How to Check for Errors in Bash Process Substitution

Last week, I wrote on twitter:

Programming in shell feels like programming in R: a lot of convenience to get started, but poor error checking and many dark corners.

— Luis Pedro Coelho (@luispedrocoelho) November 22, 2013

This is a general pattern, but what was really upsetting me was that while using process substitution can be very convenient, it is hard to get an error from the subshells.

Here is what I mean:

Some pre-processing generated two outputs file1.gz and file2.gz, both of which are sorted. Now, I want to merge the results (in sorted order) and run a Python dosummary.pyscript.

Option 1:

gunzip file1.gz
gunzip file2.gz
sort --merge file1 file2 > intermediate
python < intermediate

This works, but it generates a lot of slow disk IO.

Note that I used sort --merge which merges already sorted files. This is a really neat option in my usage as I can often sort the intermediate small files at the source.

Options 2:

sort --merge <(gunzip -c file1.gz) <(gunzip -c file2.gz) | python

This is great! No intermediate files, all done with the magic of Unix pipes.


Except if there is an error! What if one of the input files is corrupt? Or the merge fails?

Option 1 with error checking:

set -e
gunzip file1.gz
gunzip file2.gz
sort --merge file1 file2 > intermediate
python < intermediate

Now, if any of the steps fails, then the whole script fails with an error.

What about the pipe version? Let’s try this (I also use the pipefail option to make the whole pipe fail if any of the elements fails [surprisingly, this is not the default]).

Option 2 with error checking:

set -e
set -o pipefail
sort --merge <(gunzip -c file1.gz) <(gunzip -c file2.gz) | python

However, if file1.gz is corrupt, the whole thing will seem to work just fine. Even with pipefail, a process substitution error will not trigger the whole script to fail.

I did not find any way to get an error out of process substitutions. In fact the title of this post is a bit of a lie, all I found was a work-around: I replaced <(gunzip -c file.gz) by <(gunzip -c file.gz || touch GUNZIP_FAILED). And later, in the script, check whether this file exists!

Option 2 with hard-core error checking:

set -e
set -o pipefail
sort --merge <(gunzip -c file1.gz || touch GUNZIP_FAILED) <(gunzip -c file2.gz || touch GUNZIP_FAILED) | python
if [[ -e GUNZIP_FAILED ]] ; then
    exit 2

Now, any error will cause the whole thing to fail.


The issue is that, for this simple example, if I was running this one time, then I could just check the error logs. If gunzip fails, then it’ll print a message to standard error.

However, if this is a single-step in a longer pipeline, being run on many data sets; then an error here may easily get lost in the noise [1]. Besides, I don’t want to look at error logs, I want failing processes to announce themselves.

[1] Especially as many bioinformatics tools are not good Unix citizens and output a lot of messages even if there are no errors. Also, they don’t always distinguish between stdout & stderr correctly.



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