Mixing
Test command and redirection bash
Why does test > file exit with 1 while test arg1 > arg2 exit with 0 ?
I thought that in test arg1 > arg2, test arg1 is applied first then the output(it has none) written to file arg2 and since the last command(redirection) was successful the exit is 0. But so should the first case. I am a bit confused.
linux bash shell command-line scripting
asked Nov 19 '18 at 23:56
NamelessNameless
1236
add a comment |
Why does test > file exit with 1 while test arg1 > arg2 exit with 0 ?
I thought that in test arg1 > arg2, test arg1 is applied first then the output(it has none) written to file arg2 and since the last command(redirection) was successful the exit is 0. But so should the first case. I am a bit confused.
linux bash shell command-line scripting
asked Nov 19 '18 at 23:56
NamelessNameless
1236
2
test > filereturns non-zero for the same reasontestreturns non-zero. The redirection is irrelevant.
– William Pursell
Nov 20 '18 at 0:50
1
"The last command (redirection)" indicates a flaw in your thinking. The redirection is not a command.
– William Pursell
Nov 20 '18 at 0:50
add a comment |
Why does test > file exit with 1 while test arg1 > arg2 exit with 0 ?
I thought that in test arg1 > arg2, test arg1 is applied first then the output(it has none) written to file arg2 and since the last command(redirection) was successful the exit is 0. But so should the first case. I am a bit confused.
linux bash shell command-line scripting
asked Nov 19 '18 at 23:56
NamelessNameless
1236
Why does test > file exit with 1 while test arg1 > arg2 exit with 0 ?
I thought that in test arg1 > arg2, test arg1 is applied first then the output(it has none) written to file arg2 and since the last command(redirection) was successful the exit is 0. But so should the first case. I am a bit confused.
linux bash shell command-line scripting
linux bash shell command-line scripting
asked Nov 19 '18 at 23:56
NamelessNameless
1236
asked Nov 19 '18 at 23:56
NamelessNameless
1236
asked Nov 19 '18 at 23:56
NamelessNameless
1236
asked Nov 19 '18 at 23:56
NamelessNameless
1236
asked Nov 19 '18 at 23:56
NamelessNameless
1236
1236
2
test > filereturns non-zero for the same reasontestreturns non-zero. The redirection is irrelevant.
– William Pursell
Nov 20 '18 at 0:50
1
"The last command (redirection)" indicates a flaw in your thinking. The redirection is not a command.
– William Pursell
Nov 20 '18 at 0:50
add a comment |
2
test > filereturns non-zero for the same reasontestreturns non-zero. The redirection is irrelevant.
– William Pursell
Nov 20 '18 at 0:50
1
"The last command (redirection)" indicates a flaw in your thinking. The redirection is not a command.
– William Pursell
Nov 20 '18 at 0:50
2
2
test > file returns non-zero for the same reason test returns non-zero. The redirection is irrelevant.– William Pursell
Nov 20 '18 at 0:50
test > file returns non-zero for the same reason test returns non-zero. The redirection is irrelevant.– William Pursell
Nov 20 '18 at 0:50
1
1
"The last command (redirection)" indicates a flaw in your thinking. The redirection is not a command.
– William Pursell
Nov 20 '18 at 0:50
"The last command (redirection)" indicates a flaw in your thinking. The redirection is not a command.
– William Pursell
Nov 20 '18 at 0:50
add a comment |
1 Answer
1
active
oldest
votes
Redirection is not a command.
| creates a new command, because not only the is output of the previous command "piped" somewhere else, but also another command is executed on that output. For example,
ls | grep 'abc' will exit 1 if nothing matches that pattern, regardless of whether ls completed successfully or not). The exit status you see with $? is the status of the last pipe.
Redirection is not the same, because it doesn't give the output to the next command in line, just to a file. Note that redirection can make you return 1 when your command would otherwise have worked because you don't have permissions to write to the file. In that case, your command won't be executed.
TL;DR: A command requires a valid redirection to run. If it can run, the exit status will be the exit status of the command. Exit status of redirection is the OR of the command and the redirection (0 OR 1, 1 OR 1, 1 OR 0 will all return 1; only a successful redirection and successful command returns 0)
answered Nov 20 '18 at 0:20
jeremysprofilejeremysprofile
1,6331517
2
Nit: if the redirection fails, the command is never executed so it is not correct to say that "redirection can make you return 1 when your command worked". The command never executes, so it cannot be said to have worked.
– William Pursell
Nov 20 '18 at 0:53
@WilliamPursell, thanks, edited
– jeremysprofile
Nov 20 '18 at 2:52
add a comment |
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1 Answer
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1 Answer
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oldest
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oldest
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oldest
votes
Redirection is not a command.
| creates a new command, because not only the is output of the previous command "piped" somewhere else, but also another command is executed on that output. For example,
ls | grep 'abc' will exit 1 if nothing matches that pattern, regardless of whether ls completed successfully or not). The exit status you see with $? is the status of the last pipe.
Redirection is not the same, because it doesn't give the output to the next command in line, just to a file. Note that redirection can make you return 1 when your command would otherwise have worked because you don't have permissions to write to the file. In that case, your command won't be executed.
TL;DR: A command requires a valid redirection to run. If it can run, the exit status will be the exit status of the command. Exit status of redirection is the OR of the command and the redirection (0 OR 1, 1 OR 1, 1 OR 0 will all return 1; only a successful redirection and successful command returns 0)
answered Nov 20 '18 at 0:20
jeremysprofilejeremysprofile
1,6331517
2
Nit: if the redirection fails, the command is never executed so it is not correct to say that "redirection can make you return 1 when your command worked". The command never executes, so it cannot be said to have worked.
– William Pursell
Nov 20 '18 at 0:53
@WilliamPursell, thanks, edited
– jeremysprofile
Nov 20 '18 at 2:52
add a comment |
Redirection is not a command.
| creates a new command, because not only the is output of the previous command "piped" somewhere else, but also another command is executed on that output. For example,
ls | grep 'abc' will exit 1 if nothing matches that pattern, regardless of whether ls completed successfully or not). The exit status you see with $? is the status of the last pipe.
Redirection is not the same, because it doesn't give the output to the next command in line, just to a file. Note that redirection can make you return 1 when your command would otherwise have worked because you don't have permissions to write to the file. In that case, your command won't be executed.
TL;DR: A command requires a valid redirection to run. If it can run, the exit status will be the exit status of the command. Exit status of redirection is the OR of the command and the redirection (0 OR 1, 1 OR 1, 1 OR 0 will all return 1; only a successful redirection and successful command returns 0)
answered Nov 20 '18 at 0:20
jeremysprofilejeremysprofile
1,6331517
2
Nit: if the redirection fails, the command is never executed so it is not correct to say that "redirection can make you return 1 when your command worked". The command never executes, so it cannot be said to have worked.
– William Pursell
Nov 20 '18 at 0:53
@WilliamPursell, thanks, edited
– jeremysprofile
Nov 20 '18 at 2:52
add a comment |
Redirection is not a command.
| creates a new command, because not only the is output of the previous command "piped" somewhere else, but also another command is executed on that output. For example,
ls | grep 'abc' will exit 1 if nothing matches that pattern, regardless of whether ls completed successfully or not). The exit status you see with $? is the status of the last pipe.
Redirection is not the same, because it doesn't give the output to the next command in line, just to a file. Note that redirection can make you return 1 when your command would otherwise have worked because you don't have permissions to write to the file. In that case, your command won't be executed.
TL;DR: A command requires a valid redirection to run. If it can run, the exit status will be the exit status of the command. Exit status of redirection is the OR of the command and the redirection (0 OR 1, 1 OR 1, 1 OR 0 will all return 1; only a successful redirection and successful command returns 0)
answered Nov 20 '18 at 0:20
jeremysprofilejeremysprofile
1,6331517
Redirection is not a command.
| creates a new command, because not only the is output of the previous command "piped" somewhere else, but also another command is executed on that output. For example,
ls | grep 'abc' will exit 1 if nothing matches that pattern, regardless of whether ls completed successfully or not). The exit status you see with $? is the status of the last pipe.
Redirection is not the same, because it doesn't give the output to the next command in line, just to a file. Note that redirection can make you return 1 when your command would otherwise have worked because you don't have permissions to write to the file. In that case, your command won't be executed.
TL;DR: A command requires a valid redirection to run. If it can run, the exit status will be the exit status of the command. Exit status of redirection is the OR of the command and the redirection (0 OR 1, 1 OR 1, 1 OR 0 will all return 1; only a successful redirection and successful command returns 0)
answered Nov 20 '18 at 0:20
jeremysprofilejeremysprofile
1,6331517
edited Nov 20 '18 at 2:54
answered Nov 20 '18 at 0:20
jeremysprofilejeremysprofile
1,6331517
answered Nov 20 '18 at 0:20
jeremysprofilejeremysprofile
1,6331517
answered Nov 20 '18 at 0:20
jeremysprofilejeremysprofile
1,6331517
1,6331517
2
Nit: if the redirection fails, the command is never executed so it is not correct to say that "redirection can make you return 1 when your command worked". The command never executes, so it cannot be said to have worked.
– William Pursell
Nov 20 '18 at 0:53
@WilliamPursell, thanks, edited
– jeremysprofile
Nov 20 '18 at 2:52
add a comment |
2
Nit: if the redirection fails, the command is never executed so it is not correct to say that "redirection can make you return 1 when your command worked". The command never executes, so it cannot be said to have worked.
– William Pursell
Nov 20 '18 at 0:53
@WilliamPursell, thanks, edited
– jeremysprofile
Nov 20 '18 at 2:52
2
2
Nit: if the redirection fails, the command is never executed so it is not correct to say that "redirection can make you return 1 when your command worked". The command never executes, so it cannot be said to have worked.
– William Pursell
Nov 20 '18 at 0:53
Nit: if the redirection fails, the command is never executed so it is not correct to say that "redirection can make you return 1 when your command worked". The command never executes, so it cannot be said to have worked.
– William Pursell
Nov 20 '18 at 0:53
@WilliamPursell, thanks, edited
– jeremysprofile
Nov 20 '18 at 2:52
@WilliamPursell, thanks, edited
– jeremysprofile
Nov 20 '18 at 2:52
add a comment |
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2
test > filereturns non-zero for the same reasontestreturns non-zero. The redirection is irrelevant.– William Pursell
Nov 20 '18 at 0:50
1
"The last command (redirection)" indicates a flaw in your thinking. The redirection is not a command.
– William Pursell
Nov 20 '18 at 0:50