Bash Error Handling

https://wizardzines.com/comics/bash-errors/

by sohkamyung • 6 years ago

287 100 6 years ago

wizardzines.com

pferde• 6 years ago

Also known as "bash strict mode". It's a godsend if you're unlucky enough to have to deal with bash scripting.

http://redsymbol.net/articles/unofficial-bash-strict-mode/

EDIT: The comic strip would be better in three rows of two panels - a row for each set flag.

pzmarzly• 6 years ago

Another useful flag is -x, which enables tracing (Bash will print each command before executing it).

You can disable it again with set +x (same goes for +e, +u and afaik +o pipefail)

Also, please use shellcheck - https://www.shellcheck.net/

EDIT: Also, please don't modify scripts while they are running.

aidenn03• 6 years ago

Here's a workaround for modifying scripts while they are running; it works because function bodies are read and parsed entirely when they are defined.

  main() {
    script goes here
    exit
  }
  main "$@"

0xmohit• 6 years ago

Specifying PS4 when using the -x flag can be even more helpful while debugging. The variable PS4 denotes the value is the prompt printed before the command line is echoed when the -x option is set and defaults to : followed by space.

A number of useful debugging tips are listed at <https://wiki.bash-hackers.org/scripting/debuggingtips>.

mehrdadn4• 6 years ago

Wait until you realize

  set -e; (false && true); echo hi

does nothing, but

  set -e;  false && true ; echo hi

prints hi.

Sebb767• 6 years ago

It's a bit surprising at first, but I'd like to think most people deeper into bash are aware of Subshells.

alanbernstein• 6 years ago

I know $() invokes a subshell, but does () ?

My guess was that the () affects the order of operations between ; and &&, so the first line is three commands, while the second line is two.

mehrdadn4• 6 years ago

Merely knowing about when subshells occur doesn't explain this. You need to know that && and || suppress -e.

chubot9• 6 years ago

Yeah this is a variation on problem 2 I mentioned here, except with subshells rather than functions.

https://news.ycombinator.com/item?id=24740842

The problem actually has more to do with the definition of $? than the set -e behavior itself. And the fact that POSIX specifies that the error a the LHS of && is ignored (a fundamental confusion between true/false and success/error)

The exit code of the function is not what you expect, or the exit code of the subshell is not what you expect.

I made a note of it on the bug ... still thinking about what the solution to that one is.

(The other solutions are inherit_errexit, more_errexit, and a "catch" builtin.)

cryptonector2• 6 years ago

That one is OK. What's NOT OK is that conditionals disable set -e in functions that appear in the conditionals, so:

  function foo { false; true; }
  foo || echo foo failed

prints nothing.

mehrdadn4• 6 years ago

Ouch, that's horrible. Thanks!

chubot9• 6 years ago

Yes, that's #1 the "if myfunc" problem I mentioned here:

https://news.ycombinator.com/item?id=24740842

shopt -s strict_errexit in Oil disallows that (a runtime assertion 100% of the time). Feedback is welcome! https://github.com/oilshell/oil/issues/709

exdsq5• 6 years ago

Other tips I've found after a year or two of using Bash for many things it shouldn't be used for:

- Use shellcheck (static analysis/linter) https://www.shellcheck.net/

- Use shunit2 (unit tests) https://github.com/kward/shunit2

- Use 'local' or 'readonly' to annotate your variables

- Trap ctrl+c to gracefully exit (details here https://www.tothenew.com/blog/foolproof-your-bash-script-som...)

- Stick to long-form options for readability (--delete over -d for example)

- #!/usr/bin/env > #!/bin/bash for portability

- Consider setting variable values in a file and importing it at the top of your script to improve refactoring

- You will eventually forget how your scripts work - seriously consider if Bash is your best option for anything that needs to last a while!

mehrdadn4• 6 years ago

Do you want to trap INT, or EXIT?

exdsq5• 6 years ago

EXIT is trapped in the same way as 0, it's something that happens when your shell exits. Ctrl+C sends the SIGINT signal but you can catch it with INT too. You want to do the latter because your gracefully exiting the script, if you want to have some cleanup after that you could trap EXIT (for deleting tmp files or something).

asicsp5• 6 years ago

Some more useful links:

* https://mywiki.wooledge.org/BashFAQ

* https://mywiki.wooledge.org/BashGuide/Practices

* https://mywiki.wooledge.org/BashPitfalls

* https://devmanual.gentoo.org/tools-reference/bash/index.html

hivacruz• 6 years ago

I would also add shfmt, to format scripts: https://github.com/mvdan/sh

amarshall3• 6 years ago

Note that /bin/bash isn’t completely portable (though /bin/sh is, that’s not Bash) as some systems have it at /usr/local/bin/bash.

m4632• 6 years ago

That's what i've learned... #!/bin/sh is generally what I use and it survives porting.

If I need more power I usually switch to python. (Probably not relevant to a bash discussion though, sorry)

laumars3• 6 years ago

You’re just reiterating the GPs point of using env instead of calling bash directly from the shebang:

  #!/usr/bin/env bash

is more portable than

  #!/bin/bash

Though personally if I was worried about portability then I’d just write the script in vanilla sh instead.

shakna• 6 years ago

I believe this statement agrees with you:

#!/usr/bin/env > #!/bin/bash for portability

Perhaps you misread it? env bash is greater for portability than bin/bash.

jancsika• 6 years ago

- Trap ctrl+c to gracefully exit (details here https://www.tothenew.com/blog/foolproof-your-bash-script-som...)

I'd rather that most devs don't touch that signal. Using that binding and having a GUI or CLI program continue hanging because the dev screwed up the cleanup is a real pain. And someone writing a Bash script is highly likely for doing something "very clever" with that signal to make my life harder.

Or if you're going to do something with it, at least make it clear you're trolling me. Show me a text add that forces me to choose my favorite Korean boy band before I can exit, or something in that vein.

exdsq5• 6 years ago

That’s fair! But sometimes I want to have a hook that basically says “Are you sure?” to catch mistakes in the wrong terminal or something. One thing I wrote recently took several hours to run and it’d suck to accidentally close it because I type without looking.

jmholla• 6 years ago

In that case, you could not catch CTRL-C in the cleanup.

nemetroid• 6 years ago

Agree that developers should be very careful about messing up Ctrl-C. However, as others have pointed out, it can make sense for long-running processes (especially in cases where there's an intermediate result that can be output instead of the final result). I think a good compromise is to only ever trap Ctrl-C once, so that a double Ctrl-C always successfully interrupts.

corytheboyd• 6 years ago

Huge +1 to using long form options in scrips, even if you’re the sole maintainer of the script. Also if you have a command that takes many flags, breaking them out onto new lines can help keep it readable

kasabali4• 6 years ago

Beware long forms are not portable.

gkfasdfasdf• 6 years ago

This is a great list. Also while reading about 'readonly' bash variables I ran across this amazing project which lets you call native functions from bash [0]. My mind is spinning from the possibilities...

[0] https://github.com/taviso/ctypes.sh

txutxu• 6 years ago

- #!/usr/bin/env > #!/bin/bash for portability

Yes... but do not take that as a "cargo cult script shebang".

If you're a sysadmin writing a script for a company with 2k linux servers, that has a policy of "we only use linux version Foo X"... and we do not use other bash in the system than /bin/bash (no bash compiled by hand, no multiple versions of bash, etc)... then portability via "env" does not make sense.

If you have two laptops and a raspberry at home, with debian or arch, and you write a script for yourself... then portability via "env" does not make sense.

And last but not least... using env is slower.

See:

    strace -fc /bin/bash -c ':'

    strace -fc /usr/bin/env bash -c ':'

On my system, that's 92 syscalls and 3 errors, Vs 152 syscalls and 8 errors.

Just to start procesing.

Diferent levels of system bloat (environment, library paths, etc) can give different results than my example.

And as others said... if you're not using GNU/bash syntax and the script is really simple, the best for portbility is to go with /bin/sh.

    strace -fc /bin/sh -c ':'

On my system 41 syscalls and 1 error... (and less RAM, CPU and pagefaults).

If you're not using associative arrays, array indexes, non POSIX builtin options, and other bash extensions... if the script is just to join a few commands and variables... it pays the effort to write it in simple sh, both, for portability and performance.

rmrfrmrf• 6 years ago

Two things for me are:

  - Do I trust my code to run on a machine where /bin/bash doesn't work?

  - Do I trust my users to have their PATH configured correctly?

IME a user misconfiguring ~/.bashrc is about seven million times more likely than some theoretical argument about "portability", or even the idea that running my code on some unspecified version of bash that a mac user accidentally downloaded while screwing up a Homebrew copy/paste command is preferable to using the factory default that everyone has.

l0b03• 6 years ago

I'm working on a book about Bash scripting which is currently in the review phase, and it includes most of these. For graceful exit I recommend `trap cleanup EXIT` rather than specifically trapping SIGINT, mostly because the special exit signal is triggered no matter why the script is interrupted. I wouldn't normally recommend pulling out variables into a separate files until those variables are used by more than one script. I'd be interested in the rationale for why that helps refactoring.

heavyset_go• 6 years ago

Have a title I can look for after the book is released?

loevborg• 6 years ago

`-o pipefail` is not without its gotchas

    bash -eo pipefail -c "cat /etc/services | head -1; echo never got here"

stonewareslord• 6 years ago

What is the gotcha here? You’re telling it to pipefail and exit on fail, so it exits.

tux1968• 6 years ago

The gotcha is perhaps failing to recognize that cat will actually fail in this simple pipeline because its output is truncated "unexpectedly" by head exiting after completing its job properly. (Maybe you could argue that head should be a good citizen and read its input to completion)

Where this will work as expected:

    bash -eo pipefail -c "head < /etc/services  -1|cat ; echo got here"

aidenn03• 6 years ago

I hate pipefail.

I find set -e also has some gotchas. '|| die' added to the end of each line is uglier but more obvious in many cases.

polyrand• 6 years ago

In Bash 4.4+ you can use:

  shopt -s inherit_errexit 2>/dev/null || true

To make subprocesses inherit the errexit flag.

chinigo• 6 years ago

Lifesaver! Without this, error handling in bash functions is a gigantic, unintuitive pain. Thank you for bringing this to my attention.

amelius2• 6 years ago

And this boilerplate isn't going to be a pain?

kasabali4• 6 years ago

               inherit_errexit
                       If  set,  command  substitution  inherits  the value of the errexit option,
                       instead of unsetting it  in  the  subshell  environment.   This  option  is
                       enabled when posix mode is enabled.

So must I presume older versions were already doing that without needing an option set?

harisund• 6 years ago

I thought people shouldn't post anything about bash on HN? The minute you post something about bash immediately you will draw out a whole bunch of folks from the wood works talking about how bash sucks and should never be used for anything more than 3 or 4 lines and how they replaced bash with python or some thing else, immediately in turn drawing out a bunch of other folks talking about how bash should be replaced with power shell and how you can parse objects better ... .

michaelcampbell• 6 years ago

Any programming topic spawns a bunch of comments about readability, or examples of code-golf. It's the nature of things.

exdsq5• 6 years ago

I had a really fun project earlier in the year prototyping a load testing tool for a blockchain in Bash while 4 other developers wrote a ‘better’ one in Haskell. Bash can get results quickly, although it’s not maintainable! Still, a decent kloc or two of bash with performance results within the sprint.

heresie-dabord• 6 years ago

I had become annoyed by the Python bigots who will tell us about how easy to read their language is because its notation is clean, how all its functionality is "intuitive", how any combination of Python-based Rube-Goldberg Machine systems is the best.

But then came the PowerShell people...

asicsp5• 6 years ago

Others in this on-going series:

* https://wizardzines.com/comics/environment-variables

* https://wizardzines.com/comics/brackets-cheatsheet

* https://wizardzines.com/comics/bash-quotes

* https://wizardzines.com/comics/bash-if-statements/

JdeBP• 6 years ago

I wonder how much Clippy is to blame for the visual motif used in this comic strip.

Common motifs, elsewhere, for a shell are a dollar sign and an underscore or a greater than sign and an underscore. (The latter is somewhat odd for a shell, given that it more resembles the prompts on Microsoft/IBM command interpreters, and not the PS1 prompts of Unix shells, which are commonly dollar symbols, hashes, or percent signs rather than greater than.)

* https://www.redbubble.com/i/sticker/zsh-by-zoerab/20363330.E...

* https://commons.wikimedia.org/wiki/File:Bash_Logo_black_and_...

* https://commons.wikimedia.org/wiki/File:PowerShell_5.0_icon....

* https://icon-library.net/icon/commands-icon-5.html

* https://icon-library.com/icon/bash-icon-10.html

* https://dribbble.com/shots/6101482-Bash-Automation

GolDDranks• 6 years ago

I almost always write POSIX shell instead of bash for compatibility; it would be nice to see collections for tips and tricks specifically for POSIX shell. I know, for example, that -o pipefail doesn't exist in plain POSIX shell. I wonder what's are the best practices when you can't use it.

_0w8t• 6 years ago

I also write my scripts to stay /bin/sh compatible. If this is not enough, then a real scripting language should be used, not bash.

But I very much agree that lack of pipefail is painful. If I know that output on the left of the pipe is small, I read it into a variable and then use printf | right part. If the output can be big, I use a helper function to emulate it that I copy-paste.

asicsp5• 6 years ago

it would be nice to see collections for tips and tricks specifically for POSIX shell

Check out https://freebsdfrau.gitbook.io/serious-shell-programming/

sigzero• 6 years ago

it would be nice to see collections for tips and tricks specifically for POSIX shell

https://www.shellscript.sh/

There is a Facebook group and I have emailed the author as well with questions. Nice guy.

m4632• 6 years ago

ok some low-grade tips:

cleanup="true"

later:

  if $cleanup; then a ; else b ; fi

or slightly uglier

  $cleanup && rm -f mytempfile

I like one-line functions:

  die() { echo "$@" 1>&2; exit 1; }

peterwwillis2• 6 years ago

Read the ash or dash man pages and just use what they give you. That's pretty much Bourne shell which is pretty much POSIX.

still_grokking• 6 years ago

-o pipefail doesn't exist in plain POSIX shell

I think this changed lately[1]. No clue where's implemented, though.

[1] https://www.austingroupbugs.net/view.php?id=789

_where• 6 years ago

If it’s not by default there’s a reason. Bash is literally running commands in a shell session. Think terminal session. When a command fails, would you want the terminal session to end? That’d be annoying.

Same theory for unset variable. Referencing an undefined variable shouldn’t break your session. Why initialize it anyway? It’s more code to change if you don’t use it if you have to initialize it when it might not be needed. And, you’d have to call the script with A= just to check A wasn’t defined, and in the process now you have A assigned to an empty string, instead of only defaulting to one when called, which uses more memory and execution time.

The pipeline doesn’t die because && and || and parens are seriously helpful for one-liners.

Don’t think of it as a script. Think of it as a script for a shell.

pwdisswordfish4• 6 years ago

Sometimes the only reason is backwards compatibility.

Waterluvian• 6 years ago

Thinking of it that way, it becomes clear why you would want to switch to something like Python for more complex workflows.

Bash is brilliantly useful for a lot of things. I’m not bashing it.

IshKebab2• 6 years ago

A compelling argument for why Bash scripts shouldn't exist at all.

chubot9• 6 years ago

https://www.oilshell.org/blog/2018/01/28.html#shouldnt-we-di...

kasabali4• 6 years ago

you’d have to call the script with A= just to check A wasn’t defined, and in the process now you have A assigned to an empty string

You have ${var:-} for doing that

tpoacher• 6 years ago

I'm surprised nobody's mentioned [expect](https://likegeeks.com/expect-command/) yet in this thread :)

x87678r• 6 years ago

bash is not fit for any moderately complex task.

I dont understand its popularity. Normally sane people who like unit testing, CICD, SOLID principles, quality tools end up with a bunch of crappy scripts holding everything together. Please avoid.

chubot9• 6 years ago

https://www.oilshell.org/blog/2018/01/28.html#shouldnt-we-di...

peterwwillis2• 6 years ago

It's fantastic for simple tasks, which is why so many people use it. It also turns out that many complex tasks can be reduced to a collection of simple tasks, otherwise known as "a bunch of crappy scripts".

I encourage all my teams to avoid built-in CI/CD features and plugins and just script what they want in a Docker container. It ends up being easier to maintain, breaks less often, and is more portable.

amelius2• 6 years ago

I dont understand its popularity.

I think the only reason people use the shell for scripting is that

    ls -lsa /tmp

is simpler to write than

    execute(["ls", "-lsa", "/tmp"])

or even

    execute("ls -lsa /tmp")

> Normally sane people who like unit testing, CICD, SOLID principles, quality tools end up with a bunch of crappy scripts holding everything together. Please avoid.

Totally agree!

arendtio• 6 years ago

I like the notion of 'set -e' and at the same time I hate it.

First, because it behaves inconsistently across shells/versions [1] and second, because it doesn't always work as expected. For example, when you depend on the 'set -e' behavior in a function and call the function from within a condition, the 'set -e' has no effect at all. So you better don't count on 'set -e'.

But don't expect me to follow my own advice, as not using 'set -e' isn't a good option either...

[1] https://www.in-ulm.de/~mascheck/various/set-e/

chubot9• 6 years ago

Yup, I am fixing this with Oil: https://news.ycombinator.com/item?id=24740842 (2 of 4 problems fixed so far, more problems welcome)

Try it out on your shell scripts and let me know what happens :) OSH has the "broken" POSIX/bash behavior to maintain compatibility, while Oil opts you in to the better error semantics.

chubot9• 6 years ago

I'm working on fixing errexit / set -e in Oil:

https://github.com/oilshell/oil/issues/709

Summary of problems:

1. The "if myfunc" problem -- error checking is skipped. This is specified by POSIX, but it's undesirable.

2. The f() { test -d /tmp && echo "exists" } problem. The exit code of the function is probably not what you expect!

3. the local x=$(false) problem. This happens in all shells because of the definition of $?. (local is a shell builtin with its own exit code, not part of the language) This one is already fixed with shopt -s more_errexit in Oil.

4. the x=$(false) problem. This is a bash problem with command substitution. For example, dash and zsh don't have this problem. Test case:

    bash -c 'set -e; x=$(false); echo should not get here'

Newer versions of bash fix it with inherit_errexit. Oil also implements inherit_errexit and turns it on by default! (in Oil, not OSH)

-----

So 1 and 2 relate to the confusing combination of shell functions and errexit.

And 3 and 4 relate to command subs. Oil has fixed these with opt-in OSH shell options (on by default in Oil), but not 1 and 2.

If you know of any other problems, please chime in on the bug!

a13692099933• 6 years ago

2. The f() { test -d /tmp && echo "exists" } problem. The exit code of the function is probably not what you expect!

The exit code is 0 (assuming /tmp/, stdout, /bin/test and /bin/echo are all working correctly; with /tmp1 it's 1), as expected; is this referencing a bug in sh and/or bash that I've fixed locally and then forgotten about?

(Also, I'm pretty sure it should be:

  f() { test -d /tmp && echo "exists"; }

unless the parse error for missing ';' was your point (I haven't bothered to fix that one, but maybe Oil has).)

chubot9• 6 years ago

Yeah I didn't explain that very well. It's actually a variation on the problem here:

https://news.ycombinator.com/item?id=24738274

except with a shell function rather than a subshell.

Demo:

https://github.com/oilshell/blog-code/blob/master/errexit /demo.sh#L21

Run ./demo.sh subshell-compare and func-compare.

I have seen multiple people point it out as confusing. The problem is that in the first case, there are two exit codes:

1. false (suppressed on LHS of &&)

2. true

And in the second case, there are 3:

1. false (suppressed on LHS of &&)

2. true

3. the exit code of the function/subshell, which is the exit code of the last statement, which is nonzero. This then CAUSES THE PROGRAM TO FAIL, whereas it didn't before.

(edit: removed incorrect code)

-----

So it's kind of like a "ghost" exit code created by the subshell/function! Which has an unexpected interaction with errexit.

Does that make sense?

I am not sure exactly how to fix it, but it will probably involve limiting && to stuff like this:

    if test -d /tmp && test -d /tmp/foo; then echo yes; fi

And disallow it when standing alone, because it doesn't make much sense there, if errexit is on.

Feel free to chime in bug with any possible solutions or more problems.

----

(And yes Oil fixes the brace problem with shopt -s parse_brace, which is on by default in Oil! Try it out and let me know if you like it :) )

major505• 6 years ago

This is usefull. I know a guy who didnt treat unset vairable, so in the script where he would remove with rm -rf /$dir-old-back his script removed all directories. The problem is that all his backups were in an external drive mounted in a directory, so he removed all his backups also. A hell month for him .

JeremyBanks• 6 years ago

https://dev.to/banks/stop-ignoring-errors-in-bash-3co5

cryptonector2• 6 years ago

Except `set -e` is stupid because it is disabled all the way down when a command occurs in a conditional, meaning this:

  function foo { false; true; }
  foo || echo foo failed

prints nothing!

From the bash manual page:

| If a compound command other than a subshell returns a non-zero status because a command failed while -e was being ignored, the shell does not exit.

POSIX says the same thing, so this is true of all POSIX-y shells.

This means you really have to check for errors you care about, and `set -e` is useless. Ugh!

lazyant• 6 years ago

I'd add TRAP DEBUG (running line by line) to the toolset

bewuethr• 6 years ago

set -e and set -u both come with their own pitfalls: https://mywiki.wooledge.org/BashFAQ/105 and https://mywiki.wooledge.org/BashFAQ/112

I generally use them and think that overall, they have more benefits than drawbacks, but the odd time where I run into one of the pitfalls, debugging usually takes a while.

ShellCheck already does a pretty good job of pointing out incorrect variable names, too.

westurner• 6 years ago

From https://twitter.com/b0rk/status/1312413117436104705 :

TIL that you can use the "DEBUG" trap to step through a bash script line by line

  trap '(read -p "[$BASH_SOURCE:$LINENO] $BASH_COMMAND?")' DEBUG

> [...] it does something very different than sh -x — sh -x will just print out lines, this stops before* every single line and lets you confirm that you want to run that line*

> you can also customize the prompt with set -x

  export PS4='+(${BASH_SOURCE}:${LINENO}) '
  set -x

With a markdown_escape function, could this make for something like a notebook with ```bash fenced code blocks with syntax highlighting?

Bash Error Handling

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