What's the easiest way to get a user's full name on a Linux/POSIX system?

Question

I could grep through /etc/passwd but that seems onerous. \'finger\' isn\'t installed and I\'d like to avoid that dependency. This is for a program so it would be nice if there

Answer 1

Take 1:

$ user_name=sshd
$ awk -F: "\$1 == \"$user_name\" { print \$5 }" /etc/passwd
Secure Shell Daemon

However, passwd database supports special character '&' in the gecos, which should replaced with capitalized value of user name:

$ user_name=operator
$ awk -F: "\$1 == \"$user_name\" { print \$5 }" /etc/passwd
System &

Most of answers here (except for finger solution) do not respect &. If you want to support this case, then you'll need a more complicated script.

Take 2:

$ user_name=operator
$ awk -F: "\$1 == \"$user_name\" { u=\$1; sub(/./, toupper(substr(u,1,1)), u);
    gsub(/&/, u, \$5); print \$5 }" /etc/passwd
System Operator