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Command-Line & Shell Cheatsheet

A comprehensive cheat sheet for navigating and utilizing command-line interfaces and shell environments effectively. This guide covers essential commands, scripting techniques, and environment configurations for improved productivity.

Basic Navigation & File Management

Navigation Commands

pwd

Print working directory (shows the current directory).

cd <directory>

Change directory to <directory>. Use cd .. to go up one level.

ls

List directory contents (files and subdirectories).

ls -l

List directory contents in long format (permissions, size, etc.).

ls -a

List all files, including hidden files (starting with .).

ls -t

List files sorted by modification time (newest first).

File & Directory Manipulation

mkdir <directory>

Create a new directory named <directory>.

touch <file>

Create an empty file named <file> or update the timestamp if the file exists.

cp <source> <destination>

Copy the file or directory <source> to <destination>.

mv <source> <destination>

Move or rename the file or directory <source> to <destination>.

rm <file>

Remove (delete) the file <file>. Warning: This is permanent!

rm -r <directory>

Remove the directory <directory> and its contents recursively. Use with caution!

File Viewing

cat <file>

Display the entire contents of <file> on the terminal.

less <file>

View the contents of <file> one page at a time, allowing navigation.

head <file>

Display the first few lines of <file> (default is 10 lines).

tail <file>

Display the last few lines of <file> (default is 10 lines).

tail -f <file>

Display the last few lines of <file> and continue to display new lines as they are added (follow mode).

wc <file>

Word count - Display number of lines, words, and bytes in file.

Piping, Redirection, and Permissions

Piping and Redirection

| (pipe)

Pass the output of one command as input to another command.

Example: ls -l | grep 'txt' (list files and filter for those containing ‘txt’)

> (redirect output)

Redirect the output of a command to a file, overwriting the file if it exists.

Example: ls > files.txt (save the list of files to files.txt)

>> (append output)

Append the output of a command to a file without overwriting it.

Example: echo 'New line' >> files.txt

2> (redirect error)

Redirect standard error to a file.

Example: command 2> error.log

&> (redirect both)

Redirect standard output and standard error to a file.

Example: command &> output.log

< (redirect input)

Redirect input from a file to a command.

Example: wc < files.txt (count words in files.txt)

File Permissions

chmod <permissions> <file>

Change the permissions of a file or directory. Permissions can be specified numerically (e.g., 755) or symbolically (e.g., u+rwx,g+rx,o+rx).

chown <user>:<group> <file>

Change the owner and group of a file or directory.

ls -l output

The output shows permissions in the format -rwxr-xr--. The first character indicates the file type (e.g., - for regular file, d for directory). The next three characters are the owner’s permissions, followed by the group’s permissions, and then others’ permissions. r = read, w = write, x = execute.

Numeric Permissions

4 = read, 2 = write, 1 = execute. Add these values to set permissions. For example, 7 (4+2+1) means read, write, and execute.

Symbolic Permissions

u = user/owner, g = group, o = others, a = all. + adds a permission, - removes a permission, = sets a permission.

Example: chmod u+x <file> (add execute permission for the owner)

umask

Sets default permissions for newly created files and directories. Common value is 022.

Process Management

ps

Display a snapshot of the current processes.

ps aux

Display a comprehensive list of all processes.

top

Display a dynamic real-time view of running processes.

kill <PID>

Terminate the process with the specified process ID (PID).

Example: kill 1234 (kills process with PID 1234)

kill -9 <PID>

Forcefully terminate the process (use as a last resort).

Example: kill -9 1234

bg

Place a stopped job in the background.

fg

Move a background job to the foreground.

jobs

List active jobs.

Shell Scripting Basics

Script Structure

A shell script is a text file containing a sequence of commands.

  • The first line should specify the interpreter using a shebang (#!): 
    #!/bin/bash
  • Comments start with #.
  • Make the script executable using chmod +x <script_name>.

Variables

Defining a variable

variable_name="value" (no spaces around =)`

Accessing a variable

$variable_name or ${variable_name}

Environment variables

Accessed like regular variables. Examples: $HOME, $PATH, $USER

Read-only variables

readonly variable_name

Unsetting a variable

unset variable_name

Control Structures

If statement:

if [ condition ]; then
  commands
elif [ condition ]; then
  commands
else
  commands
fi

For loop:

for variable in word1 word2 ... wordN; do
  commands
done

While loop:

while [ condition ]; do
  commands
done

Until loop:

until [ condition ]; do
  commands
done

Functions

Defining a function

 
function_name() {
  commands
}

or

function function_name {
  commands
}

Calling a function

function_name

Passing arguments

Inside the function, access arguments using $1, $2, etc.

Returning a value

Use return value (value must be an integer between 0 and 255). Use echo to return other types of data, but capture the output.

Command Substitution

$(command)

Execute command and substitute the output into the current command line.

Example: echo "Today is $(date +%Y-%m-%d)"

`command`

(Deprecated) - An older form of command substitution (using backticks).

Advanced Shell Techniques

Regular Expressions (grep)

grep is a powerful tool for searching text using regular expressions.

  • grep 'pattern' <file>: Search for lines containing pattern in file.
  • grep -i 'pattern' <file>: Case-insensitive search.
  • grep -r 'pattern' <directory>: Recursive search in directory.
  • grep -v 'pattern' <file>: Invert the match (show lines that do not contain pattern).
  • grep -E 'pattern' <file>: Use extended regular expressions.

sed (Stream Editor)

sed is a powerful stream editor for transforming text.

  • sed 's/old/new/g' <file>: Replace all occurrences of old with new in file.
  • sed -i 's/old/new/g' <file>: Replace in-place (modifies the file directly).
  • sed '/pattern/d' <file>: Delete lines containing pattern.
  • sed '2d' <file>: Delete the second line.
  • sed '$d' <file>: Delete the last line.

awk (Pattern Scanning and Processing Language)

awk is a powerful programming language for text processing.

  • awk '{print $1}' <file>: Print the first field of each line in file (fields are separated by spaces by default).
  • awk -F',' '{print $2}' <file>: Print the second field of each line, using , as the field separator.
  • awk '/pattern/ {print $0}' <file>: Print lines containing pattern.
  • awk 'BEGIN {print "Start"} {print $1} END {print "End"}' <file>: Execute code before and after processing the file.

find

find . -name "*.txt"

Find all files with the .txt extension in the current directory and its subdirectories.

find / -type d -name "config"

Find all directories named config in the entire file system.

find . -size +1M

Find all files larger than 1MB in the current directory.

find . -mtime -7

Find files modified in the last 7 days.

find . -user <username>

Find all files owned by <username>.

find . -exec ls -l {} \;

Execute the ls -l command on each file found.
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