error handling in a bash subshell

Question

I'm writing some bash code, and I'd like the script to exit if there's any error. The set -e trick works pretty well, but not with subshells. Here's a simplified example:

set -e

chmod a=r file.txt
x=`grep value file.txt | awk '{print $2*2}'`
echo "okay, x=$x"

chmod a= file.txt
x=`grep value file.txt | awk '{print $2*2}'`
echo "should never be printed"

In this example, the second grep command fails with a non-zero exit status, but the subshell executes the awk command anyway, which completes with a status of zero. So, the subshell as a whole has an exit status of zero, and the script continues.

A websearch shows several articles about the limitations of set -e, but can anyone recommend a clean workaround?

Answer 1

 x=`grep value file.txt | awk '{print $2*2}'`

the second grep command fails with a non-zero exit status, but the subshell executes the awk command anyway

That doesn't really have anything to do with the subshell, but due to how pipelines work. The shell starts both (all) commands in the pipeline concurrently, so it doesn't even know the exit status of the left hand side before the right hand command starts.

And yes, by default, the exit status of a pipeline is that of the rightmost command. In general, that's useful, since pipelines can also finish so that the rightmost command exits without reading all input, and the left command is terminated by the SIGPIPE signal when trying to write more to the now-closed pipe.

That's not an error, since you might really not care about the rest of data. Consider something like zcat file.gz | grep -q xyz, which only looks if xyz exists somewhere in the data, at least once.

Anyway, as they say in comments, you use set -o pipefail in many shells (Bash, ksh, zsh, Busybox, at least) to have the rightmost nonzero exit status determine the exit status of the whole pipeline.

But note that grep returns with a falsy status not only in case of an error, but also if it doesn't find any matching lines. If you don't want that to be considered an error, you'll have to do more work.

In this particular case, you could have awk do the pattern matching too, e.g. x=$(awk '/value/ {print $2 * 2}' < file.txt). That should only exit with an error if the file can't be read.