List all users in Linux

cat /etc/passwd
First column tells you the username

Fig.01: List users using /etc/passwd

use awk command to filter out only the usernames:

 awk -F':' '{ print $1}' /etc/passwd

Following shell script will list all the normal users and system accounts:

#!/bin/bash
# Name: listusers.bash
# Purpose: List all normal user and system accounts in the system. Tested on RHEL / Debian Linux
# Author: Vivek Gite <www.cyberciti.biz>, under GPL v2.0+
# -----------------------------------------------------------------------------------
_l="/etc/login.defs"
_p="/etc/passwd"
 
## get mini UID limit ##
l=$(grep "^UID_MIN" $_l)
 
## get max UID limit ##
l1=$(grep "^UID_MAX" $_l)
 
## use awk to print if UID >= $MIN and UID <= $MAX and shell is not /sbin/nologin   ##
echo "----------[ Normal User Accounts ]---------------"
awk -F':' -v "min=${l##UID_MIN}" -v "max=${l1##UID_MAX}" '{ if ( $3 >= min && $3 <= max  && $7 != "/sbin/nologin" ) print $0 }' "$_p"
 
 
 
echo ""
echo "----------[ System User Accounts ]---------------"
awk -F':' -v "min=${l##UID_MIN}" -v "max=${l1##UID_MAX}" '{ if ( !($3 >= min && $3 <= max  && $7 != "/sbin/nologin")) print $0 }' "$_p"
 

----------[ Normal User Accounts ]---------------
vivek:x:500:500::/home/vivek:/bin/bash
raj:x:501:501::/home/raj:/bin/ksh
ash:x:502:502::/home/ash:/bin/zsh
jadmin:x:503:503::/home/jadmin:/bin/sh
wwwint:x:506:506::/htdocs/intranet:/bin/bash
scpftp:x:507:507::/htdocs/ftpjail:/bin/bash
rsynftp:x:508:508::/htdocs/projets:/bin/bash
mirror:x:509:509::/htdocs:/bin/bash
jony:x:510:510::/home/jony:/bin/ksh
amyk:x:511:511::/home/amyk:/bin/ksh
----------[ System User Accounts ]---------------
root:x:0:0:root:/root:/bin/bash
bin:x:1:1:bin:/bin:/sbin/nologin
daemon:x:2:2:daemon:/sbin:/sbin/nologin
adm:x:3:4:adm:/var/adm:/sbin/nologin
lp:x:4:7:lp:/var/spool/lpd:/sbin/nologin
sync:x:5:0:sync:/sbin:/bin/sync
shutdown:x:6:0:shutdown:/sbin:/sbin/shutdown
halt:x:7:0:halt:/sbin:/sbin/halt
mail:x:8:12:mail:/var/spool/mail:/sbin/nologin
uucp:x:10:14:uucp:/var/spool/uucp:/sbin/nologin
operator:x:11:0:operator:/root:/sbin/nologin
games:x:12:100:games:/usr/games:/sbin/nologin
gopher:x:13:30:gopher:/var/gopher:/sbin/nologin
ftp:x:14:50:FTP User:/var/ftp:/sbin/nologin
nobody:x:99:99:Nobody:/:/sbin/nologin
dbus:x:81:81:System message bus:/:/sbin/nologin
vcsa:x:69:69:virtual console memory owner:/dev:/sbin/nologin
abrt:x:173:173::/etc/abrt:/sbin/nologin
haldaemon:x:68:68:HAL daemon:/:/sbin/nologin
ntp:x:38:38::/etc/ntp:/sbin/nologin
saslauth:x:499:499:"Saslauthd user":/var/empty/saslauth:/sbin/nologin
postfix:x:89:89::/var/spool/postfix:/sbin/nologin
apache:x:48:48:Apache:/var/www:/sbin/nologin
webalizer:x:67:67:Webalizer:/var/www/usage:/sbin/nologin
sshd:x:74:74:Privilege-separated SSH:/var/empty/sshd:/sbin/nologin
tcpdump:x:72:72::/:/sbin/nologin
mysql:x:27:27:MySQL Server:/var/lib/mysql:/bin/bash
memcached:x:498:496:Memcached daemon:/var/run/memcached:/sbin/nologin
squid:x:23:23::/var/spool/squid:/sbin/nologin
rpc:x:32:32:Rpcbind Daemon:/var/cache/rpcbind:/sbin/nologin
rpcuser:x:29:29:RPC Service User:/var/lib/nfs:/sbin/nologin
nfsnobody:x:65534:65534:Anonymous NFS User:/var/lib/nfs:/sbin/nologin

courtesy: http://www.cyberciti.biz/faq/linux-list-users-command/