Bash

The default shell on most Linux and CI environments (macOS now defaults to Zsh). Worth knowing well because scripts run somewhere on Bash.

Startup Files

File	Loaded for
/etc/profile, ~/.bash_profile, ~/.bash_login, ~/.profile	Login shells (first match wins for the user files)
~/.bashrc	Interactive non-login shells
BASH_ENV	Non-interactive (scripts)

Common pattern: source ~/.bashrc from ~/.bash_profile so interactive login shells get both.

Script Safety Header

#!/usr/bin/env bash
set -euo pipefail   # exit on error, unset vars are errors, fail on pipe errors
IFS=$'\n\t'         # safer word splitting

Idioms

# Default value / required value
: "${NAME:=default}"          # set if unset
: "${REQUIRED:?must be set}"   # error if unset
 
# Conditionals (prefer [[ ]] in bash)
if [[ -f "$f" && "$x" == prod ]]; then ...; fi
 
# C-style loop / arrays
for ((i=0; i<5; i++)); do echo "$i"; done
arr=(a b c); echo "${arr[@]}"; echo "${#arr[@]}"
 
# Command substitution + here-doc
now=$(date +%F)
cat <<EOF
host=$(hostname)
EOF
 
# Parameter expansion
echo "${path##*/}"   # basename
echo "${path%/*}"    # dirname
echo "${var:-fallback}"

Portability

[[ ]], arrays, and ${var,,} are Bash-isms — not POSIX sh. macOS ships an old Bash 3.2 (no mapfile/declare -A); brew install bash for modern features, or target zsh.

Debugging

bash -x script.sh        # trace each command
set -x; ...; set +x       # trace a region
shellcheck script.sh      # lint (brew install shellcheck)