Linux Shell Scripting Cheat Sheet

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High-value Linux shell scripting reference covering variables, expansion, conditionals, loops, functions, redirection, pipelines, traps, getopts, and script debugging.

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Variables and Expansion

Define shell variables, expand strings, and work safely with parameters.

Assign shell variable

Set a shell variable in the current shell.

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bashLINUXshellvariablesbash

bash

name='alice'

Do not put spaces around `=` in shell variable assignments.

Export environment variable

Expose a variable to child processes.

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bashLINUXshellenvironmentexport

bash

export APP_ENV=production

Used for config values consumed by child commands and scripts.

Use default value if unset

Expand parameter with a fallback.

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bashLINUXshellparameter-expansiondefaults

bash

echo "${PORT:-8080}"

Useful in scripts where environment variables may be optional.

Require variable to be set

Fail fast if a variable is missing.

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bashLINUXshellparameter-expansionvalidation

bash

echo "${DATABASE_URL:?DATABASE_URL is required}"

Great for scripts that should stop immediately when config is missing.

Capture command output

Assign command output to a variable.

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bashLINUXshellsubstitutionvariables

bash

files=$(find . -type f | wc -l)

Use `$(...)` instead of legacy backticks.

Quote variable safely

Print variable preserving spaces.

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bashLINUXshellquotingsafety

bash

printf '%s
' "$name"

Double-quote variable expansions unless you intentionally want word splitting.

Conditionals Loops and Functions

Build shell scripts that branch, loop, and encapsulate logic.

Check whether file exists

Use a shell test in an if statement.

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bashLINUXshelliffiles

bash

if [[ -f /etc/app.conf ]]; then echo 'exists'; fi

Common script pattern for optional config files.

Check whether directory exists

Test for directory presence.

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bashLINUXshellifdirectories

bash

if [[ -d releases ]]; then ls releases; fi

Useful for deployment and housekeeping scripts.

Loop over items

Iterate through words or filenames.

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bashLINUXshellloopfor

bash

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

Quote variables inside the loop body to handle spaces safely.

Read file line by line

Process lines safely using while-read.

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bashLINUXshellwhileread

bash

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

Preferred pattern for preserving whitespace and backslashes.

Branch with case

Match input against multiple patterns.

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bashLINUXshellcasecontrol-flow

bash

case "$1" in start) echo start ;; stop) echo stop ;; *) echo usage ;; esac

Cleaner than long `if/elif` chains for command dispatch.

Define shell function

Create a reusable shell function.

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bashLINUXshellfunctionbash

bash

greet() { echo "hello $1"; }

Functions help keep scripts readable and avoid repetition.

Enable strict shell behavior

Fail script on errors and unset vars.

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bashLINUXshellstrict-modesafety

bash

set -euo pipefail

Widely used hardening pattern for production shell scripts.

Run cleanup on exit

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bashLINUXshelltrapcleanup

bash

trap 'rm -f "$tmpfile"' EXIT

Useful for temporary files, locks, and service teardown in scripts.

Input Output and Pipelines

Redirect streams, build pipelines, and control output files.

Redirect stdout to file

Write output to a file, overwriting it.

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bashLINUXshellredirectstdout

bash

echo 'hello' > greeting.txt

Use `>>` to append instead of overwrite.

Append stdout to file

Append output to an existing file.

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bashLINUXshellredirectappend

bash

echo 'next line' >> app.log

Common for quick logging in scripts.

Redirect stderr to file

Send errors to a separate file.

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bashLINUXshellredirectstderr

bash

command 2> errors.log

Useful when preserving errors separately from normal output.

Redirect stdout and stderr together

Capture all output in one file.

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bashLINUXshellredirectstderr

bash

command > command.log 2>&1

Common logging pattern in scripts and cron jobs.

Pipe stdout to another command

Send output of one command as input to another.

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bashLINUXshellpipeunix

bash

ps aux | grep nginx

Pipelines are the foundation of Unix composition.

Use process substitution

Pass command output as a pseudo-file.

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bashLINUXshellprocess-substitutiondiff

bash

diff <(sort old.txt) <(sort new.txt)

Handy when a command expects file arguments but you want to compare pipeline output.

Prompt user for input

Read a value interactively.

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bashLINUXshellreadinteractive

bash

read -rp 'Enter environment: ' env

Useful for operator-driven scripts and local helpers.

Print formatted output

Use printf instead of echo for predictable formatting.

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bashLINUXshellprintfformat

bash

printf 'user=%s id=%s
' "$USER" "$UID"

Preferred in scripts because format control is explicit.

Script Execution and Utilities

Run scripts, parse arguments, and debug shell behavior.

Use bash shebang

Start a Bash script with an explicit interpreter.

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bashLINUXshellshebangbash

bash

#!/usr/bin/env bash

Portable way to locate Bash on many systems.

Make script executable

Add execute bit to a script.

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bashLINUXshellscriptchmod

bash

chmod +x deploy.sh

Then run it directly as `./deploy.sh`.

Run shell script

Execute a script from the current directory.

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bashLINUXshellscriptexecution

bash

./deploy.sh

Use an explicit path so the shell does not rely on `PATH`.

Trace script execution

Print commands as they execute.

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bashLINUXshelldebugbash

bash

bash -x ./deploy.sh

Very useful for debugging shell scripts and CI tasks.

Parse short options with getopts

Handle flags in shell scripts.

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bashLINUXshellgetoptsarguments

bash

while getopts ':f:n:' opt; do case "$opt" in f) file="$OPTARG" ;; n) count="$OPTARG" ;; esac; done

Useful for small production scripts with predictable CLI flags.

Lint shell script

Static-analyze shell scripts for bugs.

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bashLINUXshelllintshellcheck

bash

shellcheck deploy.sh

One of the highest-value tools for improving script quality.

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