Extracting substring from environment variable

Question

In a bash or zsh script, how might I extract the host from a url, e.g. unix.stackexchange.com from http://unix.stackexchange.com/questions/ask, if the latter is in an environment variable?

Answer 1

You can use parameter expansion, which is available in any POSIX compliant shell.

$ export FOO=http://unix.stackexchange.com/questions/ask
$ tmp="${FOO#*//}" # remove http://
$ echo "${tmp%%/*}" # remove everything after the first /
unix.stackexchange.com

A more reliable, but uglier method would be to use an actual URL parser. Here is an example for python:

$ python3 -c 'import sys; from urllib.parse import urlparse; print(urlparse(sys.argv[1]).netloc)' "$FOO"
unix.stackexchange.com

Answer 2

If the URLs all follow this pattern I have this short and ugly hack for you:

echo "$FOO" | cut -d / -f 3

Answer 3

You can do it many ways, some of them being:

export _URL='http://unix.stackexchange.com/questions/ask'

echo "$_URL" | sed -ne 'y|/|\n|;s/.*\n\n/;P'

expr "$_URL" : 'http://\([^/]*\)'

echo "$_URL" |  perl -lpe '($_) = m|^http://\K[^/]+|g'

perl -le 'print+(split m{/}, $ENV{_URL})[2]'

(set -f; IFS=/; set -- $_URL; echo "$3";)

Answer 4

Can be done also with regex groups:

$ a="http://unix.stackexchange.com/questions/ask"
$ perl -pe 's|(.*//)(.*?)(/.*)|\2|' <<<"$a"
unix.stackexchange.com