Return 1 if command output is empty without capturing it
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I have a shell script which ends with a few pipes grep ... | while read ...| sort | uniq and I want to return 1 if the output is empty, but uniq always returns 0 even if its input is empty.
So far the best solution I've found is adding grep with an empty pattern ... uniq | grep '', this works perfectly but feels more like a hack.
So my question is: Is there any better/canonical way to do this?
Some restrictions:
- I don't want to capture my output in a variable since I would need to print it again afterwards:
a=$(... | uniq); printf '%sn' "$a"; [ -n "$a" ]which doesn't feel right either. - If possible I'd also prefer a standard tool (no moreutils) and something portable
Thanks!
linux shell-script shell
asked Mar 24 at 18:10
Quentin L'Hours
132
add a comment |Â
up vote
2
down vote
favorite
I have a shell script which ends with a few pipes grep ... | while read ...| sort | uniq and I want to return 1 if the output is empty, but uniq always returns 0 even if its input is empty.
So far the best solution I've found is adding grep with an empty pattern ... uniq | grep '', this works perfectly but feels more like a hack.
So my question is: Is there any better/canonical way to do this?
Some restrictions:
- I don't want to capture my output in a variable since I would need to print it again afterwards:
a=$(... | uniq); printf '%sn' "$a"; [ -n "$a" ]which doesn't feel right either. - If possible I'd also prefer a standard tool (no moreutils) and something portable
Thanks!
linux shell-script shell
asked Mar 24 at 18:10
Quentin L'Hours
132
Storing it and printing it is unavoidable, even if you use a program liketee. While some programs make it look like it does not store it and print it later, that is what you have to do to check the output of a command and then running it
– Stan Strum
Mar 24 at 18:15
In these cases I tend to usewc -land check the output but this is not better than your approach.
– Ned64
Mar 24 at 18:17
yeah store and print later is indeed probably what uniq and sort are doing, but the less I do it the better
– Quentin L'Hours
Mar 24 at 18:20
What about a function like:muniq () uniq?
– Jesse_b
Mar 24 at 18:27
@Jesse_b yes it's easier to read but it's still a hack
– Quentin L'Hours
Mar 24 at 18:29
add a comment |Â
up vote
2
down vote
favorite
up vote
2
down vote
favorite
I have a shell script which ends with a few pipes grep ... | while read ...| sort | uniq and I want to return 1 if the output is empty, but uniq always returns 0 even if its input is empty.
So far the best solution I've found is adding grep with an empty pattern ... uniq | grep '', this works perfectly but feels more like a hack.
So my question is: Is there any better/canonical way to do this?
Some restrictions:
- I don't want to capture my output in a variable since I would need to print it again afterwards:
a=$(... | uniq); printf '%sn' "$a"; [ -n "$a" ]which doesn't feel right either. - If possible I'd also prefer a standard tool (no moreutils) and something portable
Thanks!
linux shell-script shell
asked Mar 24 at 18:10
Quentin L'Hours
132
I have a shell script which ends with a few pipes grep ... | while read ...| sort | uniq and I want to return 1 if the output is empty, but uniq always returns 0 even if its input is empty.
So far the best solution I've found is adding grep with an empty pattern ... uniq | grep '', this works perfectly but feels more like a hack.
So my question is: Is there any better/canonical way to do this?
Some restrictions:
- I don't want to capture my output in a variable since I would need to print it again afterwards:
a=$(... | uniq); printf '%sn' "$a"; [ -n "$a" ]which doesn't feel right either. - If possible I'd also prefer a standard tool (no moreutils) and something portable
Thanks!
linux shell-script shell
asked Mar 24 at 18:10
Quentin L'Hours
132
asked Mar 24 at 18:10
Quentin L'Hours
132
asked Mar 24 at 18:10
Quentin L'Hours
132
asked Mar 24 at 18:10
Quentin L'Hours
132
132
Storing it and printing it is unavoidable, even if you use a program liketee. While some programs make it look like it does not store it and print it later, that is what you have to do to check the output of a command and then running it
– Stan Strum
Mar 24 at 18:15
In these cases I tend to usewc -land check the output but this is not better than your approach.
– Ned64
Mar 24 at 18:17
yeah store and print later is indeed probably what uniq and sort are doing, but the less I do it the better
– Quentin L'Hours
Mar 24 at 18:20
What about a function like:muniq () uniq?
– Jesse_b
Mar 24 at 18:27
@Jesse_b yes it's easier to read but it's still a hack
– Quentin L'Hours
Mar 24 at 18:29
add a comment |Â
Storing it and printing it is unavoidable, even if you use a program liketee. While some programs make it look like it does not store it and print it later, that is what you have to do to check the output of a command and then running it
– Stan Strum
Mar 24 at 18:15
In these cases I tend to usewc -land check the output but this is not better than your approach.
– Ned64
Mar 24 at 18:17
yeah store and print later is indeed probably what uniq and sort are doing, but the less I do it the better
– Quentin L'Hours
Mar 24 at 18:20
What about a function like:muniq () uniq?
– Jesse_b
Mar 24 at 18:27
@Jesse_b yes it's easier to read but it's still a hack
– Quentin L'Hours
Mar 24 at 18:29
Storing it and printing it is unavoidable, even if you use a program like
tee. While some programs make it look like it does not store it and print it later, that is what you have to do to check the output of a command and then running it– Stan Strum
Mar 24 at 18:15
Storing it and printing it is unavoidable, even if you use a program like
tee. While some programs make it look like it does not store it and print it later, that is what you have to do to check the output of a command and then running it– Stan Strum
Mar 24 at 18:15
In these cases I tend to use
wc -l and check the output but this is not better than your approach.– Ned64
Mar 24 at 18:17
In these cases I tend to use
wc -l and check the output but this is not better than your approach.– Ned64
Mar 24 at 18:17
yeah store and print later is indeed probably what uniq and sort are doing, but the less I do it the better
– Quentin L'Hours
Mar 24 at 18:20
yeah store and print later is indeed probably what uniq and sort are doing, but the less I do it the better
– Quentin L'Hours
Mar 24 at 18:20
What about a function like:
muniq () uniq ?– Jesse_b
Mar 24 at 18:27
What about a function like:
muniq () uniq ?– Jesse_b
Mar 24 at 18:27
@Jesse_b yes it's easier to read but it's still a hack
– Quentin L'Hours
Mar 24 at 18:29
@Jesse_b yes it's easier to read but it's still a hack
– Quentin L'Hours
Mar 24 at 18:29
add a comment |Â
2 Answers
2
active
oldest
votes
up vote
0
down vote
accepted
Frankly, I think | grep '' is just fine. You've already used a bunch of programs in the pipeline, so forking out another one isn't going to hurt. And you already know what grep does, so there's no debugging with regard to writing some "smart" shell snippet to do it.
The comments mentioned also | grep ., which might be another possibility. grep . doesn't match on an empty line, grep '' does, so choose depending on which one you want.
Though, using grep like that might not be immediately obvious to a casual reader, so you might want to add a comment about its purpose in any case.
answered Mar 24 at 19:45
ilkkachu
48.9k671136
Yeah I guess I'll stick with this, too bad there's nothing more obvious than grep.
– Quentin L'Hours
Mar 24 at 21:29
add a comment |Â
up vote
0
down vote
I think this should do the work:
[ -n "$(command | tee /dev/tty)" ]
tee will send command's output to both stdout (where it's evaluated by the test command) and your terminal (/dev/tty), so you can see the output without capturing it in a variable.
It's a POSIX-compliant solution AFAIK.
answered Mar 24 at 18:33
nxnev
2,4522423
and that only works if the output from the script is going to a tty, and not redirected somewhere else... also the comsub will eat tailing newlines, making an output that contains just empty lines seem like there was no output
– ilkkachu
Mar 24 at 19:39
Thanks, it would work in my case but I feel this is too much compared to just using grep, even if that's not grep's primary purpose.
– Quentin L'Hours
Mar 24 at 21:27
add a comment |Â
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
0
down vote
accepted
Frankly, I think | grep '' is just fine. You've already used a bunch of programs in the pipeline, so forking out another one isn't going to hurt. And you already know what grep does, so there's no debugging with regard to writing some "smart" shell snippet to do it.
The comments mentioned also | grep ., which might be another possibility. grep . doesn't match on an empty line, grep '' does, so choose depending on which one you want.
Though, using grep like that might not be immediately obvious to a casual reader, so you might want to add a comment about its purpose in any case.
answered Mar 24 at 19:45
ilkkachu
48.9k671136
Yeah I guess I'll stick with this, too bad there's nothing more obvious than grep.
– Quentin L'Hours
Mar 24 at 21:29
add a comment |Â
up vote
0
down vote
accepted
Frankly, I think | grep '' is just fine. You've already used a bunch of programs in the pipeline, so forking out another one isn't going to hurt. And you already know what grep does, so there's no debugging with regard to writing some "smart" shell snippet to do it.
The comments mentioned also | grep ., which might be another possibility. grep . doesn't match on an empty line, grep '' does, so choose depending on which one you want.
Though, using grep like that might not be immediately obvious to a casual reader, so you might want to add a comment about its purpose in any case.
answered Mar 24 at 19:45
ilkkachu
48.9k671136
Yeah I guess I'll stick with this, too bad there's nothing more obvious than grep.
– Quentin L'Hours
Mar 24 at 21:29
add a comment |Â
up vote
0
down vote
accepted
up vote
0
down vote
accepted
Frankly, I think | grep '' is just fine. You've already used a bunch of programs in the pipeline, so forking out another one isn't going to hurt. And you already know what grep does, so there's no debugging with regard to writing some "smart" shell snippet to do it.
The comments mentioned also | grep ., which might be another possibility. grep . doesn't match on an empty line, grep '' does, so choose depending on which one you want.
Though, using grep like that might not be immediately obvious to a casual reader, so you might want to add a comment about its purpose in any case.
answered Mar 24 at 19:45
ilkkachu
48.9k671136
Frankly, I think | grep '' is just fine. You've already used a bunch of programs in the pipeline, so forking out another one isn't going to hurt. And you already know what grep does, so there's no debugging with regard to writing some "smart" shell snippet to do it.
The comments mentioned also | grep ., which might be another possibility. grep . doesn't match on an empty line, grep '' does, so choose depending on which one you want.
Though, using grep like that might not be immediately obvious to a casual reader, so you might want to add a comment about its purpose in any case.
answered Mar 24 at 19:45
ilkkachu
48.9k671136
answered Mar 24 at 19:45
ilkkachu
48.9k671136
answered Mar 24 at 19:45
ilkkachu
48.9k671136
answered Mar 24 at 19:45
ilkkachu
48.9k671136
48.9k671136
Yeah I guess I'll stick with this, too bad there's nothing more obvious than grep.
– Quentin L'Hours
Mar 24 at 21:29
add a comment |Â
Yeah I guess I'll stick with this, too bad there's nothing more obvious than grep.
– Quentin L'Hours
Mar 24 at 21:29
Yeah I guess I'll stick with this, too bad there's nothing more obvious than grep.
– Quentin L'Hours
Mar 24 at 21:29
Yeah I guess I'll stick with this, too bad there's nothing more obvious than grep.
– Quentin L'Hours
Mar 24 at 21:29
add a comment |Â
up vote
0
down vote
I think this should do the work:
[ -n "$(command | tee /dev/tty)" ]
tee will send command's output to both stdout (where it's evaluated by the test command) and your terminal (/dev/tty), so you can see the output without capturing it in a variable.
It's a POSIX-compliant solution AFAIK.
answered Mar 24 at 18:33
nxnev
2,4522423
and that only works if the output from the script is going to a tty, and not redirected somewhere else... also the comsub will eat tailing newlines, making an output that contains just empty lines seem like there was no output
– ilkkachu
Mar 24 at 19:39
Thanks, it would work in my case but I feel this is too much compared to just using grep, even if that's not grep's primary purpose.
– Quentin L'Hours
Mar 24 at 21:27
add a comment |Â
up vote
0
down vote
I think this should do the work:
[ -n "$(command | tee /dev/tty)" ]
tee will send command's output to both stdout (where it's evaluated by the test command) and your terminal (/dev/tty), so you can see the output without capturing it in a variable.
It's a POSIX-compliant solution AFAIK.
answered Mar 24 at 18:33
nxnev
2,4522423
and that only works if the output from the script is going to a tty, and not redirected somewhere else... also the comsub will eat tailing newlines, making an output that contains just empty lines seem like there was no output
– ilkkachu
Mar 24 at 19:39
Thanks, it would work in my case but I feel this is too much compared to just using grep, even if that's not grep's primary purpose.
– Quentin L'Hours
Mar 24 at 21:27
add a comment |Â
up vote
0
down vote
up vote
0
down vote
I think this should do the work:
[ -n "$(command | tee /dev/tty)" ]
tee will send command's output to both stdout (where it's evaluated by the test command) and your terminal (/dev/tty), so you can see the output without capturing it in a variable.
It's a POSIX-compliant solution AFAIK.
answered Mar 24 at 18:33
nxnev
2,4522423
I think this should do the work:
[ -n "$(command | tee /dev/tty)" ]
tee will send command's output to both stdout (where it's evaluated by the test command) and your terminal (/dev/tty), so you can see the output without capturing it in a variable.
It's a POSIX-compliant solution AFAIK.
answered Mar 24 at 18:33
nxnev
2,4522423
edited Mar 24 at 19:10
answered Mar 24 at 18:33
nxnev
2,4522423
answered Mar 24 at 18:33
nxnev
2,4522423
answered Mar 24 at 18:33
nxnev
2,4522423
2,4522423
and that only works if the output from the script is going to a tty, and not redirected somewhere else... also the comsub will eat tailing newlines, making an output that contains just empty lines seem like there was no output
– ilkkachu
Mar 24 at 19:39
Thanks, it would work in my case but I feel this is too much compared to just using grep, even if that's not grep's primary purpose.
– Quentin L'Hours
Mar 24 at 21:27
add a comment |Â
and that only works if the output from the script is going to a tty, and not redirected somewhere else... also the comsub will eat tailing newlines, making an output that contains just empty lines seem like there was no output
– ilkkachu
Mar 24 at 19:39
Thanks, it would work in my case but I feel this is too much compared to just using grep, even if that's not grep's primary purpose.
– Quentin L'Hours
Mar 24 at 21:27
and that only works if the output from the script is going to a tty, and not redirected somewhere else... also the comsub will eat tailing newlines, making an output that contains just empty lines seem like there was no output
– ilkkachu
Mar 24 at 19:39
and that only works if the output from the script is going to a tty, and not redirected somewhere else... also the comsub will eat tailing newlines, making an output that contains just empty lines seem like there was no output
– ilkkachu
Mar 24 at 19:39
Thanks, it would work in my case but I feel this is too much compared to just using grep, even if that's not grep's primary purpose.
– Quentin L'Hours
Mar 24 at 21:27
Thanks, it would work in my case but I feel this is too much compared to just using grep, even if that's not grep's primary purpose.
– Quentin L'Hours
Mar 24 at 21:27
add a comment |Â
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Storing it and printing it is unavoidable, even if you use a program like
tee. While some programs make it look like it does not store it and print it later, that is what you have to do to check the output of a command and then running it– Stan Strum
Mar 24 at 18:15
In these cases I tend to use
wc -land check the output but this is not better than your approach.– Ned64
Mar 24 at 18:17
yeah store and print later is indeed probably what uniq and sort are doing, but the less I do it the better
– Quentin L'Hours
Mar 24 at 18:20
What about a function like:
muniq () uniq?– Jesse_b
Mar 24 at 18:27
@Jesse_b yes it's easier to read but it's still a hack
– Quentin L'Hours
Mar 24 at 18:29