List only directories (Shallow Thoughts)

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

Sat, 03 Sep 2011

List only directories

Fairly often, I want a list of subdirectories inside a particular directory. For instance, when posting blog entries, I may need to decide whether an entry belongs under "linux" or some sub-category, like "linux/cmdline" -- so I need to remind myself what categories I have under linux.

But strangely, Linux offers no straightforward way to ask that question. The ls command lists directories -- along with the files. There's no way to list just the directories. You can list the directories first, with the --group-directories-first option. Or you can flag the directories specially: ls -F appends a slash to each directory name, so instead of linux you'd see linux/. But you still have to pick the directories out of a long list of files. You can do that with grep, of course:

ls -1F ~/web/blog/linux | grep /
That's a one, not an ell: it tells ls to list files one per line. So now you get a list of directories, one per line, with a slash appended to each one. Not perfect, but it's a start.

Or you can use the find program, which has an option -type d that lists only directories. Perfect, right?

find ~/web/blog/linux -maxdepth 1 -type d

Except that lists everything with full pathnames: /home/akkana/web/blog/linux, /home/akkana/web/blog/linux/editors, /home/akkana/web/blog/linux/cmdline and so forth. Way too much noise to read quickly.

What I'd really like is to have just a list of directory names -- no slashes, no newlines. How do we get from ls or find output to that? Either we can start with find and strip off all the path information, either in a loop with basename or with a sed command; or start with ls -F, pick only the lines with slashes, then strip off those slashes. The latter sounds easier.

So let's go back to that ls -1F ~/web/blog/linux | grep / command. To strip off the slashes, you can use sed's s (substitute) command. Normally the syntax is sed 's/oldpat/newpat/'. But since slashes are the pattern we're substituting, it's better to use something else as the separator character. I'll use an underscore.

The old pattern, the one I want to replace, is / -- but I only want to replace the last slash on the line, so I'll add a $ after it, representing end-of-line. The new pattern I want instead of the slash is -- nothing.

So my sed argument is 's_/$__' and the command becomes:

ls -1F ~/web/blog/linux | grep / | sed 's_/$__'

That does what I want. If I don't want them listed one per line, I can fudge that using backquotes to pass the output of the whole command to the shell's echo command:

echo `ls -1F ~/web/blog/linux | grep / | sed 's_/$__'`

If you have a lot of directories to list and you want ls's nice columnar format, that's a little harder. You can ls the list of directories (the names inside the backquotes), ls `your long command` -- except that now that you've stripped off the path information, ls won't know where to find the files. So you'd have to change directory first:

cd ~/web/blog/linux; ls -d `ls -1F | grep / | sed 's_/$__'`

That's not so good, though, because now you've changed directories from wherever you were before. To get around that, use parentheses to run the commands inside a subshell:

(cd ~/web/blog/linux; ls -d `ls -1F | grep / | sed 's_/$__'`)

Now the cd only applies within the subshell, and when the command finishes, your own shell will still be wherever you started.

Finally, I don't want to have to go through this discovery process every time I want a list of directories. So I turned it into a couple of shell functions, where $* represents all the arguments I pass to the command, and $1 is just the first argument.

lsdirs() { 
  (cd $1; /bin/ls -d `/bin/ls -1F | grep / | sed 's_/$__'`)
}

lsdirs2() { 
  echo `/bin/ls -1F $* | grep / | sed 's_/$__'` 
}
I specify /bin/ls because I have a function overriding ls in my .zshrc. Most people won't need to, but it doesn't hurt.

Now I can type lsdirs ~/web/blog/linux and get a nice list of directories.

Update, shortly after posting: In zsh (which I use), there's yet another way: */ matches only directories. It appends a trailing slash to them, but *(/) matches directories and omits the trailing slash. So you can say

echo ~/web/blog/linux/*(/:t)
:t strips the directory part of each match. To see other useful : modifiers, type ls *(: then hit TAB.

Thanks to Mikachu for the zsh tips. Zsh can do anything, if you can just figure out how ...

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[ 10:22 Sep 03, 2011    More linux/cmdline | permalink to this entry | comments ]
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