CODEWITHSUNDEEP
bash
spraff
spraff

Reputation: 33445

`cat <does_not_exist | perl` succeeds

I have a makefile which succeeds where it should fail because a line like this

 ./preprocess.sh <PARTIAL_SOURCE | perl >FINAL_SOURCE

succeeds even though PARTIAL_SOURCE doesn't exist yet.

This isn't a quirk of preprocess.sh, it seems to be something to do with bash/sh

$> cat <does_not_exist && echo ok || echo no
bash: does_not_exist: No such file or directory
no
$> cat <does_not_exist | perl && echo ok || echo no
bash: does_not_exist: No such file or directory
ok

Why does the first fail but the second succeed?

Upvotes: 1

Views: 43

Answers (1)

janos
janos

Reputation: 124724

The second succeeds because perl succeeds. The exit code of a pipeline is the exit code of the last command in the pipeline. In this example it's perl. It receives empty input, does nothing with it, so it exits with 0, as it should.

As another example, this is successful too:

$ a | b | perl && echo ok || echo no
-bash: a: command not found
-bash: b: command not found
ok

If you don't want perl to get executed before PARTIAL_SOURCE is ready, you need to test for it before the pipeline:

if [ ! -f "$input" ]; then
    ./preprocess.sh < "$input" | perl >FINAL_SOURCE
fi

Or wait for the input to be ready:

while [ ! -f "$input" ]; do sleep 1; done

Upvotes: 2

Related Questions