Linux Shell Commands - 02 Env Vars & Users

This post is Part Two of the summary of some common Linux Shell Commands.

Environment Variables

The Path var determines where shell finds those commands, or processes.

The vars point to directories, and processes find related files based on the assigned directories.

System Vars

Two paths:

/etc/profile: First to execute when log into shell
/etc/environment: System environment var. assignment

e.g. 两种写法：

1	sudo sh -c 'echo "export <VAR>=<path>" >> /etc/profile'

1 2	sudo vim /etc/profile export <VAR>='<path 1>:<path 2>...'

User Vars

Two paths:

~/.profile: Execute once each time the user logs in
~/.bashrc: Every time when a new shell window is opened

e.g. 两种写法：

1	sudo sh -c 'echo "export <VAR>=<path>" >> .bashrc'

1 2	vim ~/.bashrc export <VAR>='<path 1>:<path 2>...'

Permanent & Temporary Vars

Permanent: Add vars in system files
Temp: Use export in bash

e.g.

# Set new var
export NAME='hello'		

# Change existing var
NAME='world'

env: Check all environmental vars
set: Check all vars in the current shell
unset <VAR>: delete vars
vimdiff: Compare differences
e.g. ./bashrc: Make changes effective immediately

Common Vars

PATH
HOME: User’s main directory
HISTSIZE: Number of stored commands
LOGNAME: Current username
HOSTNAME
SHELL
PS1: Prompt String 1. # for root, $ for normal users

Users & Usergroups

Userinfo & UID

1	sudo cat /etc/passwd

e.g. root : x : 0 : 0 : root : /root : /bin/bash

7 parts in total.

Username : Password : User ID (UID) : Group ID (GID) : Comments : Main dir. : Shell

Password Info

1	sudo cat /etc/shadow

root : * : 17590 : 0 : 99999 : 7 : : :

9 parts in total.

Username : Password : Last change : Min : Max : Warn : Inactive time : Expire time : (Reserved)

Last change: Last password change
Min / Max: Minimum / Maximum days for the password to be effective
Warn: Warnings before password expires

Usergroup Info:

1	sudo cat /etc/group

root : x : 0 :

4 parts in total.

Group name : Password: GID : List of users (1, 2, …)

Manage Users & Usergroups:

Create Users:

Methods: adduser <NAME> or sudo useradd
Set home directory: useradd -d /home/<USER> <NAME>
Set usergroup: useradd -g <GROUP> <NAME>
Set login shell: useradd -s /bin/bash <NAME>
Modify existing user info: usermod (same options with useradd)

sudo passwd:

Modify password: passwd <NAME>
Delete password: passwd -d <NAME>
Block user from logging in: passwd -l <NAME>
Unlock: passwd -u <NAME>
Password status: passwd -S <NAME>

Check Status:

sudo tail -5 /etc/passwd: System file
id <NAME>: User info
w: Current user
lastlog: login info
Switch user: su <NAME>
Check user’s all groups: groups <NAME>

Usergroup:

Add user to multiple groups: usermod -aG <Group1, Group2, ...> <NAME> (a: append)
Add new GID: e.g. groupadd -g 1024 <GROUP>
Modify group name: groupdmod -n <NEW> <OLD>