Pipes connected to stdio print output after new terminal prompt
I'm having trouble understanding how to do basic piping in C. I looked at a couple other questions on this topic, and either they were for subtly different issues, or I'm so far off the mark on this subject I couldn't understand why the answers are good for my problem.
This program below is just a simple test I made, where I'm trying to get behaviour equivalent to typing "ls | grep a" into my shell. (I have a homework assignment where I have to build a shell that can handle piping, but this is my first step towards understanding pipes to even attempt the homework). I get the correct output, but the terminal prompt ends up appearing before the output, making it look like it did not properly terminate. Since this is connected to a shell homework, I'm worried that will impact the grade (and it just feels wrong to let it look like that anyway). Any advice?
#include <unistd.h>
#include <sys/wait.h>
#include <stdio.h>
int main()
{
int fdpipe[2];
pipe(fdpipe);
int f1 = fork();
if(f1 == 0)
{
close(fdpipe[1]);
dup2(fdpipe[0],0);
close(fdpipe[0]);
execlp("/bin/grep","grep","a",NULL);
}
else
{
close(fdpipe[0]);
dup2(fdpipe[1],1);
close(fdpipe[1]);
execlp("/bin/ls","ls",NULL);
wait(NULL);
}
return 0;
}
Here's an example of my terminal output.
1067: ls
a.out test.c test.cc
NathanE: ~/Desktop/playground
1068: ./a.out
NathanE: ~/Desktop/playground
1069: a.out
(The beginning of this line is where my cursor is)
What I'm expecting would be:
1067: ls
a.out test.c test.cc
NathanE: ~/Desktop/playground
1068: ./a.out
a.out
NathanE: ~/Desktop/playground
1069: (my cursor would go here)
c pipe stdio
asked Nov 19 '18 at 6:55
Nathan EganNathan Egan
284
add a comment |
I'm having trouble understanding how to do basic piping in C. I looked at a couple other questions on this topic, and either they were for subtly different issues, or I'm so far off the mark on this subject I couldn't understand why the answers are good for my problem.
This program below is just a simple test I made, where I'm trying to get behaviour equivalent to typing "ls | grep a" into my shell. (I have a homework assignment where I have to build a shell that can handle piping, but this is my first step towards understanding pipes to even attempt the homework). I get the correct output, but the terminal prompt ends up appearing before the output, making it look like it did not properly terminate. Since this is connected to a shell homework, I'm worried that will impact the grade (and it just feels wrong to let it look like that anyway). Any advice?
#include <unistd.h>
#include <sys/wait.h>
#include <stdio.h>
int main()
{
int fdpipe[2];
pipe(fdpipe);
int f1 = fork();
if(f1 == 0)
{
close(fdpipe[1]);
dup2(fdpipe[0],0);
close(fdpipe[0]);
execlp("/bin/grep","grep","a",NULL);
}
else
{
close(fdpipe[0]);
dup2(fdpipe[1],1);
close(fdpipe[1]);
execlp("/bin/ls","ls",NULL);
wait(NULL);
}
return 0;
}
Here's an example of my terminal output.
1067: ls
a.out test.c test.cc
NathanE: ~/Desktop/playground
1068: ./a.out
NathanE: ~/Desktop/playground
1069: a.out
(The beginning of this line is where my cursor is)
What I'm expecting would be:
1067: ls
a.out test.c test.cc
NathanE: ~/Desktop/playground
1068: ./a.out
a.out
NathanE: ~/Desktop/playground
1069: (my cursor would go here)
c pipe stdio
asked Nov 19 '18 at 6:55
Nathan EganNathan Egan
284
@kabanus Oh...I guess it terminates, it just doesn't look like it does. I'm getting my terminal prompt before my output, so my cursor gets left on an empty line. My bad, let me edit the question to reflect that.
– Nathan Egan
Nov 19 '18 at 7:09
@kabanus Okay, I added that. I'm using the basic terminal with bash that comes with Ubuntu 18.04. The only modifications to it are that I customized my prompt in .bashrc.
– Nathan Egan
Nov 19 '18 at 7:35
add a comment |
I'm having trouble understanding how to do basic piping in C. I looked at a couple other questions on this topic, and either they were for subtly different issues, or I'm so far off the mark on this subject I couldn't understand why the answers are good for my problem.
This program below is just a simple test I made, where I'm trying to get behaviour equivalent to typing "ls | grep a" into my shell. (I have a homework assignment where I have to build a shell that can handle piping, but this is my first step towards understanding pipes to even attempt the homework). I get the correct output, but the terminal prompt ends up appearing before the output, making it look like it did not properly terminate. Since this is connected to a shell homework, I'm worried that will impact the grade (and it just feels wrong to let it look like that anyway). Any advice?
#include <unistd.h>
#include <sys/wait.h>
#include <stdio.h>
int main()
{
int fdpipe[2];
pipe(fdpipe);
int f1 = fork();
if(f1 == 0)
{
close(fdpipe[1]);
dup2(fdpipe[0],0);
close(fdpipe[0]);
execlp("/bin/grep","grep","a",NULL);
}
else
{
close(fdpipe[0]);
dup2(fdpipe[1],1);
close(fdpipe[1]);
execlp("/bin/ls","ls",NULL);
wait(NULL);
}
return 0;
}
Here's an example of my terminal output.
1067: ls
a.out test.c test.cc
NathanE: ~/Desktop/playground
1068: ./a.out
NathanE: ~/Desktop/playground
1069: a.out
(The beginning of this line is where my cursor is)
What I'm expecting would be:
1067: ls
a.out test.c test.cc
NathanE: ~/Desktop/playground
1068: ./a.out
a.out
NathanE: ~/Desktop/playground
1069: (my cursor would go here)
c pipe stdio
asked Nov 19 '18 at 6:55
Nathan EganNathan Egan
284
I'm having trouble understanding how to do basic piping in C. I looked at a couple other questions on this topic, and either they were for subtly different issues, or I'm so far off the mark on this subject I couldn't understand why the answers are good for my problem.
This program below is just a simple test I made, where I'm trying to get behaviour equivalent to typing "ls | grep a" into my shell. (I have a homework assignment where I have to build a shell that can handle piping, but this is my first step towards understanding pipes to even attempt the homework). I get the correct output, but the terminal prompt ends up appearing before the output, making it look like it did not properly terminate. Since this is connected to a shell homework, I'm worried that will impact the grade (and it just feels wrong to let it look like that anyway). Any advice?
#include <unistd.h>
#include <sys/wait.h>
#include <stdio.h>
int main()
{
int fdpipe[2];
pipe(fdpipe);
int f1 = fork();
if(f1 == 0)
{
close(fdpipe[1]);
dup2(fdpipe[0],0);
close(fdpipe[0]);
execlp("/bin/grep","grep","a",NULL);
}
else
{
close(fdpipe[0]);
dup2(fdpipe[1],1);
close(fdpipe[1]);
execlp("/bin/ls","ls",NULL);
wait(NULL);
}
return 0;
}
Here's an example of my terminal output.
1067: ls
a.out test.c test.cc
NathanE: ~/Desktop/playground
1068: ./a.out
NathanE: ~/Desktop/playground
1069: a.out
(The beginning of this line is where my cursor is)
What I'm expecting would be:
1067: ls
a.out test.c test.cc
NathanE: ~/Desktop/playground
1068: ./a.out
a.out
NathanE: ~/Desktop/playground
1069: (my cursor would go here)
c pipe stdio
c pipe stdio
asked Nov 19 '18 at 6:55
Nathan EganNathan Egan
284
asked Nov 19 '18 at 6:55
Nathan EganNathan Egan
284
edited Nov 19 '18 at 7:34
Nathan Egan
asked Nov 19 '18 at 6:55
Nathan EganNathan Egan
284
asked Nov 19 '18 at 6:55
Nathan EganNathan Egan
284
asked Nov 19 '18 at 6:55
Nathan EganNathan Egan
284
284
@kabanus Oh...I guess it terminates, it just doesn't look like it does. I'm getting my terminal prompt before my output, so my cursor gets left on an empty line. My bad, let me edit the question to reflect that.
– Nathan Egan
Nov 19 '18 at 7:09
@kabanus Okay, I added that. I'm using the basic terminal with bash that comes with Ubuntu 18.04. The only modifications to it are that I customized my prompt in .bashrc.
– Nathan Egan
Nov 19 '18 at 7:35
add a comment |
@kabanus Oh...I guess it terminates, it just doesn't look like it does. I'm getting my terminal prompt before my output, so my cursor gets left on an empty line. My bad, let me edit the question to reflect that.
– Nathan Egan
Nov 19 '18 at 7:09
@kabanus Okay, I added that. I'm using the basic terminal with bash that comes with Ubuntu 18.04. The only modifications to it are that I customized my prompt in .bashrc.
– Nathan Egan
Nov 19 '18 at 7:35
@kabanus Oh...I guess it terminates, it just doesn't look like it does. I'm getting my terminal prompt before my output, so my cursor gets left on an empty line. My bad, let me edit the question to reflect that.
– Nathan Egan
Nov 19 '18 at 7:09
@kabanus Oh...I guess it terminates, it just doesn't look like it does. I'm getting my terminal prompt before my output, so my cursor gets left on an empty line. My bad, let me edit the question to reflect that.
– Nathan Egan
Nov 19 '18 at 7:09
@kabanus Okay, I added that. I'm using the basic terminal with bash that comes with Ubuntu 18.04. The only modifications to it are that I customized my prompt in .bashrc.
– Nathan Egan
Nov 19 '18 at 7:35
@kabanus Okay, I added that. I'm using the basic terminal with bash that comes with Ubuntu 18.04. The only modifications to it are that I customized my prompt in .bashrc.
– Nathan Egan
Nov 19 '18 at 7:35
add a comment |
1 Answer
1
active
oldest
votes
The child process runs grep, while the parent replaces itself with ls. The wait(NULL) does nothing, because successful exec*() never return.
Because the control returns to the shell immediately after ls completes, the shell can display the next prompt before grep completes.
There are two approaches you can use to avoid this:
fork()both child processes, andwait()for themReplace the process itself with the last process in the pipe chain
Either will ensure that control is returned to the shell only after the last process in the pipe chain completes.
answered Nov 19 '18 at 8:01
Nominal AnimalNominal Animal
30k33361
Since OP seems to have a newline inPS1I think they actually have an increased chance of seeing this effect.
– kabanus
Nov 19 '18 at 8:14
That works perfectly, thank you! I understand what's going on a lot better now, thanks to you and @kabanus. Thank you both!
– Nathan Egan
Nov 19 '18 at 8:46
add a comment |
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votes
The child process runs grep, while the parent replaces itself with ls. The wait(NULL) does nothing, because successful exec*() never return.
Because the control returns to the shell immediately after ls completes, the shell can display the next prompt before grep completes.
There are two approaches you can use to avoid this:
fork()both child processes, andwait()for themReplace the process itself with the last process in the pipe chain
Either will ensure that control is returned to the shell only after the last process in the pipe chain completes.
answered Nov 19 '18 at 8:01
Nominal AnimalNominal Animal
30k33361
Since OP seems to have a newline inPS1I think they actually have an increased chance of seeing this effect.
– kabanus
Nov 19 '18 at 8:14
That works perfectly, thank you! I understand what's going on a lot better now, thanks to you and @kabanus. Thank you both!
– Nathan Egan
Nov 19 '18 at 8:46
add a comment |
The child process runs grep, while the parent replaces itself with ls. The wait(NULL) does nothing, because successful exec*() never return.
Because the control returns to the shell immediately after ls completes, the shell can display the next prompt before grep completes.
There are two approaches you can use to avoid this:
fork()both child processes, andwait()for themReplace the process itself with the last process in the pipe chain
Either will ensure that control is returned to the shell only after the last process in the pipe chain completes.
answered Nov 19 '18 at 8:01
Nominal AnimalNominal Animal
30k33361
Since OP seems to have a newline inPS1I think they actually have an increased chance of seeing this effect.
– kabanus
Nov 19 '18 at 8:14
That works perfectly, thank you! I understand what's going on a lot better now, thanks to you and @kabanus. Thank you both!
– Nathan Egan
Nov 19 '18 at 8:46
add a comment |
The child process runs grep, while the parent replaces itself with ls. The wait(NULL) does nothing, because successful exec*() never return.
Because the control returns to the shell immediately after ls completes, the shell can display the next prompt before grep completes.
There are two approaches you can use to avoid this:
fork()both child processes, andwait()for themReplace the process itself with the last process in the pipe chain
Either will ensure that control is returned to the shell only after the last process in the pipe chain completes.
answered Nov 19 '18 at 8:01
Nominal AnimalNominal Animal
30k33361
The child process runs grep, while the parent replaces itself with ls. The wait(NULL) does nothing, because successful exec*() never return.
Because the control returns to the shell immediately after ls completes, the shell can display the next prompt before grep completes.
There are two approaches you can use to avoid this:
fork()both child processes, andwait()for themReplace the process itself with the last process in the pipe chain
Either will ensure that control is returned to the shell only after the last process in the pipe chain completes.
answered Nov 19 '18 at 8:01
Nominal AnimalNominal Animal
30k33361
answered Nov 19 '18 at 8:01
Nominal AnimalNominal Animal
30k33361
answered Nov 19 '18 at 8:01
Nominal AnimalNominal Animal
30k33361
answered Nov 19 '18 at 8:01
Nominal AnimalNominal Animal
30k33361
30k33361
Since OP seems to have a newline inPS1I think they actually have an increased chance of seeing this effect.
– kabanus
Nov 19 '18 at 8:14
That works perfectly, thank you! I understand what's going on a lot better now, thanks to you and @kabanus. Thank you both!
– Nathan Egan
Nov 19 '18 at 8:46
add a comment |
Since OP seems to have a newline inPS1I think they actually have an increased chance of seeing this effect.
– kabanus
Nov 19 '18 at 8:14
That works perfectly, thank you! I understand what's going on a lot better now, thanks to you and @kabanus. Thank you both!
– Nathan Egan
Nov 19 '18 at 8:46
Since OP seems to have a newline in
PS1 I think they actually have an increased chance of seeing this effect.– kabanus
Nov 19 '18 at 8:14
Since OP seems to have a newline in
PS1 I think they actually have an increased chance of seeing this effect.– kabanus
Nov 19 '18 at 8:14
That works perfectly, thank you! I understand what's going on a lot better now, thanks to you and @kabanus. Thank you both!
– Nathan Egan
Nov 19 '18 at 8:46
That works perfectly, thank you! I understand what's going on a lot better now, thanks to you and @kabanus. Thank you both!
– Nathan Egan
Nov 19 '18 at 8:46
add a comment |
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@kabanus Oh...I guess it terminates, it just doesn't look like it does. I'm getting my terminal prompt before my output, so my cursor gets left on an empty line. My bad, let me edit the question to reflect that.
– Nathan Egan
Nov 19 '18 at 7:09
@kabanus Okay, I added that. I'm using the basic terminal with bash that comes with Ubuntu 18.04. The only modifications to it are that I customized my prompt in .bashrc.
– Nathan Egan
Nov 19 '18 at 7:35