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Catalog / Bash Scripting Cheatsheet

Bash Scripting Cheatsheet

A comprehensive cheat sheet covering Bash scripting essentials, from basic syntax to advanced techniques. Includes variables, functions, conditionals, loops, and more, with practical examples for quick reference.

Basics & Variables

Script Header & Execution

Shebang:

#!/usr/bin/env bash

Recommended header for portability. Specifies the interpreter for the script.

Execution:

bash script.sh
./script.sh  # if executable

Make script executable: chmod +x script.sh

Exit Status:
$? contains the exit status of the last command. 0 indicates success, non-zero indicates failure.

Comments:

# Single-line comment
: 'Multi-line
comment'

Variables

Declaration:

 
name="John"
age=30

Usage:

 
echo $name
echo "$name"
echo "${name}!"

Always quote variables to prevent word splitting and globbing.

String Quotes:

 
echo "Hi $name"  # Interpolation
echo 'Hi $name'  # No interpolation

Environment Variables:

echo $PATH
Access system-defined variables.

Unsetting Variables:

 
unset name

Command Substitution

Capture output of a command:

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

Backticks (obsolescent):

echo "I'm in `pwd`"

Functions & Conditionals

Functions

Definition:

 
get_name() {
  echo "John"
}

# Alternate syntax
function my_func {
  ...
}

Calling:

 
echo "You are $(get_name)"

Returning Values:

 
myfunc() {
  local myresult='some value'
  echo "$myresult"
}

result=$(myfunc)

Raising Errors:

 
myfunc() {
  return 1
}

if myfunc; then
  echo "success"
else
  echo "failure"
fi

Arguments:

 
myfunc() {
  echo "Hello, $1"
}

myfunc "World"

Special Parameters:

$#: Number of arguments
$*: All arguments as a single word
$@: All arguments as separate strings
$1: First argument

Conditionals

Basic Syntax:

 
if [[ condition ]]; then
  # statements
elif [[ condition ]]; then
  # more statements
else
  # final statements
fi

String Conditions:

[[ -z STRING ]]: Empty string
[[ -n STRING ]]: Not empty string
[[ STRING == STRING ]]: Equal
[[ STRING != STRING ]]: Not Equal

Numeric Conditions:

[[ NUM -eq NUM ]]: Equal
[[ NUM -ne NUM ]]: Not equal
[[ NUM -lt NUM ]]: Less than
[[ NUM -le NUM ]]: Less than or equal
[[ NUM -gt NUM ]]: Greater than
[[ NUM -ge NUM ]]: Greater than or equal

File Conditions:

[[ -e FILE ]]: Exists
[[ -r FILE ]]: Readable
[[ -h FILE ]]: Symlink
[[ -d FILE ]]: Directory
[[ -w FILE ]]: Writable
[[ -s FILE ]]: Size is > 0 bytes
[[ -f FILE ]]: File
[[ -x FILE ]]: Executable

Combining Conditions:

[[ ! EXPR ]]: Not
[[ X && Y ]]: And
[[ X || Y ]]: Or

Example:

 
if [[ -e "file.txt" ]]; then
  echo "file exists"
fi

Loops & Arrays

Loops

Basic For Loop:

 
for i in /etc/rc.*; do
  echo "$i"
done

C-like For Loop:

 
for ((i = 0 ; i < 100 ; i++)); do
  echo "$i"
done

Ranges:

 
for i in {1..5}; do
  echo "Welcome $i"
done

# With step size
for i in {5..50..5}; do
  echo "Welcome $i"
done

Reading Lines:

 
while read -r line; do
  echo "$line"
done <file.txt

Forever Loop:

 
while true; do
  # statements
done

Arrays

Defining Arrays:

 
Fruits=('Apple' 'Banana' 'Orange')

# Or
Fruits[0]="Apple"
Fruits[1]="Banana"
Fruits[2]="Orange"

Accessing Elements:

echo "${Fruits[0]}": Element #0
echo "${Fruits[-1]}": Last element
echo "${Fruits[@]}": All elements, space-separated

Array Length:

echo "${#Fruits[@]}": Number of elements
echo "${#Fruits}": String length of the 1st element

Range/Slice:

echo "${Fruits[@]:1:2}": Range (from position 1, length 2)

Keys:

echo "${!Fruits[@]}": Keys of all elements, space-separated

Operations:

 
Fruits=("${Fruits[@]}" "Watermelon")  # Push
Fruits+=('Watermelon')                # Also Push
Fruits=( "${Fruits[@]/Ap*/}" )        # Remove by regex match
unset Fruits[2]                       # Remove one item
Fruits=("${Fruits[@]}")               # Duplicate

Iteration:

 
for i in "${Fruits[@]}"; do
  echo "$i"
done

Dictionaries & Options

Dictionaries (Associative Arrays)

Defining Dictionaries:

 
declare -A sounds

sounds[dog]="bark"
sounds[cow]="moo"

Accessing Values:

echo "${sounds[dog]}": Dog’s sound
echo "${sounds[@]}": All values

Accessing Keys:

echo "${!sounds[@]}": All keys

Number of Elements:

echo "${#sounds[@]}": Number of elements

Deleting Elements:

unset sounds[dog]

Iteration (Values):

 
for val in "${sounds[@]}"; do
  echo "$val"
done

Iteration (Keys):

 
for key in "${!sounds[@]}"; do
  echo "$key"
done

Options (set)

set -o noclobber

Avoid overlaying files (e.g., echo "hi" > foo will fail if foo exists).

set -o errexit

Exit immediately if a command exits with a non-zero status (helps prevent cascading errors).

set -o pipefail

If a command in a pipeline fails, the pipeline’s exit status is that of the failed command.

set -o nounset

Attempting to use an unset variable results in an error, exposing potential bugs.

Glob Options (shopt)

shopt -s nullglob

If a glob pattern doesn’t match any files, it’s removed from the argument list (e.g., '*.foo' becomes ‘’).

shopt -s failglob

If a glob pattern doesn’t match, an error is thrown.

shopt -s nocaseglob

Glob patterns are case-insensitive.

shopt -s dotglob

Wildcards match dotfiles (e.g., "*.sh" matches .foo.sh).

shopt -s globstar

Allows ** for recursive matches (e.g., lib/**/*.rb matches lib/a/b/c.rb).

Redirection and miscellaneous

Redirection

command > file: Redirect stdout to file

command >> file: Redirect and append stdout to file

command 2> file: Redirect stderr to file

command 2>&1: Redirect stderr to stdout

command > file 2>&1: Redirect both stdout and stderr to file

command &> file: Redirect both stdout and stderr to file (shorthand)

command < file: Read stdin from file

command <<< "string": Here string

Heredoc 

cat <<EOF
This is a heredoc.
It allows multi-line input.
EOF

cat <<-EOF
  This is a heredoc.
  It allows multi-line input with leading tabs.
EOF

History expansions

!!

Execute last command again

!$

Expand last parameter of most recent command

!*

Expand all parameters of most recent command

!-n

Expand nth most recent command

!n

Expand nth command in history

!<command>

Expand most recent invocation of command

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