Bash tips¶

How to do certain common-but-difficult things correctly. Plus reminders of commands I routinely forget.

General¶

Use printf, not echo¶

POSIX recommends against echo because it has serious limitations, including platform-specific behavior, inconsistent \n handling, and an n-ary argument list not terminated by --.

How to get a script’s own directory¶

Doing this correctly is a bit tricky.

Note these features:

• -- is needed because, outside a script, $0 is (e.g.) -z, which readlink assumes is an option and errors with an unhelpful message.
• The || exit $? after the readlink is needed to exit the subshell immediately. Otherwise, dirname is called but with no stdin, causing it to return ..
• The outer || exit $? catches any unlikely nonzero exits from dirname.

A tiny caveat: This will fail on a fraction of outrageously strange paths. In particular, it will break on a path that contains a newline – a character that ext4 and btrfs support and that Linux will technically accept. But this would cause more serious problems, and if you’re naming files like this, stop.

this_dir=$(dirname "$(readlink -f -- "$0" || exit $?)") || exit $?

This function seems to work in Bash 5.2 on Ubuntu 25.04 and macOS 24.

get_type() {
  local v re
  v="$(type "$1" || true)"
  re="[[:space:]]is[[:space:]]an?[[:space:]]"
  if [[ ! "$v" =~ $re ]]; then
    echo command
    return
  fi
  case "$v" in
    *alias* )
      echo alias
      ;;
    *function* )
      echo 'function'
      ;;
    *builtin* )
      echo builtin
      ;;
    *reserved* )
      echo keyword
      ;;
    *is* )
      echo command
      ;;
    *not* )
      echo undefined
      ;;
    *)
      echo unknown
      ;;
  esac
}

Miscellaneous notes¶

• man builtin covers all builtins for your shell.

• my_func() { } fails in Bash but not ZSH. Bash requires functions to have contents, so use my_func() { true }.

• For security, be extremely careful with env, export, and declare. Calling them without arguments may leak secrets.