Understanding piped commands in GNU/Linux Announcing the arrival of Valued Associate #679: Cesar Manara Planned maintenance scheduled April 23, 2019 at 23:30 UTC (7:30pm US/Eastern) 2019 Community Moderator Election Results Why I closed the “Why is Kali so hard” questionHow big is the pipe buffer?In what order do piped commands run?In what order do piped commands run?Redirecting stdin with stdout to fileUnderstanding behavior of subshell and stdout with pipeWhy do some commands not read from their standard input?“Leaky” pipes in linuxCan writing to stdout place backpressure on a process?How can pipe producer tell pipe consumer it has reached 'End of File'?" (un-named-pipe, not named-pipe)Take a command that modifies a file inline and make it accept stdin/stdoutPipeline running in parallel through creating multiple subshellsHow shell delivers user's input to program and shows program's output?
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Understanding piped commands in GNU/Linux
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Understanding piped commands in GNU/Linux
Announcing the arrival of Valued Associate #679: Cesar Manara
Planned maintenance scheduled April 23, 2019 at 23:30 UTC (7:30pm US/Eastern)
2019 Community Moderator Election Results
Why I closed the “Why is Kali so hard” questionHow big is the pipe buffer?In what order do piped commands run?In what order do piped commands run?Redirecting stdin with stdout to fileUnderstanding behavior of subshell and stdout with pipeWhy do some commands not read from their standard input?“Leaky” pipes in linuxCan writing to stdout place backpressure on a process?How can pipe producer tell pipe consumer it has reached 'End of File'?" (un-named-pipe, not named-pipe)Take a command that modifies a file inline and make it accept stdin/stdoutPipeline running in parallel through creating multiple subshellsHow shell delivers user's input to program and shows program's output?
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I have two simple C programs: A
and B
. A
would run first, then B
gets the stdout
of A
and uses it as its stdin
. Assume I am using a GNU/Linux operating system and the simplest possible way to do this would be:
./A | ./B
If I had to describe this command I would say that it is a command that takes input from a producer (A
) and writes to a consumer (B
). Is that a correct description? Is there anything that I am missing?
pipe c
New contributor
add a comment |
I have two simple C programs: A
and B
. A
would run first, then B
gets the stdout
of A
and uses it as its stdin
. Assume I am using a GNU/Linux operating system and the simplest possible way to do this would be:
./A | ./B
If I had to describe this command I would say that it is a command that takes input from a producer (A
) and writes to a consumer (B
). Is that a correct description? Is there anything that I am missing?
pipe c
New contributor
Related: In what order do piped commands run?
– G-Man
2 hours ago
add a comment |
I have two simple C programs: A
and B
. A
would run first, then B
gets the stdout
of A
and uses it as its stdin
. Assume I am using a GNU/Linux operating system and the simplest possible way to do this would be:
./A | ./B
If I had to describe this command I would say that it is a command that takes input from a producer (A
) and writes to a consumer (B
). Is that a correct description? Is there anything that I am missing?
pipe c
New contributor
I have two simple C programs: A
and B
. A
would run first, then B
gets the stdout
of A
and uses it as its stdin
. Assume I am using a GNU/Linux operating system and the simplest possible way to do this would be:
./A | ./B
If I had to describe this command I would say that it is a command that takes input from a producer (A
) and writes to a consumer (B
). Is that a correct description? Is there anything that I am missing?
pipe c
pipe c
New contributor
New contributor
edited 20 mins ago
JL2210
1033
1033
New contributor
asked 11 hours ago
nihulusnihulus
1313
1313
New contributor
New contributor
Related: In what order do piped commands run?
– G-Man
2 hours ago
add a comment |
Related: In what order do piped commands run?
– G-Man
2 hours ago
Related: In what order do piped commands run?
– G-Man
2 hours ago
Related: In what order do piped commands run?
– G-Man
2 hours ago
add a comment |
2 Answers
2
active
oldest
votes
The only thing about your question that stands out as wrong is that you say
A would run first, then B gets the stdout of A
In fact, both programs would be started at pretty much the same time. If there's no input for B
when it tries to read, it will block until there is input to read. Likewise, if there's nobody reading the output from A
, its writes will block until its output is read (some of it will be buffered by the pipe).
The only thing synchronising the processes that take part in a pipeline is the I/O, i.e. the reading and writing. If no writing or reading happens, then the two processes will run totally independent of each other. If one ignores the reading or writing of the other, the ignored process will block and eventually be killed by a SIGPIPE
signal (if writing) or get an end-of-file condition on its standard input stream (if reading) when the other process terminates.
The idiomatic way to describe A | B
is that it's a pipeline containing two programs. The output produced on standard output from the first program is available to be read on the standard input by the second ("[the output of] A
is piped into B
"). The shell does the required plumbing to allow this to happen.
If you want to use the words "consumer" and "producer", I suppose that's ok too.
The fact that these are programs written in C is not relevant. The fact that this is Linux, macOS, OpenBSD or AIX is not relevant.
Actually, we can think of having A and B running in parallel as an optimization. The command is equivalent to./A > tmp_file && ./B < tmp_file
, which first save the output of A totmp_file
and then give it as an input to B. This information is taken from: okmij.org/ftp/Computation/monadic-shell.html (I change the command slightly)
– Alex Vong
7 hours ago
What I think OP have in mind is the unoptimized implementation of pipe (which is almost never used in practice).
– Alex Vong
7 hours ago
1
Writing to a temporary file was used in DOS, as that didn't support multiple processes.
– CSM
5 hours ago
1
@AlexVong Note though that your example with a temporary file is not exactly equivalent. A program may choose to seek though the contents of a file, but data coming off a pipe is not seekable. A better examlp would be to usemkfifo
to create a named pipe, then start B in the background reading from the pipe, and then A writing to it. This is nit-picking though, as the effect would be the same.
– Kusalananda♦
5 hours ago
The&&
is another problem with Alex’s suggested implementation —B
will always run, unconditionally, so it’s more like./A > tmp_file; ./B < tmp_file
. And, of course, there’s also therm tmp_file
at the end.
– G-Man
1 hour ago
|
show 3 more comments
The term usually used in documentation is "pipeline" , which consists of one or more commands, see POSIX definition So technically speaking, that's two commands you have there, two subprocesses for the shell (either fork()+exec()
'ed external commands or subshells )
As for producer-consumer part, the pipeline can be described by that pattern, since:
- Producer and Consumer share fixed-size buffer, and at least on Linux and MacOS X, there's fixed size for pipeline buffer
- Producer and Consumer are loosely-coupled, commands in pipeline don't know of each other's existence ( unless they are actively checking
/proc/<pid>/fd
directory ). - Producers write to
stdout
and consumers readstdin
as if they were a single command being executed, aka they can exist without each other.
The difference I see here is that unlike Producer-Consumer in other languges, shell commands use buffering and they write stdout once buffer is filled, but there's no mention that Producer-Consumer has to follow that rule - only wait when queue is filled or discard data (which is something else that pipeline doesn't do).
add a comment |
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2 Answers
2
active
oldest
votes
2 Answers
2
active
oldest
votes
active
oldest
votes
active
oldest
votes
The only thing about your question that stands out as wrong is that you say
A would run first, then B gets the stdout of A
In fact, both programs would be started at pretty much the same time. If there's no input for B
when it tries to read, it will block until there is input to read. Likewise, if there's nobody reading the output from A
, its writes will block until its output is read (some of it will be buffered by the pipe).
The only thing synchronising the processes that take part in a pipeline is the I/O, i.e. the reading and writing. If no writing or reading happens, then the two processes will run totally independent of each other. If one ignores the reading or writing of the other, the ignored process will block and eventually be killed by a SIGPIPE
signal (if writing) or get an end-of-file condition on its standard input stream (if reading) when the other process terminates.
The idiomatic way to describe A | B
is that it's a pipeline containing two programs. The output produced on standard output from the first program is available to be read on the standard input by the second ("[the output of] A
is piped into B
"). The shell does the required plumbing to allow this to happen.
If you want to use the words "consumer" and "producer", I suppose that's ok too.
The fact that these are programs written in C is not relevant. The fact that this is Linux, macOS, OpenBSD or AIX is not relevant.
Actually, we can think of having A and B running in parallel as an optimization. The command is equivalent to./A > tmp_file && ./B < tmp_file
, which first save the output of A totmp_file
and then give it as an input to B. This information is taken from: okmij.org/ftp/Computation/monadic-shell.html (I change the command slightly)
– Alex Vong
7 hours ago
What I think OP have in mind is the unoptimized implementation of pipe (which is almost never used in practice).
– Alex Vong
7 hours ago
1
Writing to a temporary file was used in DOS, as that didn't support multiple processes.
– CSM
5 hours ago
1
@AlexVong Note though that your example with a temporary file is not exactly equivalent. A program may choose to seek though the contents of a file, but data coming off a pipe is not seekable. A better examlp would be to usemkfifo
to create a named pipe, then start B in the background reading from the pipe, and then A writing to it. This is nit-picking though, as the effect would be the same.
– Kusalananda♦
5 hours ago
The&&
is another problem with Alex’s suggested implementation —B
will always run, unconditionally, so it’s more like./A > tmp_file; ./B < tmp_file
. And, of course, there’s also therm tmp_file
at the end.
– G-Man
1 hour ago
|
show 3 more comments
The only thing about your question that stands out as wrong is that you say
A would run first, then B gets the stdout of A
In fact, both programs would be started at pretty much the same time. If there's no input for B
when it tries to read, it will block until there is input to read. Likewise, if there's nobody reading the output from A
, its writes will block until its output is read (some of it will be buffered by the pipe).
The only thing synchronising the processes that take part in a pipeline is the I/O, i.e. the reading and writing. If no writing or reading happens, then the two processes will run totally independent of each other. If one ignores the reading or writing of the other, the ignored process will block and eventually be killed by a SIGPIPE
signal (if writing) or get an end-of-file condition on its standard input stream (if reading) when the other process terminates.
The idiomatic way to describe A | B
is that it's a pipeline containing two programs. The output produced on standard output from the first program is available to be read on the standard input by the second ("[the output of] A
is piped into B
"). The shell does the required plumbing to allow this to happen.
If you want to use the words "consumer" and "producer", I suppose that's ok too.
The fact that these are programs written in C is not relevant. The fact that this is Linux, macOS, OpenBSD or AIX is not relevant.
Actually, we can think of having A and B running in parallel as an optimization. The command is equivalent to./A > tmp_file && ./B < tmp_file
, which first save the output of A totmp_file
and then give it as an input to B. This information is taken from: okmij.org/ftp/Computation/monadic-shell.html (I change the command slightly)
– Alex Vong
7 hours ago
What I think OP have in mind is the unoptimized implementation of pipe (which is almost never used in practice).
– Alex Vong
7 hours ago
1
Writing to a temporary file was used in DOS, as that didn't support multiple processes.
– CSM
5 hours ago
1
@AlexVong Note though that your example with a temporary file is not exactly equivalent. A program may choose to seek though the contents of a file, but data coming off a pipe is not seekable. A better examlp would be to usemkfifo
to create a named pipe, then start B in the background reading from the pipe, and then A writing to it. This is nit-picking though, as the effect would be the same.
– Kusalananda♦
5 hours ago
The&&
is another problem with Alex’s suggested implementation —B
will always run, unconditionally, so it’s more like./A > tmp_file; ./B < tmp_file
. And, of course, there’s also therm tmp_file
at the end.
– G-Man
1 hour ago
|
show 3 more comments
The only thing about your question that stands out as wrong is that you say
A would run first, then B gets the stdout of A
In fact, both programs would be started at pretty much the same time. If there's no input for B
when it tries to read, it will block until there is input to read. Likewise, if there's nobody reading the output from A
, its writes will block until its output is read (some of it will be buffered by the pipe).
The only thing synchronising the processes that take part in a pipeline is the I/O, i.e. the reading and writing. If no writing or reading happens, then the two processes will run totally independent of each other. If one ignores the reading or writing of the other, the ignored process will block and eventually be killed by a SIGPIPE
signal (if writing) or get an end-of-file condition on its standard input stream (if reading) when the other process terminates.
The idiomatic way to describe A | B
is that it's a pipeline containing two programs. The output produced on standard output from the first program is available to be read on the standard input by the second ("[the output of] A
is piped into B
"). The shell does the required plumbing to allow this to happen.
If you want to use the words "consumer" and "producer", I suppose that's ok too.
The fact that these are programs written in C is not relevant. The fact that this is Linux, macOS, OpenBSD or AIX is not relevant.
The only thing about your question that stands out as wrong is that you say
A would run first, then B gets the stdout of A
In fact, both programs would be started at pretty much the same time. If there's no input for B
when it tries to read, it will block until there is input to read. Likewise, if there's nobody reading the output from A
, its writes will block until its output is read (some of it will be buffered by the pipe).
The only thing synchronising the processes that take part in a pipeline is the I/O, i.e. the reading and writing. If no writing or reading happens, then the two processes will run totally independent of each other. If one ignores the reading or writing of the other, the ignored process will block and eventually be killed by a SIGPIPE
signal (if writing) or get an end-of-file condition on its standard input stream (if reading) when the other process terminates.
The idiomatic way to describe A | B
is that it's a pipeline containing two programs. The output produced on standard output from the first program is available to be read on the standard input by the second ("[the output of] A
is piped into B
"). The shell does the required plumbing to allow this to happen.
If you want to use the words "consumer" and "producer", I suppose that's ok too.
The fact that these are programs written in C is not relevant. The fact that this is Linux, macOS, OpenBSD or AIX is not relevant.
edited 1 hour ago
answered 11 hours ago
Kusalananda♦Kusalananda
143k18267443
143k18267443
Actually, we can think of having A and B running in parallel as an optimization. The command is equivalent to./A > tmp_file && ./B < tmp_file
, which first save the output of A totmp_file
and then give it as an input to B. This information is taken from: okmij.org/ftp/Computation/monadic-shell.html (I change the command slightly)
– Alex Vong
7 hours ago
What I think OP have in mind is the unoptimized implementation of pipe (which is almost never used in practice).
– Alex Vong
7 hours ago
1
Writing to a temporary file was used in DOS, as that didn't support multiple processes.
– CSM
5 hours ago
1
@AlexVong Note though that your example with a temporary file is not exactly equivalent. A program may choose to seek though the contents of a file, but data coming off a pipe is not seekable. A better examlp would be to usemkfifo
to create a named pipe, then start B in the background reading from the pipe, and then A writing to it. This is nit-picking though, as the effect would be the same.
– Kusalananda♦
5 hours ago
The&&
is another problem with Alex’s suggested implementation —B
will always run, unconditionally, so it’s more like./A > tmp_file; ./B < tmp_file
. And, of course, there’s also therm tmp_file
at the end.
– G-Man
1 hour ago
|
show 3 more comments
Actually, we can think of having A and B running in parallel as an optimization. The command is equivalent to./A > tmp_file && ./B < tmp_file
, which first save the output of A totmp_file
and then give it as an input to B. This information is taken from: okmij.org/ftp/Computation/monadic-shell.html (I change the command slightly)
– Alex Vong
7 hours ago
What I think OP have in mind is the unoptimized implementation of pipe (which is almost never used in practice).
– Alex Vong
7 hours ago
1
Writing to a temporary file was used in DOS, as that didn't support multiple processes.
– CSM
5 hours ago
1
@AlexVong Note though that your example with a temporary file is not exactly equivalent. A program may choose to seek though the contents of a file, but data coming off a pipe is not seekable. A better examlp would be to usemkfifo
to create a named pipe, then start B in the background reading from the pipe, and then A writing to it. This is nit-picking though, as the effect would be the same.
– Kusalananda♦
5 hours ago
The&&
is another problem with Alex’s suggested implementation —B
will always run, unconditionally, so it’s more like./A > tmp_file; ./B < tmp_file
. And, of course, there’s also therm tmp_file
at the end.
– G-Man
1 hour ago
Actually, we can think of having A and B running in parallel as an optimization. The command is equivalent to
./A > tmp_file && ./B < tmp_file
, which first save the output of A to tmp_file
and then give it as an input to B. This information is taken from: okmij.org/ftp/Computation/monadic-shell.html (I change the command slightly)– Alex Vong
7 hours ago
Actually, we can think of having A and B running in parallel as an optimization. The command is equivalent to
./A > tmp_file && ./B < tmp_file
, which first save the output of A to tmp_file
and then give it as an input to B. This information is taken from: okmij.org/ftp/Computation/monadic-shell.html (I change the command slightly)– Alex Vong
7 hours ago
What I think OP have in mind is the unoptimized implementation of pipe (which is almost never used in practice).
– Alex Vong
7 hours ago
What I think OP have in mind is the unoptimized implementation of pipe (which is almost never used in practice).
– Alex Vong
7 hours ago
1
1
Writing to a temporary file was used in DOS, as that didn't support multiple processes.
– CSM
5 hours ago
Writing to a temporary file was used in DOS, as that didn't support multiple processes.
– CSM
5 hours ago
1
1
@AlexVong Note though that your example with a temporary file is not exactly equivalent. A program may choose to seek though the contents of a file, but data coming off a pipe is not seekable. A better examlp would be to use
mkfifo
to create a named pipe, then start B in the background reading from the pipe, and then A writing to it. This is nit-picking though, as the effect would be the same.– Kusalananda♦
5 hours ago
@AlexVong Note though that your example with a temporary file is not exactly equivalent. A program may choose to seek though the contents of a file, but data coming off a pipe is not seekable. A better examlp would be to use
mkfifo
to create a named pipe, then start B in the background reading from the pipe, and then A writing to it. This is nit-picking though, as the effect would be the same.– Kusalananda♦
5 hours ago
The
&&
is another problem with Alex’s suggested implementation — B
will always run, unconditionally, so it’s more like ./A > tmp_file; ./B < tmp_file
. And, of course, there’s also the rm tmp_file
at the end.– G-Man
1 hour ago
The
&&
is another problem with Alex’s suggested implementation — B
will always run, unconditionally, so it’s more like ./A > tmp_file; ./B < tmp_file
. And, of course, there’s also the rm tmp_file
at the end.– G-Man
1 hour ago
|
show 3 more comments
The term usually used in documentation is "pipeline" , which consists of one or more commands, see POSIX definition So technically speaking, that's two commands you have there, two subprocesses for the shell (either fork()+exec()
'ed external commands or subshells )
As for producer-consumer part, the pipeline can be described by that pattern, since:
- Producer and Consumer share fixed-size buffer, and at least on Linux and MacOS X, there's fixed size for pipeline buffer
- Producer and Consumer are loosely-coupled, commands in pipeline don't know of each other's existence ( unless they are actively checking
/proc/<pid>/fd
directory ). - Producers write to
stdout
and consumers readstdin
as if they were a single command being executed, aka they can exist without each other.
The difference I see here is that unlike Producer-Consumer in other languges, shell commands use buffering and they write stdout once buffer is filled, but there's no mention that Producer-Consumer has to follow that rule - only wait when queue is filled or discard data (which is something else that pipeline doesn't do).
add a comment |
The term usually used in documentation is "pipeline" , which consists of one or more commands, see POSIX definition So technically speaking, that's two commands you have there, two subprocesses for the shell (either fork()+exec()
'ed external commands or subshells )
As for producer-consumer part, the pipeline can be described by that pattern, since:
- Producer and Consumer share fixed-size buffer, and at least on Linux and MacOS X, there's fixed size for pipeline buffer
- Producer and Consumer are loosely-coupled, commands in pipeline don't know of each other's existence ( unless they are actively checking
/proc/<pid>/fd
directory ). - Producers write to
stdout
and consumers readstdin
as if they were a single command being executed, aka they can exist without each other.
The difference I see here is that unlike Producer-Consumer in other languges, shell commands use buffering and they write stdout once buffer is filled, but there's no mention that Producer-Consumer has to follow that rule - only wait when queue is filled or discard data (which is something else that pipeline doesn't do).
add a comment |
The term usually used in documentation is "pipeline" , which consists of one or more commands, see POSIX definition So technically speaking, that's two commands you have there, two subprocesses for the shell (either fork()+exec()
'ed external commands or subshells )
As for producer-consumer part, the pipeline can be described by that pattern, since:
- Producer and Consumer share fixed-size buffer, and at least on Linux and MacOS X, there's fixed size for pipeline buffer
- Producer and Consumer are loosely-coupled, commands in pipeline don't know of each other's existence ( unless they are actively checking
/proc/<pid>/fd
directory ). - Producers write to
stdout
and consumers readstdin
as if they were a single command being executed, aka they can exist without each other.
The difference I see here is that unlike Producer-Consumer in other languges, shell commands use buffering and they write stdout once buffer is filled, but there's no mention that Producer-Consumer has to follow that rule - only wait when queue is filled or discard data (which is something else that pipeline doesn't do).
The term usually used in documentation is "pipeline" , which consists of one or more commands, see POSIX definition So technically speaking, that's two commands you have there, two subprocesses for the shell (either fork()+exec()
'ed external commands or subshells )
As for producer-consumer part, the pipeline can be described by that pattern, since:
- Producer and Consumer share fixed-size buffer, and at least on Linux and MacOS X, there's fixed size for pipeline buffer
- Producer and Consumer are loosely-coupled, commands in pipeline don't know of each other's existence ( unless they are actively checking
/proc/<pid>/fd
directory ). - Producers write to
stdout
and consumers readstdin
as if they were a single command being executed, aka they can exist without each other.
The difference I see here is that unlike Producer-Consumer in other languges, shell commands use buffering and they write stdout once buffer is filled, but there's no mention that Producer-Consumer has to follow that rule - only wait when queue is filled or discard data (which is something else that pipeline doesn't do).
answered 59 mins ago
Sergiy KolodyazhnyySergiy Kolodyazhnyy
10.7k42765
10.7k42765
add a comment |
add a comment |
nihulus is a new contributor. Be nice, and check out our Code of Conduct.
nihulus is a new contributor. Be nice, and check out our Code of Conduct.
nihulus is a new contributor. Be nice, and check out our Code of Conduct.
nihulus is a new contributor. Be nice, and check out our Code of Conduct.
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Related: In what order do piped commands run?
– G-Man
2 hours ago