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Understanding the limits of bash token substitution
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bash has token substitution, either via the $() syntax or via the `` syntax, eg:
$ $(echo "echo hi")
hi
AFAIK, that works like so:
- first evaluate the inner command:
echo "echo hi", which outputsecho hi
- then execute the resulting string as a command
echo hi, which produceshi
That being the case, I would have expected the following to write hi to file /tmp/hi
$ $(echo "echo hi > /tmp/hi")
but instead, it writes nothing to any file, and outputs:
hi > /tmp/hi
What's going on here? I originally thought it might be because there are spaces in the string, but the following disconfirms that theory:
$ $(echo "echo hi bob")
hi bob
bash
asked 17 mins ago
mulllhausenmulllhausen
95272130
add a comment |
bash has token substitution, either via the $() syntax or via the `` syntax, eg:
$ $(echo "echo hi")
hi
AFAIK, that works like so:
- first evaluate the inner command:
echo "echo hi", which outputsecho hi
- then execute the resulting string as a command
echo hi, which produceshi
That being the case, I would have expected the following to write hi to file /tmp/hi
$ $(echo "echo hi > /tmp/hi")
but instead, it writes nothing to any file, and outputs:
hi > /tmp/hi
What's going on here? I originally thought it might be because there are spaces in the string, but the following disconfirms that theory:
$ $(echo "echo hi bob")
hi bob
bash
asked 17 mins ago
mulllhausenmulllhausen
95272130
add a comment |
bash has token substitution, either via the $() syntax or via the `` syntax, eg:
$ $(echo "echo hi")
hi
AFAIK, that works like so:
- first evaluate the inner command:
echo "echo hi", which outputsecho hi
- then execute the resulting string as a command
echo hi, which produceshi
That being the case, I would have expected the following to write hi to file /tmp/hi
$ $(echo "echo hi > /tmp/hi")
but instead, it writes nothing to any file, and outputs:
hi > /tmp/hi
What's going on here? I originally thought it might be because there are spaces in the string, but the following disconfirms that theory:
$ $(echo "echo hi bob")
hi bob
bash
asked 17 mins ago
mulllhausenmulllhausen
95272130
bash has token substitution, either via the $() syntax or via the `` syntax, eg:
$ $(echo "echo hi")
hi
AFAIK, that works like so:
- first evaluate the inner command:
echo "echo hi", which outputsecho hi
- then execute the resulting string as a command
echo hi, which produceshi
That being the case, I would have expected the following to write hi to file /tmp/hi
$ $(echo "echo hi > /tmp/hi")
but instead, it writes nothing to any file, and outputs:
hi > /tmp/hi
What's going on here? I originally thought it might be because there are spaces in the string, but the following disconfirms that theory:
$ $(echo "echo hi bob")
hi bob
bash
bash
asked 17 mins ago
mulllhausenmulllhausen
95272130
asked 17 mins ago
mulllhausenmulllhausen
95272130
asked 17 mins ago
mulllhausenmulllhausen
95272130
asked 17 mins ago
mulllhausenmulllhausen
95272130
asked 17 mins ago
mulllhausenmulllhausen
95272130
95272130
add a comment |
add a comment |
1 Answer
1
active
oldest
votes
Your understanding of the order of the command expansions is right, but you can't expect the re-directions > to be expanded this way. As you can see the expansion happening, the > is treated literally
$ $(echo "echo hi > /tmp/hi")
++ echo 'echo hi > /tmp/hi'
+ echo hi '>' /tmp/hi
hi > /tmp/hi
Unless you introduce an another level of command expansion using eval (dangerous) or other equivalent ways, the re-directions won't work.
answered 3 mins ago
InianInian
5,6701632
add a comment |
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1 Answer
1
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1 Answer
1
active
oldest
votes
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oldest
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votes
Your understanding of the order of the command expansions is right, but you can't expect the re-directions > to be expanded this way. As you can see the expansion happening, the > is treated literally
$ $(echo "echo hi > /tmp/hi")
++ echo 'echo hi > /tmp/hi'
+ echo hi '>' /tmp/hi
hi > /tmp/hi
Unless you introduce an another level of command expansion using eval (dangerous) or other equivalent ways, the re-directions won't work.
answered 3 mins ago
InianInian
5,6701632
add a comment |
Your understanding of the order of the command expansions is right, but you can't expect the re-directions > to be expanded this way. As you can see the expansion happening, the > is treated literally
$ $(echo "echo hi > /tmp/hi")
++ echo 'echo hi > /tmp/hi'
+ echo hi '>' /tmp/hi
hi > /tmp/hi
Unless you introduce an another level of command expansion using eval (dangerous) or other equivalent ways, the re-directions won't work.
answered 3 mins ago
InianInian
5,6701632
add a comment |
Your understanding of the order of the command expansions is right, but you can't expect the re-directions > to be expanded this way. As you can see the expansion happening, the > is treated literally
$ $(echo "echo hi > /tmp/hi")
++ echo 'echo hi > /tmp/hi'
+ echo hi '>' /tmp/hi
hi > /tmp/hi
Unless you introduce an another level of command expansion using eval (dangerous) or other equivalent ways, the re-directions won't work.
answered 3 mins ago
InianInian
5,6701632
Your understanding of the order of the command expansions is right, but you can't expect the re-directions > to be expanded this way. As you can see the expansion happening, the > is treated literally
$ $(echo "echo hi > /tmp/hi")
++ echo 'echo hi > /tmp/hi'
+ echo hi '>' /tmp/hi
hi > /tmp/hi
Unless you introduce an another level of command expansion using eval (dangerous) or other equivalent ways, the re-directions won't work.
answered 3 mins ago
InianInian
5,6701632
answered 3 mins ago
InianInian
5,6701632
answered 3 mins ago
InianInian
5,6701632
answered 3 mins ago
InianInian
5,6701632
5,6701632
add a comment |
add a comment |
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