Should bash strict mode be recommended as good practice?

[peterjc]

Should bash strict mode be recommended as good practice? See http://redsymbol.net/articles/unofficial-bash-strict-mode/ which uses:

#!/bin/bash
set -euo pipefail
IFS=$'\n\t'

Personally I use set -e most heavily, and generally just use the one line set -euo pipefail after the hash-bang.

Originally raised as https://github.com/swcarpentry/shell-novice/issues/251 but we agreed it was too advanced for the shell novice lesson.

[kynan]

:+1: I think that's definitely appropriate for an intermediate audience.

[lexnederbragt]

:+1: from me too. Makes me wonder whether there could be a shell-scripting lesson here as well, including this, using functions, if...then statements.

[gvwilson]

I'm -0.5 on a shell scripting lesson: by the time you need conditionals and functions, you should have switched to a language with proper data structures.

[lexnederbragt]

I completely agree, and I keep saying this to many people on our lab, but getting them to give up the (false) comfort of their bash skills and scripts, and take a course in python/R/whatever that gets them to the same level of comfort, is not something I am very successful with (I need more power ;-) ).

Maybe a lesson on 'if you (would like to) do this in a shell script, the equivalent python script would look like this'?

[kynan]

@gvwilson @lexnederbragt Agree with you in principle, however there are many use cases where shell scripts are still the better choice: for running a series of commands (executables) you should really not use Python (in particular not os.system!) or another high level language. For that a bash script with set -euo pipefail is the way to go.

[lexnederbragt]

Hmmm... I am more and more convinced that a Makefile is the better way to go in such a case. But that has another set of drawbacks: not easy to learn, less transparent what is being run...

[kynan]

Sure, as soon as it gets slightly more complex and you have interdependencies a Makefile is definitely superior. But there is that narrow use case for bash scripts and for that imho strict mode is very important.

[gdevenyi]

If we recommend strict mode, we should note it could break some workflows.

I recently ran into an issue where I was some text manipulation in a subshell that can fail which was okay for the toolchain, but when I added strict the toolchain would exit if that subshell call failed.

@gvwilson When you work on a toolchain that exclusively relies on manipulating files and chaining together other command line tools, what do you suggest if not bash?

[kynan]

@gdevenyi You can always append || true to any command to ignore a specific failure.

[jttkim]

Getting back to @peterjc's initial comment of using set -e, it seems to me that discussing the elements of the "unofficial strict mode" individually may make sense. Personally I

• recommend -e and -o pipefail unreservedly,
• am generally in favour of -u as well, but would expect that this may result in learners finding that numerous examples / tutorials / code snippets don't seem to work for them, so people need to be prepared a bit,
• am not so sure about the IFS setting -- interfering with the scan / parse process like that doesn't seem right to me, although I have sympathy for the motivation of trying to write scripts that can deal with file names containing spaces without running into trouble due to tokenising these.

[HaleTom]

Also consider shopt -s failglob

[CristianCantoro]

I would recommend this version:

#!/usr/bin/env bash

SOURCED=false && [ "$0" = "$BASH_SOURCE" ] || SOURCED=true

if ! $SOURCED; then
  set -euo pipefail
  IFS=$'\n\t'
fi

(read more)

In this way you can safely source the script, if needed, without polluting your shell.

[ssbarnea]

@CristianCantoro Interesting idea but I see two problems with it: it may fail if run with non bash shell or if you already had set -u enabled before calling it, as BASH_SOURCE would not be defined. I guess is going into the right direction anyway.

[CristianCantoro]

@ssbarnea to be fair this is the bash strict mode, so I would say that using bash is implied :wink: (I use zsh as my shell, but I write almost all of my shell scripts in bash).

However, you are absolutely right, I didn't notice this issue it until because it manifests itself only if you source the script directly from a non-bash shell and you have set -u either before in the script or in your shell.

If you are using bash, BASH_SOURCE is a variable, so it is available and the script does not fail even with set -u.

This would be a fix:

#!/usr/bin/env bash

SOURCED=true
if [ ! -z "${BASH_SOURCE+x}" ]; then
  SOURCED=false && [ "$0" = "$BASH_SOURCE" ] || SOURCED=true
fi

if ! $SOURCED; then
  set -euo pipefail
  IFS=$'\n\t'
fi

A little bit more verbose, and it still does not support other shells, though