Standard/canonical way to test whether foregoing pipeline produced output?
Clash Royale CLAN TAG#URR8PPP
up vote
4
down vote
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1
The situation I have in mind has the following structure:
% some_command | [PRODUCED OUTPUT] || echo 'no output' >&2
Here [PRODUCED OUTPUT] stands for some as-yet-unspecified testing command, whose value should be true (i.e. "success") iff some_command produces any output at all.
Of course, some_command stands for an arbitrarily complex pipeline, and likewise, echo 'no output' >&2 stands for some arbitrary action to perform in case some_command produces no output.
Is there a standard test to do the job represented above by [PRODUCED OUTPUT]? grep -qm1 '.' comes close but also reports false on an input consisting of empty lines.
Unlike Check if pipe is empty and run a command on the data if it isn't I just want to discard the input if it's present, I don't need to preserve it.
shell-script zsh pipe
edited Apr 13 '17 at 12:36
Community♦
1
asked Apr 13 '16 at 14:31
kjo
3,97993662
|
show 3 more comments
up vote
4
down vote
favorite
1
The situation I have in mind has the following structure:
% some_command | [PRODUCED OUTPUT] || echo 'no output' >&2
Here [PRODUCED OUTPUT] stands for some as-yet-unspecified testing command, whose value should be true (i.e. "success") iff some_command produces any output at all.
Of course, some_command stands for an arbitrarily complex pipeline, and likewise, echo 'no output' >&2 stands for some arbitrary action to perform in case some_command produces no output.
Is there a standard test to do the job represented above by [PRODUCED OUTPUT]? grep -qm1 '.' comes close but also reports false on an input consisting of empty lines.
Unlike Check if pipe is empty and run a command on the data if it isn't I just want to discard the input if it's present, I don't need to preserve it.
shell-script zsh pipe
edited Apr 13 '17 at 12:36
Community♦
1
asked Apr 13 '16 at 14:31
kjo
3,97993662
2
What's hacky about| grep -q .? O.o
– muru
Apr 13 '16 at 14:47
@muru: hackiness, of course, is in the eye of the beholder, but in my book at least, any use of a tool for a purpose entirely different from what it was intended for, especially when the tool in question is a powerful one, and the purpose it's being used is a trivial one, qualifies as "hacky". And yes, a ton of stuff would qualify as "hacky" by that definition, and a lot of other stuff many people would call hacky does not meet this definition... Nothing wrong with a hack, if that's all one can do...
– kjo
Apr 13 '16 at 14:59
@muru (cont'd) ...or if no one is looking, but for code that will go in a script, I avoid hacks as much as possible, if I want keep the script easy-to-understand by others. YMMV, it's all a matter of opinion.
– kjo
Apr 13 '16 at 15:00
Isn't testing whether input has something the intended purpose ofgrep?
– muru
Apr 13 '16 at 15:01
I'm assuming you don't care about stderr output? (you'd redirect it after some_command, before the pipeline, if you wanted it to be involved?)
– Jeff Schaller
Apr 13 '16 at 15:04
|
show 3 more comments
up vote
4
down vote
favorite
1
up vote
4
down vote
favorite
1
1
The situation I have in mind has the following structure:
% some_command | [PRODUCED OUTPUT] || echo 'no output' >&2
Here [PRODUCED OUTPUT] stands for some as-yet-unspecified testing command, whose value should be true (i.e. "success") iff some_command produces any output at all.
Of course, some_command stands for an arbitrarily complex pipeline, and likewise, echo 'no output' >&2 stands for some arbitrary action to perform in case some_command produces no output.
Is there a standard test to do the job represented above by [PRODUCED OUTPUT]? grep -qm1 '.' comes close but also reports false on an input consisting of empty lines.
Unlike Check if pipe is empty and run a command on the data if it isn't I just want to discard the input if it's present, I don't need to preserve it.
shell-script zsh pipe
edited Apr 13 '17 at 12:36
Community♦
1
asked Apr 13 '16 at 14:31
kjo
3,97993662
The situation I have in mind has the following structure:
% some_command | [PRODUCED OUTPUT] || echo 'no output' >&2
Here [PRODUCED OUTPUT] stands for some as-yet-unspecified testing command, whose value should be true (i.e. "success") iff some_command produces any output at all.
Of course, some_command stands for an arbitrarily complex pipeline, and likewise, echo 'no output' >&2 stands for some arbitrary action to perform in case some_command produces no output.
Is there a standard test to do the job represented above by [PRODUCED OUTPUT]? grep -qm1 '.' comes close but also reports false on an input consisting of empty lines.
Unlike Check if pipe is empty and run a command on the data if it isn't I just want to discard the input if it's present, I don't need to preserve it.
shell-script zsh pipe
shell-script zsh pipe
edited Apr 13 '17 at 12:36
Community♦
1
asked Apr 13 '16 at 14:31
kjo
3,97993662
edited Apr 13 '17 at 12:36
Community♦
1
asked Apr 13 '16 at 14:31
kjo
3,97993662
edited Apr 13 '17 at 12:36
Community♦
1
edited Apr 13 '17 at 12:36
Community♦
1
edited Apr 13 '17 at 12:36
Community♦
1
1
asked Apr 13 '16 at 14:31
kjo
3,97993662
asked Apr 13 '16 at 14:31
kjo
3,97993662
asked Apr 13 '16 at 14:31
kjo
3,97993662
3,97993662
2
What's hacky about| grep -q .? O.o
– muru
Apr 13 '16 at 14:47
@muru: hackiness, of course, is in the eye of the beholder, but in my book at least, any use of a tool for a purpose entirely different from what it was intended for, especially when the tool in question is a powerful one, and the purpose it's being used is a trivial one, qualifies as "hacky". And yes, a ton of stuff would qualify as "hacky" by that definition, and a lot of other stuff many people would call hacky does not meet this definition... Nothing wrong with a hack, if that's all one can do...
– kjo
Apr 13 '16 at 14:59
@muru (cont'd) ...or if no one is looking, but for code that will go in a script, I avoid hacks as much as possible, if I want keep the script easy-to-understand by others. YMMV, it's all a matter of opinion.
– kjo
Apr 13 '16 at 15:00
Isn't testing whether input has something the intended purpose ofgrep?
– muru
Apr 13 '16 at 15:01
I'm assuming you don't care about stderr output? (you'd redirect it after some_command, before the pipeline, if you wanted it to be involved?)
– Jeff Schaller
Apr 13 '16 at 15:04
|
show 3 more comments
2
What's hacky about| grep -q .? O.o
– muru
Apr 13 '16 at 14:47
@muru: hackiness, of course, is in the eye of the beholder, but in my book at least, any use of a tool for a purpose entirely different from what it was intended for, especially when the tool in question is a powerful one, and the purpose it's being used is a trivial one, qualifies as "hacky". And yes, a ton of stuff would qualify as "hacky" by that definition, and a lot of other stuff many people would call hacky does not meet this definition... Nothing wrong with a hack, if that's all one can do...
– kjo
Apr 13 '16 at 14:59
@muru (cont'd) ...or if no one is looking, but for code that will go in a script, I avoid hacks as much as possible, if I want keep the script easy-to-understand by others. YMMV, it's all a matter of opinion.
– kjo
Apr 13 '16 at 15:00
Isn't testing whether input has something the intended purpose ofgrep?
– muru
Apr 13 '16 at 15:01
I'm assuming you don't care about stderr output? (you'd redirect it after some_command, before the pipeline, if you wanted it to be involved?)
– Jeff Schaller
Apr 13 '16 at 15:04
2
2
What's hacky about
| grep -q .? O.o– muru
Apr 13 '16 at 14:47
What's hacky about
| grep -q .? O.o– muru
Apr 13 '16 at 14:47
@muru: hackiness, of course, is in the eye of the beholder, but in my book at least, any use of a tool for a purpose entirely different from what it was intended for, especially when the tool in question is a powerful one, and the purpose it's being used is a trivial one, qualifies as "hacky". And yes, a ton of stuff would qualify as "hacky" by that definition, and a lot of other stuff many people would call hacky does not meet this definition... Nothing wrong with a hack, if that's all one can do...
– kjo
Apr 13 '16 at 14:59
@muru: hackiness, of course, is in the eye of the beholder, but in my book at least, any use of a tool for a purpose entirely different from what it was intended for, especially when the tool in question is a powerful one, and the purpose it's being used is a trivial one, qualifies as "hacky". And yes, a ton of stuff would qualify as "hacky" by that definition, and a lot of other stuff many people would call hacky does not meet this definition... Nothing wrong with a hack, if that's all one can do...
– kjo
Apr 13 '16 at 14:59
@muru (cont'd) ...or if no one is looking, but for code that will go in a script, I avoid hacks as much as possible, if I want keep the script easy-to-understand by others. YMMV, it's all a matter of opinion.
– kjo
Apr 13 '16 at 15:00
@muru (cont'd) ...or if no one is looking, but for code that will go in a script, I avoid hacks as much as possible, if I want keep the script easy-to-understand by others. YMMV, it's all a matter of opinion.
– kjo
Apr 13 '16 at 15:00
Isn't testing whether input has something the intended purpose of
grep?– muru
Apr 13 '16 at 15:01
Isn't testing whether input has something the intended purpose of
grep?– muru
Apr 13 '16 at 15:01
I'm assuming you don't care about stderr output? (you'd redirect it after some_command, before the pipeline, if you wanted it to be involved?)
– Jeff Schaller
Apr 13 '16 at 15:04
I'm assuming you don't care about stderr output? (you'd redirect it after some_command, before the pipeline, if you wanted it to be involved?)
– Jeff Schaller
Apr 13 '16 at 15:04
|
show 3 more comments
2 Answers
2
active
oldest
votes
up vote
2
down vote
accepted
How about using read?
$ cat /dev/null | read pointless || echo no output
no output
$ echo something | read pointless || echo no output
$ printf "n" | read pointless || echo no output
$ printf " n" | read pointless || echo no output
$ false | read pointless || echo no output
no output
According to the Open Group definition:
EXIT STATUS
The following exit values shall be returned:
0
Successful completion.
>0
End-of-file was detected or an error occurred.
answered Apr 13 '16 at 14:47
Jeff Schaller
36k952119
This also report no output whenreadfalse.
– cuonglm
Apr 13 '16 at 14:53
How do you trigger that?
– Jeff Schaller
Apr 13 '16 at 14:56
@cuonglm, I don't want to mislead the OP if there's a case where this would be wrong -- what scenario were you thinking of?
– Jeff Schaller
Apr 14 '16 at 15:13
Just because the read documentation said it returns false when EOF reach or error occur, so it's not 100% to make sure there's no output.
– cuonglm
Apr 14 '16 at 15:19
Fair point; I browsed through the bash source code for read and while I didn't get all the way to the bottom (stopped at zreadcintr) the only errors were for timeouts, array declaration failure, or invalid -u file descriptor. Maybe otherreadimplementations have other error situations.
– Jeff Schaller
Apr 14 '16 at 15:43
add a comment |
up vote
-1
down vote
For me worked great | read -n 1.
$ echo 'faw' | read -n 1 || echo 'no output'
$ echo | read -n 1 || echo 'no output'
$ printf '' | read -n 1 || echo 'no output'
no output
$ false | read -n 1 || echo 'no output'
no output
$
answered 2 days ago
keypress
991
New contributor
keypress is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
2
down vote
accepted
How about using read?
$ cat /dev/null | read pointless || echo no output
no output
$ echo something | read pointless || echo no output
$ printf "n" | read pointless || echo no output
$ printf " n" | read pointless || echo no output
$ false | read pointless || echo no output
no output
According to the Open Group definition:
EXIT STATUS
The following exit values shall be returned:
0
Successful completion.
>0
End-of-file was detected or an error occurred.
answered Apr 13 '16 at 14:47
Jeff Schaller
36k952119
This also report no output whenreadfalse.
– cuonglm
Apr 13 '16 at 14:53
How do you trigger that?
– Jeff Schaller
Apr 13 '16 at 14:56
@cuonglm, I don't want to mislead the OP if there's a case where this would be wrong -- what scenario were you thinking of?
– Jeff Schaller
Apr 14 '16 at 15:13
Just because the read documentation said it returns false when EOF reach or error occur, so it's not 100% to make sure there's no output.
– cuonglm
Apr 14 '16 at 15:19
Fair point; I browsed through the bash source code for read and while I didn't get all the way to the bottom (stopped at zreadcintr) the only errors were for timeouts, array declaration failure, or invalid -u file descriptor. Maybe otherreadimplementations have other error situations.
– Jeff Schaller
Apr 14 '16 at 15:43
add a comment |
up vote
2
down vote
accepted
How about using read?
$ cat /dev/null | read pointless || echo no output
no output
$ echo something | read pointless || echo no output
$ printf "n" | read pointless || echo no output
$ printf " n" | read pointless || echo no output
$ false | read pointless || echo no output
no output
According to the Open Group definition:
EXIT STATUS
The following exit values shall be returned:
0
Successful completion.
>0
End-of-file was detected or an error occurred.
answered Apr 13 '16 at 14:47
Jeff Schaller
36k952119
This also report no output whenreadfalse.
– cuonglm
Apr 13 '16 at 14:53
How do you trigger that?
– Jeff Schaller
Apr 13 '16 at 14:56
@cuonglm, I don't want to mislead the OP if there's a case where this would be wrong -- what scenario were you thinking of?
– Jeff Schaller
Apr 14 '16 at 15:13
Just because the read documentation said it returns false when EOF reach or error occur, so it's not 100% to make sure there's no output.
– cuonglm
Apr 14 '16 at 15:19
Fair point; I browsed through the bash source code for read and while I didn't get all the way to the bottom (stopped at zreadcintr) the only errors were for timeouts, array declaration failure, or invalid -u file descriptor. Maybe otherreadimplementations have other error situations.
– Jeff Schaller
Apr 14 '16 at 15:43
add a comment |
up vote
2
down vote
accepted
up vote
2
down vote
accepted
How about using read?
$ cat /dev/null | read pointless || echo no output
no output
$ echo something | read pointless || echo no output
$ printf "n" | read pointless || echo no output
$ printf " n" | read pointless || echo no output
$ false | read pointless || echo no output
no output
According to the Open Group definition:
EXIT STATUS
The following exit values shall be returned:
0
Successful completion.
>0
End-of-file was detected or an error occurred.
answered Apr 13 '16 at 14:47
Jeff Schaller
36k952119
How about using read?
$ cat /dev/null | read pointless || echo no output
no output
$ echo something | read pointless || echo no output
$ printf "n" | read pointless || echo no output
$ printf " n" | read pointless || echo no output
$ false | read pointless || echo no output
no output
According to the Open Group definition:
EXIT STATUS
The following exit values shall be returned:
0
Successful completion.
>0
End-of-file was detected or an error occurred.
answered Apr 13 '16 at 14:47
Jeff Schaller
36k952119
edited Apr 14 '16 at 15:33
answered Apr 13 '16 at 14:47
Jeff Schaller
36k952119
answered Apr 13 '16 at 14:47
Jeff Schaller
36k952119
answered Apr 13 '16 at 14:47
Jeff Schaller
36k952119
36k952119
This also report no output whenreadfalse.
– cuonglm
Apr 13 '16 at 14:53
How do you trigger that?
– Jeff Schaller
Apr 13 '16 at 14:56
@cuonglm, I don't want to mislead the OP if there's a case where this would be wrong -- what scenario were you thinking of?
– Jeff Schaller
Apr 14 '16 at 15:13
Just because the read documentation said it returns false when EOF reach or error occur, so it's not 100% to make sure there's no output.
– cuonglm
Apr 14 '16 at 15:19
Fair point; I browsed through the bash source code for read and while I didn't get all the way to the bottom (stopped at zreadcintr) the only errors were for timeouts, array declaration failure, or invalid -u file descriptor. Maybe otherreadimplementations have other error situations.
– Jeff Schaller
Apr 14 '16 at 15:43
add a comment |
This also report no output whenreadfalse.
– cuonglm
Apr 13 '16 at 14:53
How do you trigger that?
– Jeff Schaller
Apr 13 '16 at 14:56
@cuonglm, I don't want to mislead the OP if there's a case where this would be wrong -- what scenario were you thinking of?
– Jeff Schaller
Apr 14 '16 at 15:13
Just because the read documentation said it returns false when EOF reach or error occur, so it's not 100% to make sure there's no output.
– cuonglm
Apr 14 '16 at 15:19
Fair point; I browsed through the bash source code for read and while I didn't get all the way to the bottom (stopped at zreadcintr) the only errors were for timeouts, array declaration failure, or invalid -u file descriptor. Maybe otherreadimplementations have other error situations.
– Jeff Schaller
Apr 14 '16 at 15:43
This also report no output when
read false.– cuonglm
Apr 13 '16 at 14:53
This also report no output when
read false.– cuonglm
Apr 13 '16 at 14:53
How do you trigger that?
– Jeff Schaller
Apr 13 '16 at 14:56
How do you trigger that?
– Jeff Schaller
Apr 13 '16 at 14:56
@cuonglm, I don't want to mislead the OP if there's a case where this would be wrong -- what scenario were you thinking of?
– Jeff Schaller
Apr 14 '16 at 15:13
@cuonglm, I don't want to mislead the OP if there's a case where this would be wrong -- what scenario were you thinking of?
– Jeff Schaller
Apr 14 '16 at 15:13
Just because the read documentation said it returns false when EOF reach or error occur, so it's not 100% to make sure there's no output.
– cuonglm
Apr 14 '16 at 15:19
Just because the read documentation said it returns false when EOF reach or error occur, so it's not 100% to make sure there's no output.
– cuonglm
Apr 14 '16 at 15:19
Fair point; I browsed through the bash source code for read and while I didn't get all the way to the bottom (stopped at zreadcintr) the only errors were for timeouts, array declaration failure, or invalid -u file descriptor. Maybe other
read implementations have other error situations.– Jeff Schaller
Apr 14 '16 at 15:43
Fair point; I browsed through the bash source code for read and while I didn't get all the way to the bottom (stopped at zreadcintr) the only errors were for timeouts, array declaration failure, or invalid -u file descriptor. Maybe other
read implementations have other error situations.– Jeff Schaller
Apr 14 '16 at 15:43
add a comment |
up vote
-1
down vote
For me worked great | read -n 1.
$ echo 'faw' | read -n 1 || echo 'no output'
$ echo | read -n 1 || echo 'no output'
$ printf '' | read -n 1 || echo 'no output'
no output
$ false | read -n 1 || echo 'no output'
no output
$
answered 2 days ago
keypress
991
New contributor
keypress is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
up vote
-1
down vote
For me worked great | read -n 1.
$ echo 'faw' | read -n 1 || echo 'no output'
$ echo | read -n 1 || echo 'no output'
$ printf '' | read -n 1 || echo 'no output'
no output
$ false | read -n 1 || echo 'no output'
no output
$
answered 2 days ago
keypress
991
New contributor
keypress is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
up vote
-1
down vote
up vote
-1
down vote
For me worked great | read -n 1.
$ echo 'faw' | read -n 1 || echo 'no output'
$ echo | read -n 1 || echo 'no output'
$ printf '' | read -n 1 || echo 'no output'
no output
$ false | read -n 1 || echo 'no output'
no output
$
answered 2 days ago
keypress
991
New contributor
keypress is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
For me worked great | read -n 1.
$ echo 'faw' | read -n 1 || echo 'no output'
$ echo | read -n 1 || echo 'no output'
$ printf '' | read -n 1 || echo 'no output'
no output
$ false | read -n 1 || echo 'no output'
no output
$
answered 2 days ago
keypress
991
New contributor
keypress is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
answered 2 days ago
keypress
991
New contributor
keypress is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
answered 2 days ago
keypress
991
answered 2 days ago
keypress
991
991
New contributor
keypress is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
New contributor
keypress is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
keypress is a new contributor to this site. Take care in asking for clarification, commenting, and answering.
Check out our Code of Conduct.
add a comment |
add a comment |
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2
What's hacky about
| grep -q .? O.o– muru
Apr 13 '16 at 14:47
@muru: hackiness, of course, is in the eye of the beholder, but in my book at least, any use of a tool for a purpose entirely different from what it was intended for, especially when the tool in question is a powerful one, and the purpose it's being used is a trivial one, qualifies as "hacky". And yes, a ton of stuff would qualify as "hacky" by that definition, and a lot of other stuff many people would call hacky does not meet this definition... Nothing wrong with a hack, if that's all one can do...
– kjo
Apr 13 '16 at 14:59
@muru (cont'd) ...or if no one is looking, but for code that will go in a script, I avoid hacks as much as possible, if I want keep the script easy-to-understand by others. YMMV, it's all a matter of opinion.
– kjo
Apr 13 '16 at 15:00
Isn't testing whether input has something the intended purpose of
grep?– muru
Apr 13 '16 at 15:01
I'm assuming you don't care about stderr output? (you'd redirect it after some_command, before the pipeline, if you wanted it to be involved?)
– Jeff Schaller
Apr 13 '16 at 15:04