bash-scripting-safe

Safe Bash Scripting

Use when writing a shell script that must not corrupt files, delete the wrong thing, or silently fail. Bash is powerful precisely because it's dangerous — the discipline below is what makes it safe.

Always start with these three lines

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

• -e — exit on first failing command (no silent mid-script failure).
• -u — error on unset variable (catches typos like $file vs $files).
• -o pipefail — a failing command anywhere in a pipeline fails the pipeline.

Add IFS=$'\n\t' when you parse output: it changes Bash's default word-splitting (space, tab, newline) to just newline + tab, so filenames containing spaces no longer get split into multiple arguments.

Quote everything (word splitting is the #1 data-loss path)

dir="$1"                          # quote expansions
for f in "$dir"/*.txt; do         # quotes + glob
  cp "$f" "$dest/"
done

Unquoted $var undergoes word splitting (splits on spaces) and globbing (* and ? expand). The classic disaster:

# WRONG — if "$file" contains a space, rm deletes multiple files; glob expansion is worse
rm $file

# RIGHT
rm -- "$file"

A variable is only safe unquoted when you intend splitting/globbing. Prefer "$@"-style quoting everywhere else.

Redirects and atomic file writes

• Use a temp file + mv for atomic writes; never > straight onto a file you also read later in the same script (it truncates the input mid-run):

tmp="$(mktemp)"
trap 'rm -f "$tmp"' EXIT          # clean up scratch even on error
grep foo input.txt > "$tmp"
mv "$tmp" output.txt              # atomic swap

• mktemp for scratch files (safer than hard-coded /tmp/name which can collide or be a symlink). trap ... EXIT guarantees cleanup on failure.
• Beware set -e + empty globs: "$dir"/*.txt with no matches stays a literal *.txt string — check with shopt -s nullglob or test existence.

Error handling

if ! command -v jq >/dev/null 2>&1; then
  echo "jq is required" >&2
  exit 1
fi