How to fail command if zero or more than one line in output

62mkv

10/12/23, 8:02 AM

Let's say I have a command like

git log --some-options

and I expect it to produce exactly one line so I could feed it to xargs, like git log --some-options | xargs git branch --all --contains

how do I make sure that, when git log command produces 0, or more than one, lines, then my command fails and git branch never gets called?

2 + 0

command-line

text-processing

Score:3

Ubuntu

muru

10/12/23, 8:56 AM

Not running the command when there's no input to xargs is easy - it has the -r option for that. For most uses of xargs I've seen, you should be using this option.

Not running the command for more than one line of input can be trickier, but can be done by using, say, awk, to print the input only if there's one line:

... | awk 'END { if (NR == 1) print }'

For example:

% seq 1 10 | awk 'END { if (NR == 1) print }'
% # no output
% seq 1 | awk 'END { if (NR == 1) print }'
1

Then you can combine the two:

git log --some-options | awk 'END { if (NR == 1) print }' | xargs -r git branch --all --contains

+ 0

Score:1

Ubuntu

Artur Meinild

10/12/23, 8:11 AM

Use wc -l like this:

[[ $(git log --some-options | wc -l) -eq 1 ]] && git log --some-options | xargs git branch --all --contains

The condition [[ $(git log --some-options | wc -l) -eq 1 ]] checks if the number of lines in the command git log --some-options is exactly 1, and if it's true runs the command after && (logical AND in bash).

+ 2

62mkv

10/12/23, 9:03 AM

I was thinking of this but it implies running the command twice, which I would rather avoid...

Artur Meinild

10/12/23, 9:06 AM

Yes, muru's solution is actually better in several ways! ^^

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