bash
if [[ -f /etc/hosts ]]; then echo yes; fiNotes
[[ ... ]] is generally safer and more expressive than [ ... ].
Bash Conditionals, Loops, and Functions
Bash if/elif/case logic, test expressions, loops, functions, return codes, and robust control-flow patterns.
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Tests and Conditionals
Use test, [, [[, and arithmetic conditions.
bash
if [[ -d /var/log ]]; then echo yes; fiNotes
Useful before cd, mkdir, or file operations.
bash
if [[ -z "$name" ]]; then echo empty; fiNotes
Quote variable expansions inside tests.
bash
if [[ "$env" == 'prod' ]]; then echo deploy; fiNotes
[[ supports pattern matching with == as well.
bash
if (( count > 10 )); then echo large; fiNotes
Cleaner than -gt for many Bash scripts.
bash
if grep -q needle file.txt; then echo found; fiNotes
Shell conditions commonly use exit status rather than booleans.
bash
case "$ext" in
jpg|png) echo image ;;
txt) echo text ;;
*) echo other ;;
esacNotes
case is usually clearer than many if/elif checks.
Loops
for, while, until, and select loops.
bash
for f in *.log; do echo "$f"; doneNotes
Pair with nullglob when patterns may not match.
bash
for ((i=0; i<5; i++)); do echo "$i"; doneNotes
Helpful for counters and indexed arrays.
bash
while IFS= read -r line; do printf '%s
' "$line"; done < file.txtNotes
Common in parsing and scripting.
bash
until curl -sf http://localhost:8080/health; do sleep 1; doneNotes
Good for readiness checks in scripts.
bash
for n in {1..5}; do [[ $n -eq 3 ]] && continue; echo "$n"; doneNotes
Fundamental control-flow building blocks.
bash
select choice in start stop status quit; do echo "$choice"; break; doneNotes
Useful for interactive admin helpers.
Functions
Define reusable Bash functions and control return behavior.
bash
greet() { echo "Hello, $1"; }Notes
Bash supports both name() { ... } and function name { ... } forms.
bash
greet JonathanNotes
Function arguments are available as positional parameters inside the function.
Use local variable in function
Keep function internals from leaking into caller scope.
bashANYfunctionslocal
bash
work() { local dir=${1:-/tmp}; cd "$dir" || return 1; pwd; }Notes
local is a Bash builtin, not POSIX sh.
bash
validate() { [[ -n "$1" ]] || return 1; }Notes
return is for status codes, not arbitrary values.
Return data via stdout
Emit data to stdout and capture it with command substitution.
bashANYfunctionsstdoutsubstitution
bash
slugify() { tr '[:upper:] ' '[:lower:]-' <<< "$1"; }
slug=$(slugify 'Hello World')Notes
This is the most common shell pattern for function 'return values'.
bash
set -x
my_func() { echo ok; }
my_func
set +xNotes
Useful when debugging control flow and variable expansion.
Script Safety
Safer Bash defaults for production scripts.
Enable stricter script mode
Exit on errors, unset variables, and pipeline failures.
bashANYstrict-modeset
bash
set -euo pipefailNotes
Popular baseline for defensive scripting, though you should still understand edge cases.
bash
IFS=$'
\t'Notes
Sometimes used with strict scripts to reduce splitting surprises.
bash
tmp=$(mktemp)
trap 'rm -f "$tmp"' EXITNotes
Very common when using temp files or temp dirs.
bash
trap 'echo "error on line $LINENO"' ERRNotes
Helpful when paired with set -e.
Shellcheck directive example
Document intentional exceptions for static analysis.
bashANYshellchecklint
bash
# shellcheck disable=SC2086
echo $unquotedNotes
Useful when you use ShellCheck in CI.
More in Bash
Bash Scripting Patterns and Practical Recipes
Production-minded Bash script patterns for files, arguments, parsing, temp files, logging, retries, and common automation tasks.
Bash Builtins, Job Control, and Shell Behavior
Core Bash builtins, shell options, history, aliases, job control, traps, and interactive-shell behavior.
Bash Arrays, Strings, and Text Handling
Indexed and associative arrays, string handling, read/mapfile, and common Bash text-processing patterns.
Bash Variables, Quoting, and Expansion
Bash variables, parameter expansion, quoting rules, command substitution, arithmetic, and shell expansion patterns.
Bash Cheat Sheet
Comprehensive Bash commands, shell syntax, quoting, variables, redirection, pipelines, and day-to-day scripting patterns.
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