Bashv5.2INTERMEDIATE

Bash Scripting

Bash scripting reference - variables, quoting, parameter expansion, conditionals, loops, functions, arrays, and safe scripting patterns.

14 min read

bashshellscriptinglinuxclish

Getting Started

Shebang, making scripts executable, and running them.

Shebang & Running

Create, permit, and run a script.

bash

# Quick Reference
#!/usr/bin/env bash
echo "Hello, World!"

# make it runnable, then run
chmod +x script.sh
./script.sh

💡 #!/usr/bin/env bash finds bash on PATH - more portable than a hardcoded /bin/bash.

⚡ A script needs the executable bit (chmod +x) to run as ./script.sh.

📌 Run without +x via "bash script.sh" - the interpreter is explicit there.

🟢 # starts a comment to end of line; the shebang is only special on line 1.

basicsshebang

Variables & Quoting

Assigning values and controlling expansion with quotes.

Variables

Assignment, usage, and scope.

bash

# Quick Reference
name="Sam"          # NO spaces around =
echo "$name"        # use with $
echo "${name}"      # braces for clarity/adjacency

💡 No spaces around = in assignments: name="Sam" (name = "Sam" runs a command).

⚡ Use ${var} braces when the name touches other text: ${name}_backup.

📌 Declare function variables with local so they do not leak into the global scope.

🟢 export makes a variable visible to child processes; plain assignment stays in the shell.

variablesscope

Quoting

Single vs double quotes and why quoting matters.

bash

# Quick Reference
echo "$var"       # double: expands variables
echo '$var'       # single: literal, no expansion
echo "${arr[@]}"  # always quote expansions

💡 Double quotes expand $var and $(...); single quotes keep everything literal.

⚡ Always quote "$var" and "${arr[@]}" - unquoted values word-split and glob-expand.

📌 "$(cmd)" captures a command's output; prefer $(...) over legacy backticks.

🟢 Unquoted variables holding paths with spaces are the #1 source of shell bugs.

quotingexpansion

Parameters & Special Variables

Script arguments and built-in variables.

Positional & Special Params

Access arguments and shell state.

bash

# Quick Reference
$0   # script name
$1 $2 ...   # positional args
$@   # all args (each quoted)
$#   # arg count
$?   # last exit status
$$   # current PID

💡 Prefer "$@" (quoted) to iterate args - it preserves each argument's boundaries.

⚡ $? holds the previous command's exit code: 0 means success, non-zero means failure.

📌 shift drops $1 and renumbers the rest - handy for processing args one by one.

🟢 "$@" keeps arguments separate; "$*" joins them into a single word.

argumentsspecial-vars

Substitution & Arithmetic

Capturing output and doing math.

Command Substitution & Arithmetic

$(...) and $((...)).

bash

# Quick Reference
files="$(ls | wc -l)"     # capture command output
sum=$(( 3 + 4 ))          # arithmetic -> 7
(( count++ ))             # arithmetic evaluation

💡 $(( )) evaluates integer math; inside it, variables need no $ prefix.

⚡ Bash arithmetic is integer-only - use bc or awk for floating-point.

📌 (( expr )) returns success/failure by the result, so it chains with && and ||.

🟢 Prefer $(cmd) over backticks - it nests cleanly: $(dirname "$(pwd)").

arithmeticsubstitution

Conditionals

Tests, comparisons, and branching.

if & Test Expressions

if/elif/else with [[ ]].

bash

# Quick Reference
if [[ "$x" == "yes" ]]; then
    echo "ok"
elif [[ -z "$x" ]]; then
    echo "empty"
else
    echo "no"
fi

💡 Use [[ ]] (bash) over [ ] (POSIX test) - it handles empty vars, &&, and patterns safely.

⚡ == does glob-pattern matching in [[ ]]; =~ does regex matching.

📌 String tests: == != -z (empty) -n (non-empty). Numeric: -eq -ne -lt -gt -le -ge.

🟢 cmd && success || failure is a compact if/else, but beware if success itself can fail.

conditionalstest

File & String Tests

Common test operators.

bash

# Quick Reference
[[ -f file ]]   # regular file exists
[[ -d dir ]]    # directory exists
[[ -z "$s" ]]   # string is empty
[[ "$a" == "$b" ]]  # strings equal

💡 -f tests a regular file; -e tests any path; -d tests a directory.

⚡ Always quote the value in string tests ([[ -z "$s" ]]) to handle empties safely.

📌 Use -eq/-lt/-gt for numbers and ==/</> for strings - mixing them miscompares.

🟢 (( n > 5 )) reads more naturally than [[ "$n" -gt 5 ]] for pure number checks.

testsfiles

case Statements

Multi-way branching on patterns.

case

Match a value against patterns.

bash

# Quick Reference
case "$1" in
    start) echo "starting" ;;
    stop)  echo "stopping" ;;
    *)     echo "usage: start|stop" ;;
esac

💡 case matches glob patterns (not regex): * ? [..] and | for alternatives.

⚡ Each branch ends with ;; - forgetting it is a common syntax error.

📌 The *) branch is the catch-all default - put it last.

🟢 case is cleaner than a long if/elif chain when branching on one value.

casebranching

Loops

Iterating with for, while, and until.

for Loops

Iterate lists, ranges, and globs.

bash

# Quick Reference
for item in a b c; do echo "$item"; done
for i in {1..5}; do echo "$i"; done
for f in *.txt; do echo "$f"; done

💡 Loop over globs (for f in *.txt) directly - never parse ls output.

⚡ {1..5} generates a range; {0..10..2} adds a step. Ranges cannot use variables.

📌 Guard glob loops with [[ -e "$f" ]] || continue in case nothing matches.

🟢 Quote "$@" in for arg in "$@" so arguments with spaces stay intact.

loopsfor

while, until & select

Condition-driven loops and reading input.

bash

# Quick Reference
while [[ $n -lt 5 ]]; do (( n++ )); done
until [[ -f ready ]]; do sleep 1; done
while read -r line; do echo "$line"; done < file

💡 while IFS= read -r line; do ... done < file is THE safe way to read lines.

⚡ IFS= prevents trimming whitespace; -r stops backslash mangling in read.

📌 until is while's inverse - it loops until the condition succeeds.

🟢 break leaves the loop entirely; continue jumps to the next iteration.

loopswhileread

Functions

Reusable blocks with arguments and return values.

Functions

Define, pass arguments, and return results.

bash

# Quick Reference
greet() {
    local name="$1"
    echo "Hello, $name"
}
greet "Sam"
result="$(greet "Sam")"   # capture the echoed output

💡 Function args are $1, $2, ... just like script args - not named parameters.

⚡ return sets an exit status (0-255) for success/failure; echo + $(...) returns data.

📌 Declare internal variables local so functions do not clobber global state.

🟢 ${1:-default} gives a parameter a fallback when the caller omits it.

functions

Arrays

Indexed and associative arrays.

Indexed Arrays

Ordered, zero-based lists.

bash

# Quick Reference
arr=(a b c)
echo "${arr[0]}"      # a
echo "${arr[@]}"      # all elements
echo "${#arr[@]}"     # length -> 3
arr+=(d)              # append

💡 ${arr[@]} expands to all elements; ${#arr[@]} is the count.

⚡ Always quote "${arr[@]}" in loops so elements with spaces stay intact.

📌 Arrays are zero-indexed; ${arr[-1]} grabs the last element (bash 4.3+).

🟢 arr+=(item) appends; ${!arr[@]} lists the indices (useful for sparse arrays).

arraysindexed

Associative Arrays

Key-value maps (bash 4+).

bash

# Quick Reference
declare -A ages
ages[sam]=30
echo "${ages[sam]}"       # 30
echo "${!ages[@]}"        # keys

💡 Associative arrays require declare -A first - otherwise keys are treated as 0.

⚡ ${!map[@]} gives keys; ${map[@]} gives values; ${#map[@]} gives the count.

📌 Associative arrays need bash 4+ (macOS ships bash 3.2 - install a newer bash).

🟢 [[ -v map[key] ]] tests whether a key exists without inserting it.

arraysassociative

Parameter Expansion

Transform variables inline - no external commands.

Defaults, Length & Substrings

Fallbacks and slicing.

bash

# Quick Reference
${var:-default}   # use default if unset/empty
${var:=default}   # assign default if unset
${#var}           # length
${var:0:3}        # substring (offset, length)

💡 ${var:-def} substitutes a default without changing var; ${var:=def} also assigns it.

⚡ ${#var} is the length; ${var:offset:length} slices a substring.

📌 ${var:?msg} aborts the script with msg if var is unset - a cheap required-arg guard.

🟢 ${var^^} / ${var,,} upper/lowercase the whole string (bash 4+), no tr needed.

parameter-expansiondefaults

Pattern Removal & Replace

Trim prefixes/suffixes and substitute.

bash

# Quick Reference
${file#*/}     # remove shortest leading match
${file##*/}    # remove longest (-> basename)
${file%.*}     # remove shortest trailing (-> drop ext)
${var/old/new} # replace first match

💡 # / ## strip from the FRONT (shortest/longest); % / %% strip from the BACK.

⚡ ${f##*/} is a fast pure-bash basename; ${f%/*} is a pure-bash dirname.

📌 ${v/old/new} replaces the first match; ${v//old/new} replaces all (double slash).

🟢 These run in-process - far faster than calling sed/basename in a loop.

parameter-expansionstring

Input & Output

Reading input and writing/redirecting output.

read Input

Prompt for and parse user input.

bash

# Quick Reference
read -r -p "Name: " name
read -rsp "Password: " pass    # -s hides input
read -ra parts <<< "a b c"     # split into an array

💡 Always use read -r - without it, backslashes in input are silently eaten.

⚡ -p prints a prompt, -s hides typed input (passwords), -t sets a timeout.

📌 read -ra arr <<< "$str" splits a string into an array on whitespace.

🟢 read returns non-zero on EOF/timeout - chain with || for a fallback value.

inputread

Output & Redirection

echo, printf, and streams.

bash

# Quick Reference
echo "text"
printf "%s = %d\n" "x" 42
cmd > out.txt      # stdout to file (overwrite)
cmd >> out.txt     # append
cmd 2> err.txt     # stderr to file

💡 Prefer printf over echo for formatting and portability - echo flags vary across shells.

⚡ > truncates, >> appends; 2> redirects stderr; 2>&1 merges stderr into stdout.

📌 Order matters: cmd > f 2>&1 works; cmd 2>&1 > f does NOT merge as expected.

🟢 &>/dev/null silences a command completely (both stdout and stderr).

outputredirection

Here-docs & Process Substitution

Multi-line input and treating output as files.

Heredocs, Herestrings & Process Substitution

Feed blocks of text and command output.

bash

# Quick Reference
cat <<EOF
multi-line
text with $var expanded
EOF

grep foo <<< "$string"          # here-string
diff <(sort a) <(sort b)        # process substitution

💡 <<EOF expands $vars inside; <<'EOF' (quoted delimiter) keeps everything literal.

⚡ <<< "$str" (here-string) is a quick way to pipe one string into a command's stdin.

📌 <(cmd) turns command output into a temp file path - great for diff <(a) <(b).

🟢 <<-EOF strips leading tabs (only tabs, not spaces) so you can indent the block.

heredocprocess-substitution

Exit Codes & Error Handling

Fail fast and clean up reliably.

set -euo pipefail & trap

Strict mode and cleanup handlers.

bash

# Quick Reference
set -euo pipefail       # strict mode (put near the top)
trap 'echo "cleanup"' EXIT
trap 'echo "error on line $LINENO"' ERR

💡 Start scripts with set -euo pipefail so failures stop the script instead of cascading.

⚡ trap ... EXIT guarantees cleanup (temp files, locks) runs on every exit path.

📌 With -u, reference optional vars as ${VAR:-} to avoid an unbound-variable error.

🟢 Send error messages to stderr with >&2 so they are not captured by $(...).

error-handlingsettrap

Globbing & Brace Expansion

Filename patterns and generated sequences.

Globs & Braces

Match files and generate strings.

bash

# Quick Reference
*.txt        # any name ending .txt
file?.log    # single-char wildcard
{a,b,c}      # brace expansion -> a b c
{1..5}       # sequence -> 1 2 3 4 5

💡 Globs match existing files; brace expansion just generates text (files need not exist).

⚡ Enable ** recursive matching with shopt -s globstar.

📌 By default an unmatched glob stays literal - set shopt -s nullglob to get nothing instead.

🟢 mkdir -p base/{a,b,c} is a quick way to create several directories at once.

globbingbrace-expansion

Argument Parsing

Handle script options and flags.

getopts

Parse single-letter options.

bash

# Quick Reference
while getopts "vo:" opt; do
    case "$opt" in
        v) verbose=1 ;;
        o) out="$OPTARG" ;;
        *) echo "usage..."; exit 1 ;;
    esac
done

💡 In the optstring, a trailing colon (o:) means that option expects an argument in $OPTARG.

⚡ After the loop, shift $((OPTIND-1)) removes options so $@ holds the positional args.

📌 getopts handles only short options (-v); for --long flags, parse with a while/case loop.

🟢 A leading colon in the optstring (":vo:") enables silent error handling you control.

getoptsarguments

Debugging & Best Practices

Find bugs and write robust scripts.

Debugging & Safety

Tracing, linting, and guidelines.

bash

# Quick Reference
bash -x script.sh        # trace every command
set -x                   # trace on
set +x                   # trace off
shellcheck script.sh     # static analysis

💡 Run scripts through ShellCheck - it flags quoting, unset vars, and portability bugs.

⚡ bash -x (or set -x) traces each command with its expanded values - the fastest debugger.

📌 Quote every expansion, use [[ ]] over [ ], and $(...) over backticks by default.

🟢 : "${1:?usage...}" is a one-line guard that aborts when a required argument is missing.

debuggingbest-practices

Bash Scripting

Table of Contents

Getting Started

Shebang & Running

Variables & Quoting

Variables

Quoting

Parameters & Special Variables

Positional & Special Params

Substitution & Arithmetic

Command Substitution & Arithmetic

Conditionals

if & Test Expressions

File & String Tests

case Statements

case

Loops

for Loops

while, until & select

Functions

Functions

Arrays

Indexed Arrays

Associative Arrays

Parameter Expansion

Defaults, Length & Substrings

Pattern Removal & Replace

Input & Output

read Input

Output & Redirection

Here-docs & Process Substitution

Heredocs, Herestrings & Process Substitution

Exit Codes & Error Handling

set -euo pipefail & trap

Globbing & Brace Expansion

Globs & Braces

Argument Parsing

getopts

Debugging & Best Practices

Debugging & Safety