File and user management in Linux command line

Operating system Linux provides the option to work under different user accounts. That means that each user gains access to his own files and programs and settings after login. Different users have different access right, uder which are rights to read, write and run files.

Linux command line Active24 for your hosting allows web console, that you can work in the web browser like Google Chrome. You can use to manage Virtual server (VPS) as well. You can gain access to Linux command line that works independently on operating system.

File sytems

In Linux there is standard that defines that in specific folders are located specific files. Not all Linux distributions and programs strictly follow this system, but mostly this hierarchy is valid:

Path to the file	Description
/ (root)	Root directory that all other directories and files start from, because it contains all other files and directories.
/bin (as binary)	Here are located files of commands that are used by the operating system.
/boot	Here you can find static files of program, that runs the operating system Linux (so called bootloader).
/dev (as device)	Here are located device files. It is similar in directories /proc and /sys. More bellow in file types c and b.
/etc (as et cetera)	Here you an find configuration files of the local computer. For example in file /etc/passwrd are information about users.
/lib (as library)	Here are located basic shared libraries and core modules
/media	To this directories are connected portable media like USB keys. Linux does not recognize disks as Windows does (like disc C:\), but connected hard discs an other input and output devices always connect to a specific file (more bellow in file types c and b), or the content of the device is placed (mounted) under specific directory in this directory (e.g. /media/usb1).
/mnt (as mounted)	Here are connected (mounted) temporarily connected file systems
/opt (as optional)	Additional software packages (optional software)
/sbin (as super-binaries)	Here you can find important system files that usually function only under the root user.
/root	Home directory of super-administrator In Linux the user, that can access and do all, is called root. Login as the root user can be serious safety risk, so to run commands in admin mode in Linux sudo (as superuser do or newer substitute user do) command started to be used .
/home	Home directory for all users except root. User with a name Tibor would have /home/tibor, in which he would have the right to read, write and run files.
/tmp (as temp)	Temporary files. Deleted when system restarts.
/usr (as UNIX System Resources)	Here are located command files that are run by the user. If you need to see a path to the file, that you run after you enter a command, you can use command which (e.g. which ls).
/var (variable)	Variable details like temporary files, system records, shared application files or files of users mailboxes.
/proc (as processes)	Directory that contains information about cores and processes. It is similarly in directories /dev and /sys. More bellow in the types of files c and b.
/run	Here are stored temporary details for system processes.
/srv (as server)	Here are stored server details. E.g. FTP server may have directory /srv/ftp.
/sys	This directory contain information about connected directories. Similarly it is in directories /dev and /proc. More bellow in the types of files c and b.

Types of files

In Linux a file has the utmost role – in Linux everything is a file. In files we can find stored program settings, program itself and even the content created by user. One file can refer to another file or even to a directory which are in Linux just another type of file. In files are stored information about components and computer devices and via those files we are able to communicate with the computer.

In Linux there exist multiple types of files. In this guide we will focus on the most used (first three in the table, that we will create in the directory ~/test in a moment:

mkdir ~/test
cd ~/test
touch standart_file.txt
touch .hidden_file.txt
mkdir directory
ln -s standart_file.txt standart_file-link.txt

More about work with files and directories in Linux command line can be found in the Basic file and directory management in Linux command line

To find out which type of file it is from the first character of a long list of files:

ls -lah

A files and directories will be listed where you can discern the type of a file by the first character on the line:

File type indicator	Description
– (hyphen)	Normal text file. If the file has a dot at the beginning of the string, it means it is hidden and does not appear in the file list, unless the switch -a is used. Hidden are mostly configuration files, because the user usually doesn’t need a direct access to them.
d (as directory)	Directory is in Linux only a type of file, therefore hiding of directories is the same as hiding files
l (as link)	Link. Here a fileobycajny_subor-odkaz.txt links to file obycajny_subor.txt. That means, that only content of file obycajny_subor.txt exists and linking file only takes over the content
c (as character device)	Defines file of the device that communicates in a way, that it sends a character by character. For example generator of random characters: ls -lah /dev/random
b (as block device)	Defines file of the device that communicates in a way that sends characters in blocks (multiple characters at once). For example connection (mounting) of file as a blocked device: ls -lah /dev/loop0
p (as pipe)	Provides a way of one-way communication between two processes.
s (as socket)	Provides a way of two-way communication between two processes.

Managing users

Since we now know the system and file types in Linux, we can move to user management. Users are created with a command useradd:

sudo useradd -m test

We have now created user with a name test (in file /etc/passwd), that is automatically placed to its primary newly created eponymous group test (in file/etc/group). At the same time a home directory has been created home/test (switch -m). We need to create a password for the new user, so he would be able to log in:

sudo passwd test

We enter strong password twice as requested and then we make sure if the user and the group are correctly created:

id test

Numerical and verbal marks are listed:

1. user (uid as user identifier),
2. primary group (gid as group identifier) and
3. all groups (groups), that the user is included in – it here is more, they are separated by comma.

To allow the new user to access system settings with a command sudo, he has to be listed in the file/etc/sudoers. For security reasons this file is not edited in text editor but via commandvisudo like this:

sudo visudo

At the end of the file we will add a line:

test ALL=(ALL:ALL) NOPASSWD: ALL

The change can be saved with a key shortcut Ctrl + X and then by pressing Y a Enter. At that moment the user test can run all commands with admin rights of root without entering password.

This option allows the user to use any command without entering password. In a reality we would recommend to consider password necessity and restrict commands that users can use.

To delete existing user we can use commanduserdel takto:

sudo userdel test

We have just deleted usertest from file /etc/passwd and also a grouptest from file /etc/group. If it is necessary, we can delete the record about the deleted user from file /etc/sudoers with a commandsudo visudo.

If you need to create a new group (first line), to which we want to include a user (second line), we will use commandgroupadd (adds a record at the end of the file /etc/group) and in a case of deleting a group (third line) use commandgroupdel (deletes the record from the file/etc/group and /etc/passwd):

sudo groupadd group_name

sudo usermod -a -G group_name test

sudo groupdel group_name

Managing file ownership and rights

Each file in Linux belongs to a specific user and group. To which user and group the specific file belongs can be checked with a command ls:

ls -lah ~/test/standard_file.txt

Third and fourth column contain a name of the user and the group – under this account and group the file belong ~/test/obycajny_subor.txt. By a commandchown (as change owner) we can change the owner of the file (first line) and by command chgrp (as change group) group (second line):

sudo chown test ~/test/standard_file.txt

sudo chgrp test ~/test/standard_file.txt

Except that, the user and a group own the file, they possess certain rights to that file. They are listed in the first column from the second character:

• first three characters show rights for user (u as user), that owns the file
• three characters in the middle show rights to the group (g as group), that owns the file
• last three characters show rights to all others (o as others)
• letter a (as all) shows rights for all previously mentioned groups (u, g, o)

Mentioned three characters are represented as follows:

• letter r (as read) or number 4 represents reading of the content of the file
• letter w (as write) or number 2 represents writing to the content of the file
• letter x (as execute) or number 1 represents a right to run the file’s content
• hyphen (–) or nuámber 0 represents no rights
• there are some special rights for files (s a t), you can read about them in this guide

If your add the numerical rights together, we will get these combinations and single digit rights:

• 4 + 2 + 1 = 7 = rwx, the number 7 means that the user, group or other we assign all rights (read, write and execute)
• 4 + 2 = 6 = rw-, the number 6 represents that the user, group or other we assign rights to read and write
• 4 + 1 = 5 = r-x, the number 5 represents that the user, group or other we assign rights to read and execute
• 2 + 1 = 3 = –wx, the number 3 represents that the user, group or other we assign rights to write and execute

If you need to fill in missing right of the file for a user (u), group (g), others (o) or all of them (a) use the symbol of add (first line). If the existing right are removed, we will use the symbol minus (second line). If we rewrite a right with another (third line) we use equals. We use a command chmod (as change mode):

chmod u+x,g+x,o+w ~/test/standard_file.txt

chmod a-x ~/test/standard_file.txt

chmod a=rwx ~/test/standard_file.txt

If we use numerical marks for file rights, we use the rights to read, write or execute for a user (number 7), right to read and write for a group (number 6) and write to read for others (number 4):

chmod 764 ~/test/standard_file.txt

During setting file rights it is necessary to choose, if we want to mark rights numerically or verbally. The combination is not possible.

Summary

To responsibly manage rights for file it is necessary to understand the file system and types of files. That is why in this guide we have continuously unveiled and describe to what directories Linux stores specific files and what type of files Linux recognizes. At the end we learned how to change owner and group of files and we have also illustrated change of rights of users and groups and others.