Profound Linux For Administrators

By Onder Teker

This book contains topics about Linux for developers as well as advanced users or administrators. The shells & commands together with expansion & substitutionare explained in detail. Process communication with pipes & redirection are explained. Scripting is covered for both developers and system experts at both fundamental and advanced level. Topics such as files, array, strings, declarations are added. Initialization scripts & crontab are explained. MySQL/MariaDB, PostgreSQL, MongoDB databases; Apache, NGINX, Tomcat web servers; C/C++, Java, JavaScript, Python development kits are included.

Author

Önder Teker, the author of the book, develops projects since the end of the 1990s, gives courses and lectures since the beginning of the 2000s, and produces printed and electronic books and visual courses since the beginning of the 2010s.

• Language: English
• Publisher: Godoro
• Release date: Nov 26, 2023
• ISBN: 9786259947822

Book preview

Profound

Linux

For

Administrators

Önder Teker

Godoro Publishing

GODORO PUBLISHING

Publisher Certificate No: 40946

The Name Of The Book:

Profound Linux For Administrators

Copyright © 2023 Godoro Publishing

The Author Of The Book:

Önder Teker

First Edition, December 2023, Istanbul

ISBN:

978-625-99478-2-2

Cover Design & Makeup:

Önder Teker

Printing & Binding:

Godoro

Special Printing Solutions

Atikali Mah. Fatih Cad. No: 81 D: 2

Fatih / İstanbul

Telefon : [phone]

http://www.godoro.com

GODORO PUBLISHING

Atikali Mah. Fatih Cad. No: 81 D: 2

Fatih / Istanbul

Phone : [phone]

http://www.godoro.com

User Management

In this section, users, groups and permissions are covered.

Basic Terms

Here some basic terms about users and administrators is given.

User

In this section, some information about users are given.

User

A person who uses an operating system is called a user. It has a name called user name. It can be a human being, a program or a machine.

Password

The information to access an operating system is called a password. Each user is given a password. They are encoded in one way so that the others can not see the original.

Authentication

Checking whether a user exists in a system and its password is correct is called authentication. It is a process to determine if a person has the rights to use the system in general.

Current User

The authenticated user which currently uses the operating system is called the current user. There may be many users defined in a system but the active one is the current. Many operations check the current user rights to do a specific task.

Permissions

This section is about permissions.

Permission

What a user can do or can not do is called a permission. Each action is checked whether the user have rights to do it before it is performed. If the user has no permission, the access is denied. Otherwise the access is granted.

Read, Write, Execute

There are three types of permissions: read, write and execute. A read permission is to access a file without changing it. On the other hand, modification of a file needs a write permission. If a file is an executable such as a command, utility or program; the execute permission is the right to run it. The permissions are called mode. It means the access rights for specific users for specific files.

Characters

In the file system, permissions are denoted by characters rather than words. The r is to read, w is to write and x is to execute. Generally they are given together as rwx. If a permission does not exist, it is denoted by a dash (-) sign. For example, rw- means no execute permission while r-- means no write or execute permissions.

Bits

Each permission can be denoted by a bit. Here 1 means with permission and 0 means without permission. The three permissions can be denoted by a 3-bit number, which is between 0 and 7. For example, 111 in binary or 7 in decimal means rwx while 000 means ---.

All the possibilites can be given as below:

Administrator & Group

In this section, what is an administrator or a group is explained.

Admin / Super User / Root

A user with more rights, which can do almost everything is called administrator, or in short admin. In some cases, it is called super user or superuser. The name of the first user with admin rights is called root in many systems. However, there are some other names used for administrators.

The root is the first and most privileged user. There may be some other admins or superusers.

Group

A collection of users are called a group. In many cases, instead of addressing individual users, a group is referenced. In this way, each user is not handled separately. All the definitions related to a group are applied to a user as soon as it is made a part of a group.

Owner

The user created an object is called the owner. An owner naturally has more permissions than the other users. Moreover, the users are in the same group as the owner of an object also may have more permissions than the ones not in the group.

Others

The users which are not owner and not in the group of the owner are called others. It may be thought of as public or anyone. Naturally the others will have less permissions to an object.

Permissions

The permissions for a file is given for the owner, group and others. For example, rwxrw-r-- means the owner have rwx (read write execute) permissions, group has rw- (read write) and others have r-- (read).

Users & Groups

In this sections users,  groups and passwords are explained.

Users

In this section, it is shown how to learn the users defined in the system.

Password File

The user created in an operating system can be learned by the paswd (password) file. It can be found this location:

/etc/password

It can be viewed by any command or utility such as cat as below:

$ cat /etc/passord

The output will be as below:

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/mail:/sbin/nologin

news:x:9:13:news:/usr/lib/news:/sbin/nologin

uucp:x:10:14:uucp:/var/spool/uucppublic:/sbin/nologin

operator:x:11:0:operator:/root:/sbin/nologin

man:x:13:15:man:/usr/man:/sbin/nologin

postmaster:x:14:12:postmaster:/var/mail:/sbin/nologin

cron:x:16:16:cron:/var/spool/cron:/sbin/nologin

ftp:x:21:21::/var/lib/ftp:/sbin/nologin

sshd:x:22:22:sshd:/dev/null:/sbin/nologin

at:x:25:25:at:/var/spool/cron/atjobs:/sbin/nologin

squid:x:31:31:Squid:/var/cache/squid:/sbin/nologin

xfs:x:33:33:X Font Server:/etc/X11/fs:/sbin/nologin

games:x:35:35:games:/usr/games:/sbin/nologin

cyrus:x:85:12::/usr/cyrus:/sbin/nologin

vpopmail:x:89:89::/var/vpopmail:/sbin/nologin

ntp:x:123:123:NTP:/var/empty:/sbin/nologin

smmsp:x:209:209:smmsp:/var/spool/mqueue:/sbin/nologin

guest:x:405:100:guest:/dev/null:/sbin/nologin

nobody:x:65534:65534:nobody:/:/sbin/nologin

godoro:x:1000:1000::/home/godoro:/bin/bash

The first line is the first and most privileged user named root. The ones at last lines are the users created by the root or any other administrator. The ones in the middle are users to do specified task. For example, the ftp is the user to connect to the system via FTP protocol.

Properties

The properies of

User Properties

The fields in the passwd file are properties of a user. In this section, their meanings are explained.

Username

The name of the user is called username. It is different from the human name such as first name or last name. It should not contain any space.

Password

The key to access to the system is called password. It the value is x, it means it is encrypted and stored in a separate file named shadow which is located as below:

/etc/shadow

The contents in this file as below:

root:$6$9Zr2.GA9i4b3/p7W1mbyCl2wXEpg0:18759:0:99999:7:::

...

godoro:$6$kc7J2GWmdvag/qR843ImpIUFA0VrZ.:18759:0:99999:7:::

Only the first and last line is shown. The encrypted passwords are also shortened.

UID (User Id)

The UID (User Id) is the id of the user. It is a number starting from zero and is incremented by one for each user. Even if the username is changed, this one is used to access user related information. In other words, this number is internally by the operating system. However, in some cases, user name is shown as user id instead of this number. The root user has the id 0. In some systems, any user whose ID is 0 is considered root.

GID (Group Id)

The id of the group the user belongs to. It's a collection of users. Instead of handling many users one by one, many operations are performed on a group and the users in that group will be affected automatically.

GECOS

The extra information about the user such as full name, phones, room number etc. is called GECOS, which comes from the name General Comprehensive Operating System. Sometimes, the content in this field is called description or comment. Because it is a place where a description of the user or any comment about it may be put.

Home

The directory belonging to a user is called home. Normally, a user will keep its own personal files there. Many applications use this directory to keep data related to the user. In most cases, the home directory is under the directory named home directory in the root. So, the home directory of the user godoro will be at the location:

/home/godoro

Shell

The interpreter which handles commands entered in the terminal by a user is called a shell. Each user may use a different interpreter. In many operating systems, they are installed under bin directory. In most cases, there is a shell named sh (Shell). There are some others such as bash (Bourne Shell). For Alpine Linux, the default shell is ash (Alpine Shell). These are located under these directories:

/bin/sh

/bin/ash

The default one for the user is defined in the passwd file.

Groups

The groups defined in the system can be learned in a few ways. Here some of them are explained.

Primary & Secondary

A group may be primary or secondary. The primary group is the group assigned to a user when it is entered the system. The files created by a user belong to the primary group. The other type is the secondary group or supplementary group. It is assigned by some command to share files. A user can access the file of the secondary group if it is put into it.

Groups Command

The groups command returns all the primary groups. For example, the command below:

$ groups

will return something like this:

godoro adm wheel floppy cdrom tape ping

In some cases, only the group the current user is in is returned.

Cut Passwords

The cut command can be used on the passwd file to get groups. For example, the call below:

cut -d: -f4 /etc/passwd

will return the output below:

0

1

2

4

7

0

0

0

12

0

100

...

984

1001

Just the beginning and end of the groups are written above. The number 0 is the id of the root group.

Groups File

There is a file named as below:

/etc/group

to hold all the groups. Displaying its content will show all the groups. For example, the call below:

$ cat /etc/group

will show all the groups. The output will be like below:

root:x:0:

bin:x:1:

daemon:x:2:

sys:x:3:

adm:x:4:heruser

...

sshuser:x:1000:

mysql:x:27:

nginx:x:985:

tomcat:x:984:

The second field is used for the password. The letter x means it is encrypted and stored in file below:

/etc/gshadow

if the group has a password. The last field is the group id.

Get The Group Entry

Another way of getting groups is using the getent command. This command can be called the group argument and the groups in the system will be displayed. The command can be used as below:

$ getent group

Add A Group

A group can be created by the groupadd command. It can be used as below:

$ groupadd mygroup

Now, a new line will be added to the /etc/groups file as below:

mygroup:x:1002:

Delete A Group

A group may be deleted by the groupdel command. It can be used as below:

$ groupdel mygroup

Now the lines in /etc/groups and /etc/gshadow for the group are also deleted.

Id Command

In order to get information about the group of a user, the id command can be used. For example, the call below:

$ id godoro

will print the output below:

uid=1000(godoro) gid=1000(godoro) groups=1000(godoro),4(adm),10(wheel),

11(floppy),19(cdrom),26(tape),999(ping)

User Operations

In this section, the commands to add, modify and delete users are explained.

Add A User

A user can be created by the useradd command. A username is given as an argument. For example, the call below:

$ useradd myuser

will add a user. A new line is created at the end of the passwd file as below:

/etc/passwd

...

myuser:x:1001:1001::/home/myuser:/bin/bash

Shell

The s (shell) option can set the shell interpreter for a user. For example, the call below:

$ useradd myuser -s /bin/sh

will set the path of the shell interpreter. This is useful when there are many shells in the system.

If the user do not need a shell or forbidden to use one, the path of the shell is given as below:

/sbin/nologin

Example:

$ useradd myuser -s /sbin/nologin

System User

Some users, which are called the system users, are used for applications and need no login. The r (system) option can create such as user as below:

$ useradd -r myuser

Since this user will not use shell, the shell argument is set to nologin:

$ useradd -r -s /sbin/nologin myuser

Delete User

A user can be deleted by the userdel command. Its usage is as below:

$ userdel myuser

After this command, the line for the user in the password file will be deleted.

Password

The topics related to passwords are explained here.

Set Password

A password may be defined for a user by the p (pasword) option of useradd command as below:

$ useradd myuser -p mypass1234

Change Password

The password of a user can be changed by the usermod command with the p (pasword) option. Example:

$ usermod myuser -p mypass1234

Passwd Command

The password of a user can be set or changed by the passwd (password) command as