[theaustinseven]

All scripts should be run with "-eu". This prevents unbound variables from ever being used and will end the script on a failed command. If you want commands in the script to fail, just drop the "e" and run with "-u". At my last job we put "-eu" in literally all of our scripts to avoid this very problem.

[wolfgang42]

To expound on this:

"set -e" is short for "set -o errexit", that is, abort the script if a command returns with a non-zero exit code.

"set -u" is short for "set -o nounset", that is, abort the script if a variable name is dereferenced when the variable hasn't been set.

I also strongly suggest reading http://www.davidpashley.com/articles/writing-robust-shell-sc... for more things that you should be aware of when writing bash scripts.

[saurik]

I also do this. However, literally just hours ago, Greg Wooledge, author of The Bash Guide, posted the following to the GNU bug-bash mailing list.

> Errexit (a.k.a. set -e) is horrible, and you should not be using it in any new shell scripts you write. It exists solely for support of legacy scripts.

http://lists.gnu.org/archive/html/bug-bash/2017-03/msg00170....

edit: I asked after this (for whatever reason, my email isn't yet showing in the archives; maybe it will appear later), and got an interesting response!

http://lists.gnu.org/archive/html/bug-bash/2017-03/msg00171....

[vog]

> All scripts should be run with "-eu".

More specifically, every shell script should have "set -eu" at its second line, directly after the sh'bang line:

    #!...
    set -eu

For debugging or logging purposes, sometimes adding -v and/or -x comes handy:

    #!...
    set -euvx

[zwp]

> should have "set -eu" at its second line

Do you have a strong reason to prefer "set" over shebang flags? I have a slight preference for shebang flags so I can deliberately override them from the command line (but it's not a hill I'd die on):

    $ cat foo 
    #!/bin/bash -eu
    cd /nowhere
    echo 'still here'
    $ ./foo 
    ./foo: line 2: cd: nowhere: No such file or directory
    $ bash -c ./foo # thinking about system(3)
    ./foo: line 2: cd: nowhere: No such file or directory
    $ bash +e ./foo # override
    ./foo: line 2: cd: nowhere: No such file or directory
    still here
    $

EDIT: Google's shell style guide has "Executables must start with #!/bin/bash and a minimum number of flags. Use set to set shell options so that calling your script as bash <script_name> does not break its functionality."

https://google.github.io/styleguide/shell.xml?showone=Which_...

[vog]

Bash is not always /bin/bash. On some systems (BSDs) it is /usr/bin/bash.

So the portable sh'bang line for bash is:

    #!/usr/bin/env bash

And there you can't add "-eu" anymore, because only one argument is possible in sh'bang lines. (/usr/bin/env would try to find an executable named "bash -eu")

So

    #!/usr/bin/env bash
    set -eu

is the more portable approach. However, for "/bin/sh" this should work portably:

    #!/bin/sh -eu

[saghm]

> Bash is not always /bin/bash. On some systems (BSDs) it is /usr/bin/bash.

Nitpick: on every BSD system I've used, it's /usr/local/bin/bash. But I definitely agree that using /usr/bin/env bash is the preferred solution for when you want to use bash (although I don't think it's even installed by default on most BSD's); as an Arch user (where /usr/bin/python is symlinked to python3 rather than python2), I appreciate those who go out of their way to use proper, portable shebangs.

[shawabawa3]

You can also do

   #!/bin/sh -eu

Although i'm not sure if there are any advantages/disadvantages

[wahern]

Disadvantage is that nobody can use an alternative shell to run your script as intended. (Alternative here could mean a less buggy version of whatever is the system shell.)

[djsumdog]

Is there a way to do this in Fish shell? So far the only thing I could find were bugs closed as duplicates that led to this open bug:

https://github.com/fish-shell/fish-shell/issues/510

[vkjv]

I usually add `pipefail` as well.

    set -euo pipefail

[jcassee]

pipefail is bash-specific, no?

[GauntletWizard]

The other one worth recommending: -o pipefail; Which makes: false | true count as an error, where otherwise it would be ignored.

[leni536]

Agree, but be aware of subshells, these options don't propagate there.

  $ set -eu
  $ echo "$(false)"
  
  $

[teaearlgraycold]

    #!/bin/bash
    set -eu
    export SHELLOPTS

[harryjo]

That doesn't solve the problem that parent incorrectly explained.

    echo "$(false)"

swallows the exit code from $(false).

    $ cat /tmp/test.sh
    set -euvx
    export SHELLOPTS
    echo "$(false)"
    echo "echo of string with failed subcommand does not kill script"
    $(false)
    echo "but consuming exit code does"
        
    $ /tmp/test.sh        
    export SHELLOPTS
    + export SHELLOPTS
    echo "$(false)"
    false
    ++ false
    + echo ''
        
    echo "echo of string with failed subcommand does not kill script"
    + echo 'echo of string with failed subcommand does not kill script'
    echo of string with failed subcommand does not kill script
    $(false)
    false
    ++ false

[Slackwise]

These are the kinds of pitfalls, aside from everything else in this thread, that make me just use Ruby for scripting instead.

I mean, I can always just backtick pipe anything that would be convenient but avoid this nonsense.

[wahern]

I also use set -f, which disables pathname expansion. It's trivial to enable pathname expansion around the few lines where that's desirable.

Also -C so you don't accidentally clobber existing files. Useful to prevent symlink attacks, in addition to preventing stupid stuff.

My standard non-interactive shell script preamble begins with

  set -e # strict error
  set -u # don't expand unbound variable
  set -f # disable pathname expansion
  set -C # noclobber

[cellularmitosis]

I start out like that, but often I end up having to remove -u if I'm e.g. trying to check if $1 is populated. Is there a way to do that while still using -u?

[mdaniel]

Same as any potentially unbound variable, by providing a (possibly empty) default value:

    if [ "${1:-}" == "my awesome value" ]; then ...

[IgorPartola]

No no no. Clearly sh and bash are just broken. We need to write scripts in Rust or Haskell to avoid safety issues. </sarcasm>

[coldtea]

Well, Rust and Haskell might not be for the job, but sh and bash are clearly broken and in many ways.

-eu should have been the default from the start, for one.

[TorKlingberg]

Well, for scripts. set -e on an interactive shell is a bit of a challenge.

[coldtea]

A shell could, and I'm going out on a limb here, check whether it's interactive or not and just enforce the -e in the appropriate case...

[harryjo]

interactive commands should abort the command on unset variable

[tomjakubowski]

"Abort command if variables are unset" is the -u flag, not -e. Both were mentioned long ago in this thread, but GP was talking about -e being painful for interactive shells. Imagine an interactive shell exiting as soon as a user's command failed!

[dllthomas]

> Imagine an interactive shell exiting as soon as a user's command failed!

Hardcore mode.

[the_why_of_y]

sh is fundamentally broken as soon as you use a pipe in a command; bash at least has "set -o pipefail".