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

[Stephane Chazelas <stephane.chazelas@gmail.com>]

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.

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

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:)


> 
> 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}"

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

So, it's just:

for e in "$path[@]"

-- 
Stephane