UNIX / Linux Command To Check Existing Groups and Users

Find out if user exists in /etc/passwd file

/etc/passwd file stores essential information required during login. All you have to do is search this file for user name using following syntax:
$ egrep -i "^username" /etc/passwd
For, example find out if vivek user exists or not, enter:
$ egrep -i "^vivek" /etc/passwd
Sample output:

vivek:x:1000:1000:Vivek Gite,,,,:/home/vivek:/bin/bash

A quick shell script code:

#!/bin/bash
# init
USERID="$1"
#....
/bin/egrep  -i "^${USERID}" /etc/passwd
if [ $? -eq 0 ]; then
   echo "User $USERID exists in /etc/passwd"
else
   echo "User $USERID does not exists in /etc/passwd"
fi
# ....

Normally, exit status is 0 returned if user accounts (lines) are found and 1 otherwise.

Find out if group exists in /etc/group file

/etc/group is an text file which defines the groups to which users belong under Linux and UNIX operating system. Again, you have to search /etc/group file using following syntax:
$ egrep -i "^groupname" /etc/group
For, example find out if vivek group exists or not, enter:
$ egrep -i "^vivek" /etc/group

id command

id is another command to display user / group information for any USERNAME, or the current user. To find out more about user called, tom, enter:
$ id tom
Sample output

id: tom: No such user

To find out ftpuser group, enter:
$ id -g ftpuser
Sample output:

id: ftpuser: No such user

id command exit status is 0 returned if user accounts (lines) are found and 1 otherwise. A sample shell script using id command:

#!/bin/bash
USERID="$1"
/bin/id $USERID 2>/dev/null
[ $? -eq 0 ] && echo "User found" || echo "User not found"
 
/bin/id -g $USERID 2>/dev/null
[ $? -eq 0 ] && echo "Group found" || echo "Group not found"
 

reference:

http://www.cyberciti.biz/faq/linux-check-existing-groups-users/