💻 Bash Scripting Cheat Sheet

87+ examples • Variables, loops, functions • Best practices

Variables & Data Types 12 String Operations 10 Conditionals 7 Loops 9 Functions 6 Input & Output 9 Arithmetic 5 Process & Job Control 9 Text Processing 12 Script Best Practices 8

Variables & Data Types 12 ▼

Variable assignment and usage

name="Alice"
echo "Hello $name"

Read-only constant

readonly PI=3.14159

Delete a variable

unset myvar

Default value if unset

echo "${name:-default}"

Assign default if unset

echo "${name:=default}"

Alt value if set

echo "${name:+alt}"

String length

echo "${#name}"

Array basics

arr=(one two three)
echo "${arr[1]}"  # two

Array operations

echo "${arr[@]}"  # all elements
echo "${#arr[@]}" # array length

Append to array

arr+=(four five)

Associative array (dict)

declare -A map
map[name]="Alice"
map[age]=30
echo "${map[name]}"

Integer variable

declare -i num=42

String Operations 10 ▼

To uppercase

echo "${str^^}"  # UPPERCASE

To lowercase

echo "${str,,}"  # lowercase

Substring

echo "${str:0:5}"  # first 5 chars

Remove shortest prefix

echo "${str#*.}"  # remove prefix

Remove longest prefix (get extension)

echo "${str##*.}" # remove all prefix

Remove shortest suffix

echo "${str%/*}"  # remove suffix

Replace first occurrence

echo "${str/old/new}"  # first match

Replace all occurrences

echo "${str//old/new}" # all matches

Check if contains substring

if [[ "$str" == *"pattern"* ]]; then
  echo "contains"
fi

Regex match

[[ "$str" =~ ^[0-9]+$ ]] && echo "numeric"

Conditionals 7 ▼

If/elif/else

if [[ $x -gt 10 ]]; then
  echo "big"
elif [[ $x -gt 5 ]]; then
  echo "medium"
else
  echo "small"
fi

File test operators

[[ -f "file.txt" ]]  # file exists
[[ -d "dir" ]]      # directory exists
[[ -r "file" ]]     # readable
[[ -w "file" ]]     # writable
[[ -x "file" ]]     # executable
[[ -s "file" ]]     # non-empty

String comparisons

[[ "$a" == "$b" ]]   # string equal
[[ "$a" != "$b" ]]   # string not equal
[[ -z "$str" ]]      # string is empty
[[ -n "$str" ]]      # string not empty

Numeric comparisons

[[ $a -eq $b ]]  # equal
[[ $a -ne $b ]]  # not equal
[[ $a -lt $b ]]  # less than
[[ $a -gt $b ]]  # greater than
[[ $a -le $b ]]  # less or equal
[[ $a -ge $b ]]  # greater or equal

AND condition

[[ $a -gt 0 && $a -lt 100 ]]

OR condition

[[ $a -eq 0 || $a -eq 1 ]]

Case statement

case "$fruit" in
  apple)  echo "red"  ;;
  banana) echo "yellow" ;;
  *)      echo "unknown" ;;
esac

Loops 9 ▼

For loop (list)

for i in 1 2 3 4 5; do
  echo "$i"
done

For loop (range)

for i in {1..10}; do echo "$i"; done

For loop (range with step)

for i in {0..100..5}; do echo "$i"; done

C-style for loop

for ((i=0; i<10; i++)); do
  echo "$i"
done

Loop over files

for file in *.txt; do
  echo "$file"
done

While loop

while [[ $count -lt 10 ]]; do
  ((count++))
done

Read file line by line

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

Until loop

until [[ $status == "ready" ]]; do
  sleep 1
  status=$(check_status)
done

Break and continue

for i in {1..10}; do
  [[ $i -eq 5 ]] && continue
  [[ $i -eq 8 ]] && break
  echo "$i"
done

Functions 6 ▼

Basic function

greet() {
  echo "Hello, $1!"
}
greet "Alice"

Return value via echo

add() {
  local result=$(( $1 + $2 ))
  echo $result
}
sum=$(add 3 5)  # 8

Local variables & defaults

process() {
  local name="$1"
  local -i count="${2:-1}"
  echo "$name x $count"
}

Trap on exit (cleanup)

cleanup() {
  rm -f /tmp/myapp_*
  echo "Cleaned up"
}
trap cleanup EXIT

Trap signals

trap "echo Interrupted; exit 1" INT TERM

Error/die function

die() {
  echo "ERROR: $*" >&2
  exit 1
}
[[ -f config ]] || die "No config file"

Input & Output 9 ▼

Read user input

read -p "Enter name: " name

Read secret input

read -sp "Password: " pass

Read with timeout

read -t 5 -p "Quick! " answer

Redirect output

echo "hello" > file.txt   # overwrite
echo "world" >> file.txt  # append

Redirect stderr

cmd 2>/dev/null           # hide errors
cmd &>/dev/null           # hide all output
cmd 2>&1                  # stderr to stdout

Command substitution

result=$(command)

Pipe to bc (math)

echo "scale=2; 22/7" | bc

Here document

cat <<EOF
Hello $name
Today is $(date)
EOF

Formatted output

printf "%-10s %5d\n" "$name" "$age"

Arithmetic 5 ▼

Basic arithmetic

echo $(( 5 + 3 ))    # 8
echo $(( 10 % 3 ))   # 1
echo $(( 2 ** 10 ))  # 1024

Increment/compound assignment

((count++))
((total += price))

Arithmetic in conditions

if (( x > 10 )); then echo "big"; fi

Floating point with bc

echo "scale=4; 22/7" | bc   # 3.1428

Floating point with awk

awk "BEGIN {printf \"%.2f\", 22/7}"  # 3.14

Process & Job Control 9 ▼

Run in background

command &

Run parallel, wait for all

command1 & command2 & wait

List background jobs

jobs

Bring job to foreground

fg %1

Survive terminal close

nohup command &

Special variables

$$   # current PID
$!   # last background PID
$?   # last exit code

Run if previous succeeded

command1 && command2

Run if previous failed

command1 || command2

Subshell (doesn't affect parent)

(cd /tmp && do_something)

Text Processing 12 ▼

Search for pattern

grep "pattern" file.txt

Recursive search

grep -r "TODO" --include="*.py" .

Count matches

grep -c "error" log.txt

Replace all in file

sed 's/old/new/g' file.txt

Replace in-place

sed -i 's/old/new/g' file.txt

Print lines 5-10

sed -n '5,10p' file.txt

Print columns 1 and 3

awk '{print $1, $3}' file.txt

Custom delimiter

awk -F: '{print $1}' /etc/passwd

Frequency count

sort file.txt | uniq -c | sort -rn

Extract CSV columns

cut -d"," -f1,3 data.csv

Translate characters

tr "a-z" "A-Z" < file.txt

Count lines

wc -l file.txt

Script Best Practices 8 ▼

Strict mode (recommended!)

#!/usr/bin/env bash
set -euo pipefail

Set options explained

set -e   # exit on error
set -u   # error on unset vars
set -o pipefail  # catch pipe errors
set -x   # debug (print commands)

Logging function

log() {
  echo "[$(date +%Y-%m-%dT%H:%M:%S)] $*"
}

Get script directory

SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"

Argument validation

[[ $# -lt 2 ]] && { echo "Usage: $0 <src> <dst>"; exit 1; }

Safe temp files

tmpfile=$(mktemp)
trap "rm -f $tmpfile" EXIT

Check dependency exists

if ! command -v jq &>/dev/null; then
  echo "jq required"
  exit 1
fi

Parse command-line options

# getopts for flags
while getopts "vf:o:" opt; do
  case $opt in
    v) VERBOSE=1 ;;
    f) FILE="$OPTARG" ;;
    o) OUTPUT="$OPTARG" ;;
    ?) exit 1 ;;
  esac
done

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