#+TITLE: TIL: set -u in bash

#+KEYWORDS: cyborg

#+LANGUAGE: en

Trying a new /Today I learned/ section... Today, I learned about =set -u=,
which will fail a bash script if we try to use a variable that's not defined.
No more [[https://github.com/valvesoftware/steam-for-linux/issues/3671][deleting someone's files because a variable was not defined]]. Demo:

#+BEGIN_SRC bash
  #!/bin/bash
  set -u

  GREETING=Hello

  #oups I misspelt that
  echo ${GREETINGS} world
#+END_SRC

If you run this:

#+BEGIN_EXAMPLE
  $ ./script
  ./script: line 7: GREETINGS: unbound variable
#+END_EXAMPLE

That makes testing whether a variable is set slightly more complicated:

#+BEGIN_SRC bash
  set -u
  if [[ -z ${GREETINGS} ]]; then
      # oups we never get here, the previous line fails with "unbound variable"
      GREETINGS=$( ....compute greetings ...)
  fi
#+END_SRC

Instead:

#+BEGIN_SRC bash
  set -u
  if [[ ! -v GREETINGS ]]; then
      GREETINGS=$( ... compute greetings ... )
  fi
#+END_SRC

Where [[https://www.gnu.org/software/bash/manual/html_node/Bash-Conditional-Expressions.html][=-v= is a Bash Conditional Expression]] described thusly:

#+begin_quote
- -v /varname/ :: True if the shell variable varname is set (has been assigned
  a value).
#+end_quote

I suppose the actually equivalent test would be:
=\[\[ ! -v GREETINGS ]] || \[\[ -z ${GREETINGS} ]]=,
to check whether the variable is not only set but also non-null.

I've always used =set -e= and =set -o pipefail= in shell scripts. The first
will exit the entire script if any command (more specifically: [[https://www.gnu.org/software/bash/manual/html_node/Pipelines.html][any pipeline]]
fails). However by default, quoting the manual: the "exit status of a pipeline
is the exit status of the last command in the pipeline", meaning the following
succeeds:

#+begin_example
$ false | true
$ echo $?
0
#+end_example

But, if "=pipefail= is enabled, the pipeline's return status is the value of
the last (rightmost) command to exit with a non-zero status, or zero if all
commands exit successfully", so:

#+begin_example
$ set -o pipefail
$ false | true
$ echo $?
1
#+end_example

TL;DR: Use =set -euo pipefail= at the top of your bash scripts. [[https://wizardzines.com/comics/bash-errors/][Julia Evans
agrees]].