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Shell Scripting Cheat Sheet

Scannable Bash reference for the Shell Scripting for DevOps Engineers track. Prefer tutorials when you need why, not only how.

Overview

Audience When to use
Linux admins, DevOps, SRE, platform engineers Daily scripting, cron jobs, CI glue, quick ops automation

Quick reference

Topic Pattern
Shebang #!/usr/bin/env bash
Strict mode set -euo pipefail
Quote "$var" · "$@" · never bare $var for paths
Defaults ${var:-default} · ${var:?required}
Arithmetic $(( i + 1 )) · (( i++ ))
Tests [[ -f $f ]] · [[ $a == "$b" ]]
case case "$1" in start) ;; *) usage; exit 2;; esac
Loops for f in "${arr[@]}"; do · while IFS= read -r line; do
Functions fn() { local x="$1"; }
Arrays a=(x y) · a+=("z") · declare -A m=([k]=v)
Redirection cmd >out 2>err · cmd >>log 2>&1 · cmd \| tee
Trace bash -x ./script.sh · PS4='+${LINENO}: '
Temp/clean tmp=$(mktemp -d) · trap 'rm -rf "$tmp"' EXIT
Lock exec 9>lockfile · flock -n 9 \|\| exit 0
Cron set PATH=; absolute paths; no aliases

Bash syntax

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

usage() { echo "Usage: $0 <arg>" >&2; exit 2; }
[[ $# -ge 1 ]] || usage
Construct Notes
[[ ]] Bash conditional — prefer over [ ] in Bash scripts
(( )) Integer arithmetic / numeric tests
$( ) Command substitution (quote when needed)
source file / . file Load library functions

Bash vs sh

On many Debian/Ubuntu systems /bin/sh is dash. Avoid Bashisms when the shebang is #!/bin/sh.

Variables

name="Ada"
readonly CONST="prod"
export PATH="/usr/local/bin:/usr/bin:/bin"

echo "${name:-world}"
: "${REQUIRED:?REQUIRED must be set}"

# Indirect / lengths — avoid MkDocs-breaking length macros in docs;
# in real scripts, prefer counting via loops or tools when publishing Markdown.
count=0
for _ in "${items[@]+"${items[@]}"}"; do count=$((count + 1)); done
Form Meaning
"$var" Safe expansion
"$@" All positional params, separately quoted
"$*" Joined as one word (rarely what you want)
${var#prefix} Strip shortest prefix
${var##prefix} Strip longest prefix
${var%suffix} Strip shortest suffix
${var/pattern/repl} Replace once

Loops

for host in web db cache; do
  echo "$host"
done

while IFS= read -r line; do
  printf 'line=%s\n' "$line"
done < file.txt

until ping -c1 -W1 "$host" >/dev/null; do
  sleep 2
done
Keyword Use
break Leave loop
continue Next iteration
nullglob shopt -s nullglob so unmatched globs expand empty

Functions

greet() {
  local whom="${1:-world}"
  echo "Hello, ${whom}"
}

log() { printf '%s %s\n' "$(date -u +%Y-%m-%dT%H:%M:%SZ)" "$*" >&2; }
die() { log "ERROR: $*"; exit "${2:-1}"; }
  • Prefer local for function variables
  • Return status via return / exit; print data on stdout
  • Share helpers with source "${REPO_ROOT}/lib/common.sh"

Redirection and pipes

Pattern Effect
>file stdout overwrite
>>file stdout append
2>file stderr only
&>file / >file 2>&1 both streams
cmd1 \| cmd2 pipe stdout
cmd <file stdin from file
cmd <<< "text" here-string 
grep ERROR app.log | tee -a errors.txt | wc -l
# Always enable pipefail in automation so mid-pipe failures surface

Text processing

Tool Typical use
grep -E Match lines
sed -E Stream edit
awk Columns / reports
cut -d -f Field split
tr Translate/delete chars
sort \| uniq -c Frequency
xargs -r Build command lines (GNU) 
df -P -x tmpfs | awk 'NR>1 {print $5, $6}'
journalctl -u nginx -n 200 --no-pager | grep -E 'error|crit'

jq (JSON)

jq -r '.env' status.json
jq -r '.services[] | select(.status!="ok") | .name' status.json
jq -r '.services[] | [.name,.status] | @tsv' status.json
Filter Meaning
.key Field
.[] Iterate array
select(expr) Filter
@tsv / @csv Encode row
-e Exit non-zero if false/null result (jq 1.6+)

yq (YAML)

Prefer mikefarah/yq v4 syntax:

yq -r '.app.name' app.yaml
yq -r '.app.image + ":" + .app.tag' app.yaml
yq -e '.app.replicas >= 1' app.yaml

Multiple yq tools exist

Confirm yq --version. Python yq wrappers differ from mikefarah/yq.

Cron and timers

PATH=/usr/local/bin:/usr/bin:/bin
SHELL=/bin/bash
*/5 * * * * /home/ops/bin/check-disk.sh 80 >>/home/ops/logs/disk.log 2>&1
Tip Why
Absolute paths Cron PATH is minimal
Redirect both streams Capture failures
flock Prevent overlap
systemd timer Better calendar + journaling on modern hosts 
# oneshot sketch
# ExecStart=/usr/local/bin/backup.sh
# OnCalendar=*-*-* 02:30:00

Shell debugging

Technique Command / pattern
Trace bash -x script.sh
Verbose set -v / set -x regionally
Line prompt export PS4='+${BASH_SOURCE}:${LINENO}: '
Syntax check bash -n script.sh
Static analysis shellcheck script.sh
Exit probe echo exit=$? after commands

Troubleshooting table

Problem Cause Resolution
Works in terminal, fails in cron PATH/cwd/env Set PATH, use absolute paths
command not found mid-script set -e after failed lookup Check command -v
Empty vars blow up set -u Provide defaults or ${var-} carefully
Pipeline “succeeds” wrongly No pipefail set -o pipefail
Word-splitting bugs Unquoted $var Quote; use arrays / read -r

Production tips

  • Log humans to stderr; emit RESULT key=value on stdout for CI
  • Validate paths with prefix checks before rm/mv
  • Prefer idempotent installs and dry-run flags
  • Keep secrets in the environment or a secrets manager — never in git
  • Run ShellCheck in CI

Common mistakes

Mistake Correct approach
rm -rf $TARGET rm -rf -- "$TARGET" after validation
Bashisms under #!/bin/sh Use env bash or stay POSIX
Relying on aliases in scripts Use full commands / functions
Interactive read in cron Fail fast; require args/config

Interview nuggets

  • Explain set -euo pipefail in one minute
  • Difference between $@ and $*
  • Why cron jobs see a different environment
  • When to leave Bash for Python

References