Shell - Users and Groups

Users and Groups Shell Commands

check current user uid/gid:

# List IDs, e.g. as root
$ id
uid=0(root) gid=0(root) groups=0(root)

# List IDs of a user, e.g. ubuntu (one uid, one gid, a list of groups)
$ id ubuntu
uid=1000(ubuntu) gid=1000(ubuntu) groups=1000(ubuntu),4(adm),20(dialout),24(cdrom),25(floppy),27(sudo),29(audio),30(dip),44(video),46(plugdev),109(netdev),110(lxd),999(docker)

# Get uid only
$ id -u
1000

# Get gid only
$ id -g
1000

# List groups of the user
$ groups ubuntu
ubuntu : ubuntu adm dialout cdrom floppy sudo audio dip video plugdev netdev lxd docker

# List all groups
$ cat /etc/group

# List all users
$ cat /etc/passwd

# Get the read-only, effective user ID
$ echo $EUID
1000

# Get the current user name
$ whoami
ubuntu

# Or check `$USER`
$ echo $USER
ubuntu

getent

Find out the members of a group with the name developers you would use the following command:

$ getent group developers

Manage Users and Groups

groupadd: add a new group

$ sudo groupadd docker

usermod: modify user, e.g. add user to group docker

$ sudo usermod -aG docker $USER

root

How to change to root user?

$ sudo su

$ su - root

Why some commands are only available with sudo?

The shell will lookup commands in PATH.

$ echo $PATH
/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

It is possible that the root user has a different PATH from a normal user (check your config files like /etc/profile, ~/.bashrc etc). E.g. root user may have /usr/local/sbin or /usr/sbin or /sbin while normal user only has the normal bin folders in the PATH.

The other case may also happen: you installed some program in PATH as a normal user, but needs root permission to execute, for example, if you install node in your own folder while you need $ sudo node bin/www to serve it, you will get an error saying node cannot be found.

Solution:

$ sudo env PATH=$PATH [COMMAND]

this will use your own PATH when executing the COMMAND, e.g.

$ sudo env PATH=$PATH node bin/www

or add this to ~/.bashrc

alias sudo='sudo env PATH=$PATH'