Bash Scripting Cheat Sheet — Scripts, Loops & Functions

Interactive Bash scripting reference with variables, loops, conditions, functions, arrays, and string operations. Searchable examples. Free, no signup required.

Variables

Declare variable

name="Alice"
age=30

Read-only variable

readonly PI=3.14159

Unset variable

unset name

Default value

${var:-default}   # use default if var unset
${var:=default}   # assign default if var unset
${var:?error msg} # error if var unset

String length

${#str}

Substring

${str:offset:length}  # e.g. ${str:2:4}

Uppercase / Lowercase

${str^^}  # uppercase
${str,,}  # lowercase

String Operations

Concatenation

full="$first $last"

Remove prefix

${str#prefix}   # shortest
${str##prefix}  # longest

Remove suffix

${str%suffix}   # shortest
${str%%suffix}  # longest

Replace

${str/old/new}   # first occurrence
${str//old/new}  # all occurrences

Check if contains substring

[[ "$str" == *"sub"* ]] && echo "found"

Split string

IFS="," read -ra arr <<< "$csv"

Arrays

Declare array

arr=(one two three)
declare -a arr

Access element

${arr[0]}

All elements

${arr[@]}

Array length

${#arr[@]}

Append

arr+=(four)

Slice

${arr[@]:1:2}  # elements 1 and 2

Iterate

for item in "${arr[@]}"; do
  echo "$item"
done

Associative array

declare -A map
map["key"]="val"
echo ${map["key"]}

Conditionals

if / elif / else

if [ "$x" -eq 1 ]; then
  echo "one"
elif [ "$x" -eq 2 ]; then
  echo "two"
else
  echo "other"
fi

Test operators

# Numeric: -eq -ne -lt -le -gt -ge
# String:  = != -z (empty) -n (not empty)
# File:    -f -d -e -r -w -x -s

case statement

case "$opt" in
  start) echo "Starting";;
  stop)  echo "Stopping";;
  *)     echo "Unknown";;
esac

Double bracket [[ ]]

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

Ternary-like

[[ condition ]] && echo "yes" || echo "no"

Loops

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

while loop

while [ $count -gt 0 ]; do
  echo $count
  ((count--))
done

until loop

until [ $done -eq 1 ]; do
  check_condition
done

Loop over files

for f in /var/log/*.log; do
  echo "$f"
done

read lines from file

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

break / continue

break     # exit loop
continue  # skip to next iteration

Functions

Define function

greet() {
  local name="$1"
  echo "Hello, $name!"
}

Call function

greet "Alice"

Return value

add() { echo $(( $1 + $2 )); }
result=$(add 3 5)

Return exit code

is_root() {
  [ "$(id -u)" -eq 0 ]
}
is_root && echo "root"

Local variables

func() {
  local x=10  # scoped to function
}

File Tests

Test operators

-e file   # exists
-f file   # is regular file
-d dir    # is directory
-r file   # readable
-w file   # writable
-x file   # executable
-s file   # size > 0
-L file   # is symlink
f1 -nt f2 # f1 newer than f2
f1 -ot f2 # f1 older than f2

Example usage

if [ -f "/etc/nginx/nginx.conf" ]; then
  echo "nginx config exists"
fi

Special Variables

Exit status

$?   # exit code of last command (0=success)

Script PID

$$   # PID of current shell/script

Argument count

$#   # number of positional arguments

All arguments

$@   # all args as separate words
$*   # all args as single string

Script name

$0   # name of the script

Positional params

$1 $2 $3 ... # individual arguments

Last background PID

$!   # PID of last background job

IFS

$IFS  # Internal Field Separator (default: space/tab/newline)

Heredoc

Basic heredoc

cat <<EOF
This is line 1
This is line 2
EOF

Heredoc to file

cat > /etc/motd <<EOF
Welcome to my server!
EOF

Suppress tab indent

cat <<-EOF
	indented content
	EOF

No variable expansion

cat <<'EOF'
$PATH won't expand here
EOF

Process Substitution & Redirection

Redirect stdout

cmd > file.txt

Redirect stderr

cmd 2> err.txt

Both stdout & stderr

cmd > out.txt 2>&1
cmd &> all.txt

Append

cmd >> file.txt

Process substitution

diff <(sort file1) <(sort file2)

Pipe

cmd1 | cmd2 | cmd3

Named pipe (FIFO)

mkfifo mypipe
cmd1 > mypipe & cmd2 < mypipe

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Frequently Asked Questions

How do I use Bash Cheat Sheet?

Browse by category (variables, loops, functions) or search for a specific syntax. Each example is runnable and copyable.

Is Bash Cheat Sheet free?

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Is my data safe?

This is a static reference tool. No data is entered or transmitted.

What is the difference between sh and bash?

sh is the original POSIX shell. bash is a superset with arrays, [[]], string manipulation, and more. Use #!/bin/bash if you need bash-specific features.

How do I run a bash script?

Make it executable with chmod +x script.sh then run with ./script.sh. Or run directly with bash script.sh without needing execute permission.

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