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Shell Scripting Cheatsheet

A quick reference guide to shell scripting, covering essential syntax, commands, and best practices for automating tasks in Unix-like environments.

Basic Syntax & Structure

Script Structure

Every shell script starts with a shebang line to specify the interpreter:
#!/bin/bash

Followed by comments, variables, and commands.

Comments are denoted by #.

Example:
# This is a comment

Variables are assigned using = without spaces.

Example:
NAME="John Doe"
echo $NAME

Variables

$VAR or ${VAR}

Accessing a variable’s value.
${VAR} is useful for variable expansion.

Example:
echo "Hello, ${NAME}!"

$0

Name of the script.

$1, $2, ...

Arguments passed to the script.

$#

Number of arguments passed to the script.

$@ or $*

All arguments as a single string or separate words.

$?

Exit status of the last executed command.

Input/Output

echo - Displays text.

Example:
echo "Hello, world!"

read - Reads input from the user.

Example:
read -p "Enter your name: " NAME

> - Redirects output to a file (overwrites).

Example:
echo "Hello" > file.txt

>> - Redirects output to a file (appends).

Example:
echo "Hello" >> file.txt

< - Redirects input from a file.

Example:
wc -l < file.txt

Control Flow

Conditional Statements

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

Example:

if [ $AGE -gt 18 ]; then
  echo "Adult"
elif [ $AGE -lt 13 ]; then
  echo "Child"
else
  echo "Teenager"
fi

Looping

for VAR in item1 item2 ...; do commands done

Iterates over a list of items.

Example:

for i in 1 2 3; do
  echo $i
done

while [ condition ]; do commands done

Executes commands while a condition is true.

Example:

i=0
while [ $i -lt 5 ]; do
  echo $i
  i=$((i+1))
done

until [ condition ]; do commands done

Executes commands until a condition is true.

Example:

i=0
until [ $i -ge 5 ]; do
  echo $i
  i=$((i+1))
done

Case Statements

case VAR in pattern1) commands ;; pattern2) commands ;; *) commands ;; esac

Example:

case $OS in
  "Linux") echo "Linux OS" ;;
  "Windows") echo "Windows OS" ;;
  *) echo "Other OS" ;;
esac

Functions & Commands

Function Definition

function_name () { commands } or function function_name { commands }

Example:

my_function () {
  echo "Hello from my_function"
}
my_function

Passing arguments to functions:
function_name arg1 arg2
Access arguments inside the function using $1, $2, etc.

Essential Commands

ls

List directory contents.

cd

Change directory.

mkdir

Create directory.

rm

Remove files or directories.

cp

Copy files or directories.

mv

Move files or directories.

cat

Concatenate and display files.

grep

Search for patterns in files.

find

Search for files based on criteria.

String Manipulation

substring=${string:position:length} - Extracts a substring.
Example:

string="Hello World"
substring=${string:0:5}  # Hello

length=${#string} - Gets the length of a string.
Example:

string="Hello"
length=${#string}  # 5

string//pattern/replacement - Replaces all occurrences of a pattern.
Example:

string="Hello World"
new_string=${string//World/Universe}  # Hello Universe

Advanced Techniques

Error Handling

set -e - Exit immediately if a command exits with a non-zero status.

Example:

set -e
command_that_might_fail
echo "This will not be executed if the command fails"

|| - Execute a command only if the previous command fails.

Example:

command_that_might_fail || echo "Command failed"

&& - Execute a command only if the previous command succeeds.

Example:

command_that_must_succeed && echo "Command succeeded"

Process Substitution

<(command) - Provides the output of a command as if it were a file.

Example:

diff <(ls dir1) <(ls dir2)

>(command) - Redirects output to a command.

Example:

ls > >(tee output.txt)

Debugging

set -x - Display commands and their arguments as they are executed.
set +x - Disable command tracing.

echo "Debugging message" >&2 - Print debugging messages to stderr.
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