Re: for loop with parameter expansion in zsh vs. bash

[Alexander Skwar <alexanders.mailinglists+nospam@xxxxxxxxx>]

Hello

2015-11-03 8:35 GMT+01:00 Stephane Chazelas <stephane.chazelas@xxxxxxxxx>:

> 2015-11-03 07:52:51 +0100, Alexander Skwar:
> > Hello
> >
> > I've got a variable, where seperate values are limited with a delimiter.
> > Let's say PATH with : (but the question is general).
> >
> > With bash, I can easily create a for loop which loops over all the
> > elements, when I have bash replace the ":" with a " ", like so:
> ${PATH//:/
> > }.
> >
> > a@ubuntu-notebook:~$ echo $PATH
> >
> /home/a/Applications/go/bin:/home/a/bin:/opt/bin:/opt/sbin:/usr/local/sbin:/usr/local/bin:/home/a/Applications/copy/x86_64:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/home/a/Applications/adt-bundle-linux-x86_64/sdk/platform-tools:/home/a/Applications/btsync:/home/a/.rvm/bin
> >
> > a@ubuntu-notebook:~$ for e in ${PATH//:/ }; do echo e: $e; done
> > e: /home/a/Applications/go/bin
> > e: /home/a/bin
> > e: /opt/bin
> > e: /opt/sbin
> > e: /usr/local/sbin
> > e: /usr/local/bin
> > e: /home/a/Applications/copy/x86_64
> > e: /usr/sbin
> > e: /usr/bin
> > e: /sbin
> > e: /bin
> > e: /usr/games
> > e: /usr/local/games
> > e: /home/a/Applications/adt-bundle-linux-x86_64/sdk/platform-tools
> > e: /home/a/Applications/btsync
> > e: /home/a/.rvm/bin
>
> That approach is wrong. You're replacing : with space and
> invoking the split+glob operator (leaving an expansion unquoted
> in ksh/bash/sh, not zsh) with the default value of IFS which
> contains space, tab and newline.
>
> That means that approach only works if $PATH components don't
> contain blanks or wildcards.
>

​Well, but I do know that my variable does not contain blanks or wildcards.
As I said, PATH was just an example, so that everybody can easily follow.
So, no, ​the approach was not at all wrong. Your assumption was, though.



>
> Here, you could use the split+glob operator, but you want to
> split on :, not blanks, and not invoke the glob part.
>
> So it would be (bash/ksh/sh, not zsh unless in sh/ksh emulation):
>
> IFS=: # split on :
> set -f # disabe glob
> for e in $PATH
>

​Thanks a lot. Works great.​



>
> However with bash/ksh/sh (not zsh), it's still wrong, because it
> would discard a trailing empty component (like for a $PATH value
> of /bin:/usr/bin:)
>


​Good catch ;)​



>
>
> >
> > In zsh, the same for loop does not work (like it does in bash):
>
> No, because and that's the most FAQ for zsh, leaving a variable
> unquoted is not the split+glob operator (like it is in most
> other shells and the number one source of bugs and security
> vulnerabilities in them,
> (
> http://unix.stackexchange.com/questions/171346/security-implications-of-forgetting-to-quote-a-variable-in-bash-posix-shells
> )
>
> In zsh, you invoke the split ($=var) or glob ($~var) operators
> explicitely, so:
>
> IFS=:
> for e in $~PATH
>
> Or use the "s" expansion flag:
>
> for e in "${(s(:))PATH}"
>
>
​Great. Thanks. Appreciated.


Too bad, that there's such a difference between the shells. Makes it hard
to share snippets with co-workers, who (for reasons, that I fail to
understand) don't use zsh.

Because of that, I'm actually using something along the lines of:

  for e in $(echo "$PATH" | tr ':' ' '); do echo e $e; done

This works everywhere.​



> For PATH however, you don't need to as zsh ties the $PATH
> variable to the $path array.
>

​I shouldn't have used PATH as an example… :/​ But thanks for the heads up
for that as well.


Alexander
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