Bash

Run and capture output

output=$(command arg1 arg2 2>&1)
exit_code=$?

Check command exists

if command -v jq &>/dev/null; then
  echo "jq is available"
else
  echo "jq is not installed" >&2
fi

Variable expansion with defaults

port="${PORT:-8080}"              # Default if unset
dir="${WORKDIR:?WORKDIR required}" # Error if unset
name="${FULL_NAME%% *}"           # Remove after first space
ext="${filename##*.}"             # Extract extension

Conditional execution

# Chain — stop on failure
mkdir -p build && cd build && cmake ..

# Test file existence
[[ -f config.json ]] && echo "Config found"

# Pattern match
[[ "$url" =~ ^https?:// ]] && echo "Valid URL"

Globbing

# Standard globs
ls *.ts                       # All .ts files
ls src/**/*.test.ts           # Recursive (needs shopt -s globstar)

# Brace expansion
mkdir -p src/{components,utils,hooks}
cp file.{js,bak}             # Copy file.js to file.bak

Edge Cases

• Always double-quote variable expansions: "$var", "$@", "$(cmd)"
• [[ ]] is Bash-specific; [ ] is POSIX but splits words and expands globs
• Backticks `cmd` are legacy — use $(cmd) which nests cleanly
• set -e makes scripts exit on any unhandled failure — sometimes too aggressive for interactive use
• Empty globs: by default *.xyz returns the literal string if no match; use shopt -s nullglob to get empty results
• Word splitting on unquoted $(cmd) can cause bugs with filenames containing spaces
• eval is dangerous — avoid unless absolutely necessary; prefer arrays for dynamic commands