Bash Conditionals, Loops, and Functions

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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.

Test whether file exists

Check whether a regular file exists.

↗#

bashANYiffile-tests

bash

if [[ -f /etc/hosts ]]; then echo yes; fi

[[ ... ]] is generally safer and more expressive than [ ... ].

Test whether directory exists

Check whether a directory exists.

↗#

bashANYifdir-tests

bash

if [[ -d /var/log ]]; then echo yes; fi

Useful before cd, mkdir, or file operations.

Test for empty string

Check whether a string is empty.

↗#

bashANYifstrings

bash

if [[ -z "$name" ]]; then echo empty; fi

Quote variable expansions inside tests.

Compare strings

Compare string values.

↗#

bashANYifstrings

bash

if [[ "$env" == 'prod' ]]; then echo deploy; fi

[[ supports pattern matching with == as well.

Compare integers

Use arithmetic context for integer comparisons.

↗#

bashANYifarithmetic

bash

if (( count > 10 )); then echo large; fi

Cleaner than -gt for many Bash scripts.

Test command success

Use a command directly as the condition.

↗#

bashANYifcommands

bash

if grep -q needle file.txt; then echo found; fi

Shell conditions commonly use exit status rather than booleans.

Pattern matching with case

Match one variable against several patterns.

↗#

bashANYcasepatterns

bash

case "$ext" in
  jpg|png) echo image ;;
  txt) echo text ;;
  *) echo other ;;
esac

case is usually clearer than many if/elif checks.

Loops

for, while, until, and select loops.

For loop over words

Iterate over a list or glob.

↗#

bashANYforloops

bash

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

Pair with nullglob when patterns may not match.

C-style arithmetic for loop

Use arithmetic syntax in Bash for loops.

↗#

bashANYforarithmetic

bash

for ((i=0; i<5; i++)); do echo "$i"; done

Helpful for counters and indexed arrays.

While read loop

Read input line by line safely.

↗#

bashANYwhileread

bash

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

Common in parsing and scripting.

Until loop

Run loop until command succeeds.

↗#

bashANYuntilretry

bash

until curl -sf http://localhost:8080/health; do sleep 1; done

Good for readiness checks in scripts.

Use break and continue

Skip or exit loop iterations.

↗#

bashANYbreakcontinueloops

bash

for n in {1..5}; do [[ $n -eq 3 ]] && continue; echo "$n"; done

Fundamental control-flow building blocks.

Functions

Define reusable Bash functions and control return behavior.

Define a function

Define a shell function.

↗#

bashANYfunctionsdefinition

bash

greet() { echo "Hello, $1"; }

Bash supports both name() { ... } and function name { ... } forms.

Call a function

Invoke a shell function with arguments.

↗#

bashANYfunctionscall

bash

greet Jonathan

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; }

local is a Bash builtin, not POSIX sh.

Return status from function

Return a status code from a function.

↗#

bashANYfunctionsreturn

bash

validate() { [[ -n "$1" ]] || return 1; }

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')

This is the most common shell pattern for function 'return values'.

Trace shell functions

Trace function execution during debugging.

↗#

bashANYfunctionsdebug

bash

set -x
my_func() { echo ok; }
my_func
set +x

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 pipefail

Popular baseline for defensive scripting, though you should still understand edge cases.

Set safer IFS

Restrict word splitting to newline and tab.

↗#

bashANYifsstrict-mode

bash

IFS=$'
\t'

Sometimes used with strict scripts to reduce splitting surprises.

Run cleanup on exit

↗#

bashANYtrapcleanup

bash

tmp=$(mktemp)
trap 'rm -f "$tmp"' EXIT

Very common when using temp files or temp dirs.

Print error location

Add basic error diagnostics to a script.

↗#

bashANYtraperrdebug

bash

trap 'echo "error on line $LINENO"' ERR

Helpful when paired with set -e.

Shellcheck directive example

Document intentional exceptions for static analysis.

↗#

bashANYshellchecklint

bash

# shellcheck disable=SC2086
echo $unquoted

Useful when you use ShellCheck in CI.

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Tests and Conditionals

Test whether file exists

Test whether directory exists

Test for empty string

Compare strings

Compare integers

Test command success

Pattern matching with case

Loops

For loop over words

C-style arithmetic for loop

While read loop

Until loop

Use break and continue

Simple select menu

Functions

Define a function

Call a function

Use local variable in function

Return status from function

Return data via stdout

Trace shell functions

Script Safety

Enable stricter script mode

Set safer IFS

Run cleanup on exit

Print error location

Shellcheck directive example

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