How to check if $PWD is a subdirectory of a given path

To test if a string is a prefix of another, in any Bourne-style shell:

case $PWD/ in
  /home/*) echo "home sweet home";;
  *) echo "away from home";;

The same principle works for a suffix or substring test. Note that in case constructs, unlike in file names, * matches any character, including a / or an initial ..

In shells that implement the [[ … ]] syntax (i.e. bash, ksh and zsh), it can be used to match a string against a pattern. (Note that the [ command can only test strings for equality.)

if [[ $PWD/ = /home/* ]]; then …

If you're specifically testing whether the current directory is underneath /home, a simple substring test is not enough, because of symbolic links.

If /home is a filesystem of its own, test whether the current directory (.) is on that filesystem.

if [ "$(df -P . | awk 'NR==2 {print $6}')" = "/home" ]; then
  echo 'The current directory is on the /home filesystem'

If you have the NetBSD, OpenBSD or GNU (i.e. Linux) readlink, you can use readlink -f to strip symbolic links from a path.

case $(readlink -f .)/ in $(readlink -f /home)/*) …

Otherwise, you can use pwd to show the current directory. But you must take care not to use a shell built-in if your shell tracks cd commands and keeps the name you used to reach the directory rather than its “actual” location.

case $(pwd -P 2>/dev/null || env PWD= pwd)/ in
  "$(cd /home && { pwd -P 2>/dev/null || env PWD= pwd; })"/*) …

If you want to reliably test whether a directory is a subdirectory of another, you'll need more than just a string prefix check. Gilles' answer describes in detail how to do this test properly.

But if you do want a simple string prefix check (maybe you've already normalized your paths?), this is a good one:

test "${PWD##/home/}" != "${PWD}"

If $PWD starts with "/home/", it gets stripped off in the left side, which means it won't match the right side, so "!=" returns true.

Crude version:

[ ${PWD:0:6} = "/home/" ]

Has the disadvantage that one has to count characters first and one can't replace /home/ by something general like $1.

edit (thanks @Michael) for the generalization to compare with $VAR one can use

[ "${PWD:0:${#VAR}}" = $VAR ]