Bash Hardening

Bash Hardening Guide

Concise correctness pointers for non-trivial bash. Surface the traps LLMs reliably miss when emitting "robust" scripts.

Assumes you can already write if/while/for, run chmod +x, and read a stack trace. This skill covers the correctness layer — the parts models gloss over: set -e exceptions, IFS, quoting, parameter expansion, trap inheritance, array gotchas, version differences.

When to use

Load when the task is:

• Writing/auditing installers, CI scripts, ops tooling, release scripts
• Diagnosing a script that "should work" but silently does the wrong thing
• Adding set -euo pipefail or strict-mode hardening
• Trap / cleanup / signal handling
• Array manipulation with empty-array / strict-mode interactions
• Cross-platform bash (macOS 3.2 vs Linux 4+/5+)
• ShellCheck triage (SC2086, SC2046, SC2155, SC2015, SC2128, SC2178, SC2207)

Do NOT load for: trivial one-liners, interactive shell use, "what is $PATH", POSIX-sh-only constraints (dash/ash), zsh/fish — those don't need this skill.

set -euo pipefail — what each option misses

set -e (errexit) does not trigger when the failing command is:

• Inside && / || / ! chains. cmd1 && cmd2 — if cmd1 fails, script continues.
• The condition of if, while, until. if cmd; then ... masks cmd's failure.
• Inside a function called from any of the above contexts. Once disabled at the call site, errexit stops applying inside the function body.
• Inside command substitution: foo=$(cmd_that_fails) — exit status discarded. Use set -o errexit; foo=$(cmd) || exit or check $? immediately.
• Inside process substitution: cat <(cmd_that_fails) — exit silently discarded.

set -E (errtrace) propagates the ERR trap into functions, command substitutions, subshells. Pair set -eE if you have an ERR trap — without -E, the trap fires only at top level.

set -u (nounset) edge cases:

• ${var:-default} and ${var-default} are safe under -u.
• "${arr[@]}" of an empty array errors under -u in bash 4.3 and earlier (and was buggy through 5.0). Defensive form: "${arr[@]:+${arr[@]}}" or "${arr[@]+${arr[@]}}".
• Always initialise arrays explicitly: arr=() — never assume declare -a arr is enough under -u.

set -o pipefail:

• Without it: false | true exits 0 (only last command's status counts).
• With it: pipeline exits with the rightmost non-zero status.
• For per-segment exit codes use ${PIPESTATUS[@]} immediately after the pipeline (resets on next command).

(( expr )) and let return non-zero when the result is zero. Under set -e, (( count = 0 )) aborts the script. Defensive form: (( count = 0 )) || true or count=0 plain assignment.

local var=$(cmd) masks cmd's exit code — local itself returns 0. Same for declare, export, readonly. Split: local var; var=$(cmd).

Recommended top-of-script: set -Eeuo pipefail; IFS=$'\n\t'.

IFS and word splitting

• Default IFS is $' \t\n' (space, tab, newline). Unquoted expansions are split on any of these, then glob-expanded.
• IFS=$'\n\t' (Bash strict-mode idiom) defangs space-splitting — filenames with spaces survive for f in $files. Cost: any code that genuinely splits on spaces breaks.
• IFS is global — set inside a function, it leaks. Use local IFS=... inside functions.
• "${arr[*]}" joins elements with the first character of IFS (so with default IFS, a space). "${arr[@]}" keeps elements separate. Different operations, never interchangeable.
• "$@" (quoted) preserves each positional arg as one word; $@ unquoted word-splits — same trap as $*.
• read honours IFS. while IFS= read -r line strips nothing — quoted-empty IFS preserves leading/trailing whitespace; -r disables backslash interpretation.

Quoting reflexes

• Always quote variable expansions: "$var", "${arr[@]}", "$(cmd)", "${var/old/new}". Unquoted = word-split + glob.
• [[ ]] does not word-split RHS — [[ $foo = bar ]] works without quotes around $foo. [ ] does word-split — quote or you'll get "too many arguments" on values with spaces or empty values.
• Inside [[ $a = $b ]], $b is treated as a glob pattern. Quote to compare literally: [[ $a = "$b" ]]. Inside =~, quoting the RHS makes it literal — for regex, leave RHS unquoted (or store the pattern in a variable and reference unquoted).
• Backticks `cmd` — use $(cmd) instead. Nestable, no backslash-escape weirdness.
• Tilde expansion does not happen inside double quotes: "~/file" is literal. Use "$HOME/file".
• Heredoc: <<EOF expands $var and $(cmd). <<'EOF' (quoted delimiter) treats body literally. <<-EOF strips leading tabs only (not spaces) — useful for indentation in scripts.

[[ ]] vs [ ] vs (( ))

• [[ ]] — bash builtin. No word splitting on RHS, regex =~, glob match in ==/!= (don't quote pattern), short-circuit && / ||. Bash/ksh/zsh only — not POSIX.
• [ ] — equivalent to /usr/bin/test. Subject to word splitting; must quote vars. POSIX. Avoid -a/-o (deprecated, ambiguous) — chain with &&/||: [ -e "$f" ] && [ -r "$f" ].
• (( )) — arithmetic. Integer math, C-style (++, **, %, ternary). Variables auto-dereferenced — (( x = y + 1 )) not (( x = $y + 1 )). Returns non-zero when result is 0 — see set -e interaction above.

Parameter expansion (replace sed/awk for simple ops)

• ${var:-default} — default if unset or empty.
• ${var-default} — default if unset only (empty stays empty).
• ${var:=default} — assign default if unset/empty.
• ${var:?msg} — print msg to stderr and exit if unset/empty. Useful for required-arg checks.
• ${var:+alt} — alt if set/non-empty (inverse of :-).
• ${#var} — string length. ${#arr[@]} — array element count.
• ${var:offset:length} — substring. Negative offset needs space or parens: ${var: -3} or ${var:(-3)}.
• ${var#pat} — strip shortest prefix match. ${var##pat} — longest. (Mnemonic: # is at start of comments.)
• ${var%pat} — strip shortest suffix. ${var%%pat} — longest. (Mnemonic: % is at the end.)
• ${var/pat/repl} — replace first match. ${var//pat/repl} — replace all. ${var/#pat/repl} — match at start. ${var/%pat/repl} — match at end.
• ${var^^} / ${var,,} — upper/lower (bash 4+). ${var^} / ${var,} — first char only.
• ${!prefix*} / ${!prefix@} — names of variables matching prefix. ${!arr[@]} — array indices/keys.

Arrays

• Indexed: arr=(a b "c d"). Access: ${arr[0]}, all elements "${arr[@]}", indices "${!arr[@]}", count ${#arr[@]}.
• Associative: declare -A m; m[key]=val — bash 4.0+. macOS default bash is 3.2.57 (Apple stuck at last GPLv2 release) — brew install bash for 5+ or fall back to parallel indexed arrays.
• Append: arr+=(d e). Single-element append: arr+=(x) not arr+=x (the latter concatenates to element 0).
• "${arr[@]}" — each element a separate word. "${arr[*]}" — single string joined by IFS[0]. Different.
• mapfile -t lines < file (or readarray) — read file into array, one line per element, -t trims newline. Bash 4+.
• A bare arr=value assigns to index 0 of an existing array — silent footgun if you meant to overwrite.

Trap semantics

• Syntax: trap 'cmd' EXIT INT TERM HUP ERR. Last trap for a signal wins — they don't stack. To chain, read trap -p SIG and prepend.
• EXIT fires on any exit (clean, error, signal-after-handler). Idiomatic cleanup:

  tmpdir=$(mktemp -d)
  trap 'rm -rf -- "$tmpdir"' EXIT
  

• ERR fires only when set -e would. Without set -E (errtrace), it does not fire inside functions, command substitutions, or subshells — even though the failing command is "in" the script.
• set -T (functrace) similarly inherits DEBUG and RETURN traps into functions.
• Subshells (( cmd ), command substitution, pipeline segments) reset signal traps to their inherited disposition unless -E/-T set; INT/TERM are reset to default unless explicitly re-trapped.
• Ignore a signal during a critical section: trap '' INT; critical_cmd; trap - INT (- restores default).

Process substitution and pipelines

• <(cmd) — bash makes cmd's stdout available as a file path (/dev/fd/N or named FIFO). >(cmd) — same but for stdin. Not POSIX — bash/ksh/zsh only; don't use under #!/bin/sh.
• Exit status of the substituted command is not propagated. cat <(false) — cat reports success.
• Use it to fix the subshell-loop trap: cmd | while read x; do count=$((count+1)); done runs the loop in a subshell — count outside is unchanged. Rewrite as while read x; do ...; done < <(cmd) to keep the loop in the parent shell.
• Alternative: shopt -s lastpipe (bash 4.2+, only when job control is off — i.e. non-interactive scripts) makes the last pipeline segment run in the parent.

Globbing options

• Default: a glob with no matches expands to the literal pattern — for f in *.txt iterates once with f="*.txt" if no .txt files exist.
• shopt -s nullglob — unmatched glob expands to nothing. Loop iterates zero times. Sane default for scripts.
• shopt -s failglob — unmatched glob is a hard error.
• shopt -s globstar — ** matches recursively. Bash 4+.
• shopt -s extglob — ?(...), *(...), +(...), @(...), !(...) extended patterns.
• shopt -s dotglob — globs match dotfiles.

Common dangerous patterns

• cmd | while read; do ...; done — loop runs in subshell; mutations don't escape. Fix: while read; do ...; done < <(cmd) or shopt -s lastpipe.
• eval "$user_input" — almost always wrong. Replace with arrays, declare -n namerefs (bash 4.3+), or restructure.
• [ "$a" = "$b" -a "$c" = "$d" ] — -a/-o are deprecated and ambiguous. Use [ "$a" = "$b" ] && [ "$c" = "$d" ] or [[ $a = $b && $c = $d ]].
• cd $dir; rm -rf * without quotes — word-splits and globs $dir. Quote: cd "$dir".
• A && B || C is not if-then-else. If A succeeds and B fails, C runs anyway. Use a real if.
• local var=$(cmd) — local masks cmd's exit. Split into two lines.
• echo -n / echo -e — non-portable (BSD echo differs). Use printf '%s' "$var" / printf '%s\n' "$var".
• find . -name '*.tmp' | xargs rm — breaks on filenames with spaces/newlines. Use find ... -print0 | xargs -0 rm or find ... -delete or find ... -exec rm {} +.
• Backticks. Use $( ).

Locale and portability

• LC_ALL=C sort (and grep, comm, uniq, awk) — deterministic byte-order, faster, avoids locale-dependent collation surprises (A vs a ordering, [a-z] not matching what you expect).
• LC_NUMERIC=C printf '%f' 1.5 — decimal point stays .. Without it, locales like de_DE use , and printf errors.
• #!/bin/bash vs #!/usr/bin/env bash — the latter follows $PATH, picks up brew install bash's 5.x on macOS instead of the 3.2.57 in /bin/bash.
• macOS-shipped bash is 3.2.57 — no associative arrays, no mapfile, no ${var^^}, no ** globstar, no &>>. Either require bash 4+ in your shebang/preamble or write to 3.2 floor.

Tooling

• ShellCheck — run on every script. Top rules to internalise:
  • SC2086 — quote variable expansions to prevent globbing/word-splitting.
  • SC2046 — quote command substitution to prevent same.
  • SC2155 — declare-and-assign masks exit code (local x=$(cmd)).
  • SC2015 — A && B || C is not if-then-else.
  • SC2128 — using array name without index gets element 0 only.
  • SC2178 — assigning string to array variable.
  • SC2207 — use mapfile or array assignment instead of arr=( $(cmd) ).
  • SC2034 — variable defined but never used (often a typo).
• shfmt — formatter; pin a version in CI, run shfmt -d for diff-only check.
• bats-core — bash test framework when scripts get non-trivial.
• set -x — trace. Pair with PS4='+ ${BASH_SOURCE}:${LINENO}:${FUNCNAME[0]:-main}: ' for richer output.

Authoritative references

Official Bash manual (gnu.org/software/bash/manual):

• Shell Parameter Expansion
• The Set Builtin
• Arrays
• The Shopt Builtin

Greg's Wiki / BashFAQ (mywiki.wooledge.org):

• BashFAQ/105 — Why doesn't set -e do what I expected?
• BashPitfalls
• IFS
• BashFAQ/100 — String manipulation
• SignalTrap
• ProcessSubstitution
• ArithmeticExpression
• BashGuide/Practices

ShellCheck wiki — every SCxxxx has its own page at shellcheck.net/wiki/SCxxxx (e.g. SC2155, SC2015).

Guardrails

Before recommending a non-trivial bash pattern (strict-mode preamble, trap, array idiom, parameter-expansion trick):

1. Quote the specific option / builtin / expansion form by name.
2. State the bash version floor if it matters (3.2 macOS / 4.0 / 4.3 / 4.4 / 5.0).
3. Cite the BashFAQ / Bash manual / ShellCheck rule for non-obvious behaviour.
4. Run shellcheck on the produced script and address every finding — silence only with an inline # shellcheck disable=SCxxxx plus a one-line reason.

A bash script that passes ShellCheck is the floor, not the ceiling.