6 ms·

What is this magic doing? $(cd "${0%/*}")

by nodesocket 12y ago

endgame 12y ago
It's a bashism. Open up "man bash" and search for "Remove matching suffix pattern".
- clebio 12y ago
  which, regrettably, is not something you can do with `man` if you find `${0%/*}` in a script you're reading that someone else wrote.
  - endgame 12y ago
    No, but you can find it by searching for "%", because that's exactly what I did.
DangerousPie 12y ago
$0 is the path of the script. ${0%/* } takes $0, deletes the smallest substring matching /* from the right (the filename) and returns the rest, which would be the directory of the script. So this changes the directory to the directory the script is located in.
- rjgray 12y ago
  Is there any good reason to use this approach over the more readable 'dirname $0'? Not that it would have made any difference to the bug in question of course.
- barrkel 12y ago
  $0 is not the path of the script. $0 is the path passed to the shell used to execute the script. If run via the #! line, it will contain a path of some kind. But if it's passed directly using "bash myscript.sh", then it won't. And yes, dirname is a way out of this. I'd do this: "$(cd "$(dirname "$0")"; pwd)" if I wanted the path to the script. I would also sanity-check the path by testing for the existence of some files or directories that are expected to exist under it, before trying to delete it all.
amputect 12y ago
Per a subsequent comment in the github thread: "the ${0%/*} pulls the path out of $0, cd's into the specified directory, then uses $PWD to figure out where that directory lives - and all this in a subshell, so we don't affect $PWD" So for example, that would take /user/amputect/seam_setup.sh and change directories into into /user/amputect.
pwg 12y ago
Assuming the shell script is for Bash, that incantation removes the shortest matching suffix which matches the pattern "/*". So it is attempting to remove the filename component from a pathname string in the "0" positional parameter. Since $0 is usually the name of the shellscript itself, this would be trying to obtain the directory path of the shellscript.

What is this magic doing? $(cd "${0%/*}")

endgame 12y ago

It's a bashism. Open up "man bash" and search for "Remove matching suffix pattern".

clebio 12y ago
which, regrettably, is not something you can do with `man` if you find `${0%/*}` in a script you're reading that someone else wrote.
- endgame 12y ago
  No, but you can find it by searching for "%", because that's exactly what I did.

DangerousPie 12y ago

$0 is the path of the script. ${0%/* } takes $0, deletes the smallest substring matching /* from the right (the filename) and returns the rest, which would be the directory of the script. So this changes the directory to the directory the script is located in.

rjgray 12y ago
Is there any good reason to use this approach over the more readable 'dirname $0'? Not that it would have made any difference to the bug in question of course.
barrkel 12y ago
$0 is not the path of the script. $0 is the path passed to the shell used to execute the script. If run via the #! line, it will contain a path of some kind. But if it's passed directly using "bash myscript.sh", then it won't. And yes, dirname is a way out of this. I'd do this: "$(cd "$(dirname "$0")"; pwd)" if I wanted the path to the script. I would also sanity-check the path by testing for the existence of some files or directories that are expected to exist under it, before trying to delete it all.

amputect 12y ago

Per a subsequent comment in the github thread: "the ${0%/*} pulls the path out of $0, cd's into the specified directory, then uses $PWD to figure out where that directory lives - and all this in a subshell, so we don't affect $PWD" So for example, that would take /user/amputect/seam_setup.sh and change directories into into /user/amputect.

pwg 12y ago

Assuming the shell script is for Bash, that incantation removes the shortest matching suffix which matches the pattern "/*". So it is attempting to remove the filename component from a pathname string in the "0" positional parameter. Since $0 is usually the name of the shellscript itself, this would be trying to obtain the directory path of the shellscript.