Check if a user is in a group

Question

I have a server running where I use php to run a bash script to verify certain information of a user. For example, I have a webhosting server set up, and in order to be able to

Answer 1

A while ago, I wrote a shell function to check if a user is a member of a group. To maximise portability, I wanted it be POSIX-compatible (while this question is tagged as bash, this function will still work). For performance, I wanted to use builtin shell features as much as possible: the only external command it uses is id, the POSIX-standardised utility for getting data about a user’s identity.

is_in_group()
{
  groupname="$1"
  # The second argument is optional -- defaults to current user.
  current_user="$(id -un)"
  user="${2:-$current_user}"
  for group in $(id -Gn "$user") ; do
    if [ "$group" = "$groupname" ]; then
      return 0
    fi
  done
  # If it reaches this point, the user is not in the group.
  return 1
}

Example usage to test both positive and negative cases – and ensure it handles a non-existent username gracefully:

g=mail
userlist="anthony postfix xxx"
for u in $userlist; do
  if is_in_group "$g" "$u"; then
    printf "%s is in ‘%s’\n" "$u" "$g"
  else
    printf "%s is NOT in ‘%s’\n" "$u" "$g"
  fi
done

Running the above command prints the following output:

anthony is NOT in ‘mail’
postfix is in ‘mail’
id: ‘xxx’: no such user
xxx is NOT in ‘mail’

---

It hasn’t been tested for the case where a group or user has a space or other unusual characters in their name but some research shows that such names are not legal: the POSIX Base Definition for Group Name states that

To be portable across conforming systems, the value is composed of characters from the portable filename character set.

The Portable Filename Character Set is specified as the alphanumeric characters, A-Z, a-z, 0-9 along with the period, underscore, and hyphen-minus characters.