Linux useful commands

# to find os details
uname -a

# 1.  Add / update / delete / list - users in linux os

# list all user in linux
cat /etc/passwd # also  /etc/passwd, /etc/group and /etc/shadow or /etc/master.passwd  could be used

# sample output for list users:
root:x:0:0:root:/root:/bin/bash - root is the username
daemon:x:1:1:daemon:/usr/sbin - daemon is the username

# to create new user login for linux os.
sudo useradd -d /path/to/home/USERNAME USERNAME
sudo passwd USERNAME # asks for new password for the account.

# to create new user login and add to existing group
sudo useradd -G group1,group2 -d /path/to/home USERNAME

# to remove user login from linux os.
userdel -r USERNAME    # -r option remove user owned home folder

# to update user password for linux.
sudo passwd USERNAME

# 2. Add / update / delete / list - groups in linux os

# to list all groups
groups

# to create a new user group
groupadd GROUPNAME



# 3. SSH stuff

# update remote host key in ~/.ssh/known_hosts
ssh-keygen -R "ip address"