes entries from the directory stack
`pushd`	Adds a directory to the directory stack
`printf`	Prints formatted text
`source`	Reads and executes commands from a file in the current shell
`times`	Shows the accumulated user and system times
`wait`	Causes the shell to wait for a job completion

Common Utilities and Switches

Commands	Explanation
`ls -l`	Displays files with their types and permissions
`ls -a`	Displays all files, including hidden ones
`pwd`	Shows the current working directory
`whoami`	Reveals the username of the current user
`last`	Shows the latest user login information
`find /home -name *.txt`	Searches for all text files in the /home directory
`find . -size 10k -print`	Searches for all files larger than 10k in the current directory
`egrep “(foo	bar)” file.txt`
`sed s/foo/bar/g file.txt`	Replaces “foo” with “bar” in file.txt
`locate file.txt`	Finds the location of file.txt quickly
`grep foo file.txt`	Searches for the word “foo” in file.txt
`ps -ef`	Lists all currently running services
`netstat -ant`	Displays all network connections
`netstat -ent`	Shows all established network connections
`ifconfig`	Displays all network interfaces, IPs, and MAC addresses
`ping`	Tests host reachability
`nslookup`	Used for DNS query purposes
`ssh`	Logs in to a remote Linux system
`scp -r dir user@remote-ip:/opt/`	Recursively copies all files and directories from the local system to a remote system
`scp -r user@remote-ip:/opt/ dir/`	Recursively copies all files and directories from a remote system to the local system
`rsync -avz localdir user@remote-ip:/backup`	Synchronizes files/directories between the local and remote systems
`df -h`	Displays free and used space on mounted filesystems
`du -sh`	Shows the total disk usage of the current directory
`free -m`	Displays free and used memory and swap space
`lsof`	Lists files that are open by running processes
`chown user:group filename`	Changes the owner and group of a file or directory
`chmod ugo file.txt`	Modifies the permissions of file.txt for user, group, and others
`kill pid`	Terminates a specified running process
`passwd username`	Sets or resets the password for a user
`top`	Displays all running processes, memory usage, and CPU usage in real-time