current community

your communities

more stack exchange communities

company blog
Stack Exchange Inbox Reputation and Badges
tour help
Unix & Linux Stack Exchange is a question and answer site for users of Linux, FreeBSD and other Un*x-like operating systems. Join them; it only takes a minute:

Sign up
Here's how it works:
  1. Anybody can ask a question
  2. Anybody can answer
  3. The best answers are voted up and rise to the top
up vote 10 down vote favorite

I am currently exploring Debian packages, and I have been reading some code samples. And on every line in, for example, the postinst script is a pattern.

some command || true
another command || true

So if some command fails, then the line returns true but I don't see how this affects the output of the program. Usually in these scripts there is a set -e at the beginning.

share|improve this question
up vote 14 down vote accepted

While it does not affect the output of the program just run - it permits the caller to proceed as if all is okay aka affects future logic.

Rephrased: it masks the error status of the previous command.

michael@x071:[/usr/sbin]cat /tmp/false.sh
#!/bin/sh
false

michael@x071:[/usr/sbin]cat /tmp/true.sh 
#!/bin/sh
false || true

michael@x071:[/usr/sbin]sh /tmp/false.sh; echo $?
1
michael@x071:[/usr/sbin]sh /tmp/true.sh; echo $? 
0
share|improve this answer
1  
That's true, but it doesn't answer why masking a command's exit status is useful. – moopet 39 mins ago
up vote 23 down vote

The reason for this pattern is that maintainer scripts in Debian packages tend to start with set -e, which causes the shell to exit as soon as any command (strictly speaking, pipeline, list or compound command) exits with a non-zero status. This ensures that errors don't accumulate: as soon as something goes wrong, the script aborts.

In cases where a command in the script is allowed to fail, adding || true ensures that the resulting compound command always exits with status zero, so the script doesn't abort. For example, removing a directory shouldn't be a fatal error (preventing a package from being removed); so we'd use

rmdir ... || true

since rmdir doesn't have an option to tell it to ignore errors.

share|improve this answer
    
ah - not knowing debian I did not know of this 'debian installer' convention (-e). Your answer improves on context! And may help me understand why things finish with no error status - while things actually were not error free (and effect stability and/or conflicts with what is documented as 'expected results' when I play 'sandbox' with Debian on Power. – Michael Felt 4 hours ago
1  
Well, without set -e there's no need for || true at all, I thought it important to provide the context. If you notice odd things on POWER, I strongly encourage you to file bugs (reportbug)! – Stephen Kitt 3 hours ago
2  
@MichaelFelt, actually set -e is not only "Debian convention", but a good programming pattern one should always use. See. e.g. davidpashley.com/articles/writing-robust-shell-scripts – Kay 2 hours ago
    
per the question here - why use || true set -e is the likely context and likely the most common. I bow to this answer! Literally though, it is useful anytime exit status is deemed irrelevant AND (as you article link adds) I am not using exit status as part of my script control. I see utility (in set -e) but would not go so far as the article does and say "Every script you write should include set -e at the top". It is a style of programming. "ALWAYS | Every" includes it own set of traps - aka - absolutes re: wild-card solutions will ALWAYS backfire eventually aka - no free rides. – Michael Felt 1 hour ago

Your Answer

 
discard

By posting your answer, you agree to the privacy policy and terms of service.

Not the answer you're looking for? Browse other questions tagged or ask your own question.