Command line user interface¶
General¶
- System administration requires the use of command line interface (CLI) since graphical user interface (GUI) may not be available
- Most system administration tasks are easier and quicker to execute through CLI
- In this course we focus only for CLI
- All commands are run through shell in Linux
Terminal¶
- Terminal is a text based command line interface
- Computer hardware is utilized and installed programs are run by inputting text commands sequentially
Command line management¶
- Below is the list of some of the general hot keys for command line management
| Key combination | Action |
|---|---|
| Ctrl + a | Move cursor to the beginning of the line |
| Ctrl + e | Move cursor to the end of the line |
| Ctrl + p | Previous command |
| Ctrl + n | Next command |
| Ctrl + u | Remove all characters between cursor and the beginning of the line |
| Ctrl + k | Remove all characters between cursor and the end of the line |
| Ctrl + l | Clear screen |
- The following table includes some of the basic keys for interacting with command line
| Key | Action |
|---|---|
| TAB | Autocomplete the command |
| ↑ | Previous command from command history |
| ↓ | Next command from command history |
| Del | Remove character from the position of cursor |
| Backspace | Remove character from the left of the cursor |
Autocompleting commands - example¶
- Command can be autocompleted with TAB key
- This feature is supported for example in bash command shell
- Example:
- Write the word who in command line and press TAB button twice
testuser@ubuntu-pc:~$ who
who whoami
testuser@ubuntu-pc:~$
- As can be seen from the example, two commands beginning with the word who are presented
Shell¶
- Shell is a text based program for controlling the operating system
- Login is required for user authentication
- Shell starts automatically when user logs in to the operating system
- During shell startup both common and user settings are loaded
- After startup an empty command prompt is printed to the screen
- User is now able to enter commands through this prompt
- Current Linux distributions use bash as the default shell
- Other shells can be chosen as well
- All valid shells can be found from the file /etc/shells with the following command:
testuser@ubuntu-pc:~$ cat /etc/shells
- Some of the most common shells are listed in the table below
| Shell | Description |
|---|---|
| sh (Bourne shell) | Primitive shell from the 1970s |
| bash (Bourne again shell) | Added the option for command autocompletion, command line editing and command history |
| csh (C shell) | Shell with syntax resembling the C programming language |
| tcsh (Enhanced C shell) | Further developed version of csh with modern features |
| ksh (Korn shell) | Shell with command line editing and command history |
| zsh (Z shell) | Modern shell with Bourne shell compatibility |
Shell functionality¶
- The following sequence describes how shell works
- User inputs command through command prompt and this command is transferred to the shell
- Shell interprets the command and calls for the corresponding program
- If program cannot be found shell returns error message
- If program can be found, new process is created for the execution of the program
- After the execution of the program shell prints new empty command prompt for the user
Shell functionality - example¶
- Below is the example where user enters valid and invalid command
testuser@ubuntu-pc:~$ cd ..
testuser@ubuntu-pc:/home$ cd..
cd..: command not found
testuser@ubuntu-pc:/home$
- The description of the action above:
- User inputs a valid command cd ..
- User's current directory changes (changes to the parent directory)
- User inputs invalid command cd..
- Shell return error command not found
- New empty prompt is printed to the screen
Command prompt¶
- The structure of Linux command prompt is presented below
User types in Linux¶
- Regular user can perform tasks not requiring special permissions
- Super user / root has rights to perform system wide tasks
- Bypass all system security restrictions
- Delete all files, read other user files etc.
- Install, customize and remove programs
- etc.
- For the security point of view it is important to use regular user and use root user rights only if necessary
- Below is the comparison of regular and root user
Change the user¶
- Current logged in user can be printed with whoami command
testuser@ubuntu-pc:~$ whoami
testuser
- This user is regular user without any permissions for performing system wide tasks
- To login with superuser, use su (switch user) command (superuser password is required)
testuser@ubuntu-pc:~$ su
Password:
root@ubuntu-pc:~#
- To change user to some other user than root, user account name will be given as a parameter for the previously presented command
testuser@ubuntu-pc:~$ su - sulo
Password:
sulo@ubuntu-pc:~$
- To logout from current user session use exit or logout commands
testuser@ubuntu-pc:~$ su - sulo
Password:
sulo@ubuntu-pc:~$ exit
testuser@ubuntu-pc:~$ su - root
Password:
root@ubuntu-pc:~# logout
testuser@ubuntu-pc:~$
Important: If su command is used without - character, current directory is not changed and only the following environment variables will be changed: HOME and SHELL (In addition, USER and LOGNAME will also be changed if target user is not root)!
Setting / changing password for the user¶
- Root user does not password by default
- Password can be set with the following command
testuser@ubuntu-pc:~$ sudo passwd root
Enter new UNIX password:
Retype new UNIX password:
passwd: password updated successfully
Sudo¶
- Sudo command (substitute user do) allows regular user to execute commands with administrative permissions
- Regular user is a member of a group with administrative permissions
- Sudo command can be found from most of the current Linux distributions
- Sudo command is used for tasks that require administrative permissions
- Below is an example where system is shutdown with sudo command
testuser@ubuntu-pc:~$ sudo /sbin/shutdown –r now
[sudo] password for testuser:
.
System is restarting...
.
Where to get help with Linux commands? - Man¶
- Each command has a dedicated page in man(ual)
- Manual can be run with man command and by giving command name as a parameter
- Below is an example where whoami command section is searched from the manual
testuser@ubuntu-pc:~$ man whoami
WHOAMI(1) User Commands WHOAMI(1)
NAME
whoami - print effective userid
SYNOPSIS
whoami [OPTION]...
DESCRIPTION
Print the user name associated with the current effective user ID. Same as id -un.
--help display this help and exit
--version
output version information and exit
AUTHOR
Written by Richard Mlynarik.
REPORTING BUGS
GNU coreutils online help: <http://www.gnu.org/software/coreutils/>
Report whoami translation bugs to <http://translationproject.org/team/>
COPYRIGHT
Copyright © 2017 Free Software Foundation, Inc. License GPLv3+: GNU GPL version 3 or later <http://gnu.org/licenses/gpl.html>.
This is free software: you are free to change and redistribute it. There is NO WARRANTY, to the extent permitted by law.
SEE ALSO
Full documentation at: <http://www.gnu.org/software/coreutils/whoami>
or available locally via: info '(coreutils) whoami invocation'
GNU coreutils 8.28 January 2018 WHOAMI(1)
-
Manual can be navigated with the following keys:
- Page up / down: manual is navigated up / down
- q: exists from the manual
-
Man includes a lot of useful information about the command, but the command needs to be known beforehand
- If user cannot remember the command name, parts of the name or command description can be searched from the manual by using the command man -k word
- Below is an example where the word who is searched from the manual
testuser@ubuntu-pc:~$ man –k who
at.allow (5) - determine who can submit jobs via at or batch
at.deny (5) - determine who can submit jobs via at or batch
bsd-from (1) - print names of those who have sent mail
btrfs-filesystem (8)- command group other primarily does work on the whole filesystems
from (1) - print names of those who have sent mail
w (1) - Show who is logged on and what they are doing.
w.Procps (1) - Show who is logged on and what they are doing.
who(1) - Show who is logged on
whoami (1) - print effective userid
--help option¶
- Almost all commands have --help option available
- Option is given after the command and it prints a short description of the command
testuser@ubuntu-pc:~$ whoami –help
Usage: whoami [OPTION]...
Print the user name associated with the current effective user ID.
Same as id –un.
--help display this help and exit
--version output version information and exit
GNU coreutils online help:
<http://www.gnu.org/software/coreutils/>
Full documentation at:
<http://www.gnu.org/software/coreutils/whoami>
Or available locally via: ’(coreutils) whoami invocation’
testuser@ubuntu-pc:~$
info¶
- Online documentation about the command can be read with info command
testuser@ubuntu-pc:~$ info whoami
Next: groups invocation, Prev: logname invocation, Up: User information
20.3. ’whoami’: Print effective user ID
==============================
’whoami’ prints the user name associated with the current effective user ID. It is equivalent to the command ’id -un’.
The only options are ’--help’ and ’--version’. *Note Common options::.
An exit status of zero indicates success, and a nonzero value indicates failure.
Apropos and Whatis¶
- Apropos offers a short description for the command that matches the keyword used in search (this is similar to man -k word command)
testuser@ubuntu-pc:~$ apropos who
at.allow (5) - determine who can submit jobs via at or batch
at.deny (5) - determine who can submit jobs via at or batch
bsd-from (1) - print names of those who have sent mail
btrfs-filesystem (8)- command group other primarily does work on the whole filesystems
from (1) - print names of those who have sent mail
w (1) - Show who is logged on and what they are doing.
w.Procps (1) - Show who is logged on and what they are doing.
who(1) - Show who is logged on
whoami (1) - print effective userid
- The command whatis offers a short description for a given command
testuser@ubuntu-pc:~$ whatis whoami
whoami (1) -print effective userid