Bash Scripting Cheatsheet — Complete Quick Reference (2026)

NAME="Alice"                  # no spaces around =
readonly NAME="Bob"           # constant
echo "$NAME"                  # interpolation (use quotes!)
echo "Length: ${#NAME}"       # string length
default=${1:-"fallback"}      # default value
export PATH="$PATH:/custom"   # environment variable

if [ "$NAME" = "Alice" ]; then
    echo "hello"
elif [ "$NAME" = "Bob" ]; then
    echo "bye"
else
    echo "who?"
fi

# test flags
[ -f "file" ]    # file exists
[ -d "dir" ]     # directory exists
[ -z "$var" ]    # empty string
[ -n "$var" ]    # non-empty string
[ "$a" -eq "$b" ]  # numeric equal (-ne, -gt, -lt, -ge, -le)
[ "$a" = "$b" ]    # string equal (!= for not equal)

# modern [[ ]] (safer, supports && ||)
[[ "$NAME" == A* ]] && echo "starts with A"
[[ -f "file" && -r "file" ]] && echo "exists and readable"

# for
for i in {1..5}; do echo "$i"; done
for file in *.txt; do echo "$file"; done
for ((i=0; i<5; i++)); do echo "$i"; done

# while
while IFS= read -r line; do echo "$line"; done < file.txt
while [ "$count" -lt 5 ]; do ((count++)); done

# until
until ping -c1 example.com &>/dev/null; do sleep 1; done

function greet() {
    local name="$1"            # $1, $2, ... positional args
    local greeting="${2:-Hi}"  # default value
    echo "$greeting, $name!"
    return 0                   # exit status
}
greet "Alice"                  # call
greet "Bob" "Hey"

# return values (stdout-based)
get_name() { echo "Alice"; }
name=$(get_name)

arr=("a" "b" "c")
echo "${arr[0]}"              # element
echo "${arr[@]}"              # all elements
echo "${#arr[@]}"             # length
echo "${!arr[@]}"             # indices
arr+=("d")                    # append
unset arr[1]                  # remove element

# associative arrays (Bash 4+)
declare -A map
map[name]="Alice"
echo "${map[name]}"

s="hello world"
echo "${s:0:5}"               # "hello" — substring
echo "${s/world/there}"       # replace first
echo "${s//l/x}"              # replace all
echo "${s^^}"                 # uppercase
echo "${s,,}"                 # lowercase
echo "${s#hel}"               # remove prefix
echo "${s%orld}"              # remove suffix

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

# write
echo "text" > file.txt        # overwrite
echo "text" >> file.txt       # append

# check
[ -s "file" ]                 # non-empty file
[ -r "file" ]                 # readable
[ -w "file" ]                 # writable
[ -x "file" ]                 # executable

# find and replace in files
find . -type f -name "*.txt" -exec sed -i 's/old/new/g' {} +

# backup with timestamp
cp file.txt{,.bak.$(date +%Y%m%d)}

# kill processes matching name
pkill -f "node app"

# disk usage by directory
du -sh */ | sort -rh

# watch a command every 2 seconds
watch -n2 'ps aux | grep python'

# line count (excluding blank)
grep -c . file.txt