How to use locate to search for folders only

Actually, locate has what it takes if you use the --regexp option and you don't mind it spitting out files that have the same name as the directories you seek. The "end of line" position marker gets the job done:

locate -r '/dirname$'

locate also supports --ignore-case if that's what you want.

locate itself can't do it for you. So the UNIX way to do it is to filter the output of locate:

locate --null something | xargs -r0 sh -c 'for i do [ -d "$i" ] && printf "%s\n" "$i"; done' sh {} +

Why not use the find command ?

find . -name YOUR_SEARCH_NAME -type d

find as suggested in Scott Wilson's answer is what I would have used. However, if you really need to use the locate DB, a hackish solution could be

sudo strings /var/lib/mlocate/mlocate.db | grep -E '^/.*dirname'

• sudo since the database is not directly readable by regular users.
• strings to strip metadata (this makes you also find directories to which you don't have read permission, which locate usually hinders).
• /var/lib/mlocate/mlocate.db is the DB path on Ubuntu, apparently (as an example. Other distributions might have it in other places, e.g. /var/lib/slocate/slocate.db).
• grep -E to enable regular expressions.
• ^/.*dirname will match all lines that start with a /, which all directories in the DB happen to do, followed by any character a number of times, followed by your search word.

Positive sides of this solution:

• it is faster than find,
• you can use all the bells and whistles of grep (or other favourite text processing tools).

Negative sides:

• the same as locate in general (DB must be updated),
• you need root access.

Putting Oliver Salzburg's neat line into your .bashrc:

# locate directories:
# -------------------
locd () {
    locate -0 -b -A "$@" | xargs -0 -I {} bash -c '[ -d "{}" ] && echo "{}"'
}

then you can type locd something everytime you want to locate just directories.

Some breakdown:

• -0 provides better support for filenames containing spaces or newlines.

• -b or --basename matches only the last part of the path.

• -A matches all inputs, so if you provide multiple arguments, all must be present.