Getting ls to show symlinks (and stripping terminal slashes in shells) (Shallow Thoughts)

Akkana's Musings on Open Source Computing, Science, and Nature.

Wed, 15 Aug 2012

Getting ls to show symlinks (and stripping terminal slashes in shells)

The Linux file listing program, ls, has been frustrating me for some time with its ever-changing behavior on symbolic links.

For instance, suppose I have a symlink named Maps that points to a directory on another disk called /data/Maps. If I say ls ~/Maps, I might want to see where the link points:

lrwxrwxrwx   1 akkana users              12 Jun 17  2009 Maps -> /data/Maps/
or I might equally want to see the contents of the /data/Maps directory.

Many years ago, the Unix ls program magically seemed to infer when I wanted to see the link and what it points to, versus when I wanted to see the contents of the directory the link points to. I'm not even sure any more what the rule was; just that I was always pleasantly surprised that it did what I wanted. Now, in modern Linux, it usually manages to do the opposite of what I want. But the behavior has changed several times until, I confess, I'm no longer even sure of what I want it to do.

So if I'm not sure whether I usually want it to show the symlink or follow it ... why not make it do both?

There's no ls flag that will do that. But that's okay -- I can make a shell function to do what I want..

Current ls flags

First let's review man ls to see the relevant flags we do have, searching for the string "deref".

I find three different flags to tell ls to dereference a link: -H (dereference any link explicitly mentioned on the command line -- even though ls does that by default); --dereference-command-line-symlink-to-dir (do the same if it's a directory -- even though -H already does that, and even though ls without any flags also already does that); and -L (dereference links even if they aren't mentioned on the command line). The GNU ls maintainers are clearly enamored with dereferencing symlinks.

In contrast, there's one flag, -d, that says not to dereference links (when used in combination with -l). And -d isn't useful in general (you can't make it part of a normal ls alias) because -d also has another, more primary meaning: it also prevents you from listing the contents of normal, non-symlinked directories.

Solution: a shell function

Let's move on to the problem of how to show both the link information and the dereferenced file.

Since there's no ls flag to do it, I'll have to do it by looping over the arguments of my shell function. In a shell test, you can use -h to tell if a file is a symlink. So my first approach was to call ls -ld on all the symlinks to show what the point to:

ll() {
    /bin/ls -laFH $*
    for f in $*; do
        if [[ -h $f ]]; then
            echo -n Symlink:
            /bin/ls -ld $f
        fi
    done
}

Terminally slashed

That worked on a few simple tests. But when I tried to use it for real I hit another snag: terminal slashes.

In real life, I normally run this with autocompletion. I don't type ll ~/Maps -- I'm more likely to type like ll Ma<tab> -- the tab looks for files beginning with Ma and obligingly completes it as Maps/ -- note the slash at the end.

And, well, it turns out /bin/ls -ld Maps/ no longer shows the symlink, but derefernces it instead -- yes, never mind that the man page says -d won't dereference symlinks. As I said, those ls maintainers really love dereferencing.

Okay, so if I want to not dereference, since there's no ls flag that means really don't dereference, I mean it -- my little zsh function needs to find a way of stripping any terminal slash on each directory name. Of course, I could do it with sed:

        f=`echo $f | sed 's/\/$//'`
and that works fine, but ... ick. Surely zsh has a better way?

In fact, there's a better way that even works in bash (thanks to zsh wizard Mikachu for this gem):

        f=${f%/}

That "remove terminal slash" trick has already come in handy in a couple of other shell functions I use -- definitely a useful trick if you use autocompletion a lot.

Making the link line more readable

But wait: one more tweak, as long as I'm tweaking. That long ls -ld line,

lrwxrwxrwx   1 akkana users              12 Jun 17  2009 Maps -> /data/Maps/
is way too long and full of things I don't really care about (the permissions, ownership and last-modified date on a symlink aren't very interesting). I really only want the last three words,
Maps -> /data/Maps/

Of course I could use something like awk to get that. But zsh has everything -- I bet it has a clever way to separate words.

And indeed it does: arrays. The documentation isn't very clear and not all the array functions worked as the docs implied, but here's what ended up working: you can set an array variable by using parentheses after the equals sign in a normal variable-setting statement, and after that, you can refer to it using square brackets. You can even use negative indices, like in python, to count back from the end of an array. That made it easy to do what I wanted:

            line=( $(/bin/ls -ld $f ) )
            echo -E Symlink: $line[-3,-1]

Hooray zsh! Though it turned out that -3 didn't work for directories with spaces in the name, so I had to use [9, -1] instead. The echo -E is to prevent strange things happening if there are things like backslashes in the filename.

The completed shell function

I moved the symlink-showing function into a separate function, so I can call it from several different ls aliases, and here's the final result:

show_symlinks() {
    for f in $*; do
        # Remove terminal slash.
        f=${f%/}
        if [[ -h $f ]]; then
            line=( $(/bin/ls -ld $f ) )
            echo -E Symlink: $line[9,-1]
        fi
    done
}

ll() {
    /bin/ls -laFH $*
    show_symlinks $*
}

Bash doesn't have arrays like zsh, so replace those two lines with

            echo -n 'Symlink: '
            /bin/ls -ld $f | cut -d ' ' -f 10-
and the rest of the function should work just fine.

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[ 19:22 Aug 15, 2012    More linux/cmdline | permalink to this entry | comments ]
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