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Basic Linux Commands

Basic Linux Commands
Hostman Team
Technical writer
Linux
12.07.2024
Reading time: 9 min
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Linux is an open-source operating system commonly used in server solutions that require high reliability, fault tolerance, and customization flexibility. To interact with a Linux-based OS, you execute commands in a console terminal, especially when managing remote hosts such as cloud servers.

This guide will cover the most basic terminal commands necessary for working with most known Linux distributions:

  • Debian

  • Ubuntu

  • Alt Linux

  • Kali Linux

This guide uses Ubuntu 22.04, installed on a Hostman cloud server.

Basic Linux commands vary in their purposes but  are most commonly needed for the following tasks:

  • Managing files and directories: creating, deleting, moving, copying, and changing permissions of files and directories.

  • System administration: managing processes, installing, and removing programs.

  • Managing network: managing network connections, checking network status, and configuring network interfaces.

Command Input and Output Streams

Before diving into the commands, it is important to understand a few basic principles of how Linux handles data.

Every process (program) in Linux has three standard data streams:

  • stdin (number 0): input stream.

  • stdout (number 1): output stream.

  • stderr (number 2): error stream.

The most basic are stdin and stdout.

Using standard data streams, Linux allows you to build data processing pipelines. In other words, instead of displaying the application's (launched by a command) result in the console, you can pass the data as input to another application, and so on.

For example, a trivial pipeline might look like this:

ls .. | grep 32

Output:

lib32
libx32

First, we execute the ls command, which displays a list of all system files and folders, and then pass its output to the grep command, which searches for directories whose names contain 32.

Environment, Directories, and File Management

This section covers the basic commands in Linux for navigating directories and managing files.

Current Directory Address (pwd)

To see which directory you are currently in, use:

pwd

For example, if you are in the /root directory, the console will display:

/root

Changing to a Specific Directory (cd)

Instead of specifying absolute paths when executing commands, it may be convenient to manually navigate between directories:

cd /usr

In this example, we have navigated to the system directory /usr. You can verify this by explicitly requesting the current directory:

pwd

Output:

/usr

To return to the root directory, execute the cd command without specifying a path:

cd

Let's navigate to another system directory:

cd /sys/devices

To go up one level, use two dots (..):

cd ..

Now we are in the /sys directory.

Listing Files and Directories in the Current Directory (ls)

To explore the contents of the file system, you can request a list of files and directories in the current directory by using the ls command in Linux.

ls

Output:

bin  games  include  lib  lib32  lib64  libexec  libx32  local  sbin  share  src

To see hidden files and directories, add the -A flag:

ls -A

Output:

.ansible  ._history  .rc  .cache  .lesshst  .profile  resize.log  snap  .ssh

A similar command that provides a bit more information about the file system contents, adding pointers to the current and higher levels, is:

ls -a

Output:

.  ..  .ansible  ._history  .rc  .cache  .lesshst  .profile  resize.log  snap  .ssh

You can check the contents of a directory without navigating into it:

ls /var

The console output will contain a list of files and directories of the specified directory, not the user's current location:

backups  cache  crash  lib  local  lock  log  mail  opt  run  snap  spool  tmp

To request a list of all system files and directories, add a slash (/) or two dots (..):

ls /

Or:

ls ..

Creating a Directory (mkdir)

When creating a directory, specify its name:

mkdir mywork

Verify that the directory was created:

ls

Output will show the new directory among others:

mywork  resize.log  snap

Directories can be created recursively (one inside another) using the -p flag:

mkdir myjob/inner -p

Check for the nested directory:

ls ./myjob

Output:

inner

The -p flag also prevents an error when creating an existing directory. For example, try creating the directory again in the usual way:

mkdir mywork

The console will display an error:

mkdir: cannot create directory 'mywork': File exists

However, with the -p flag, there will be no error:

mkdir mywork -p

Creating a File (nano)

Creating a file and filling it with content is done through the console text editor nano:

sudo nano myfile

The console terminal will switch to text editing mode. After finishing work with the file, press Ctrl + S to save it and Ctrl + X to close it.

The content of the created file be as follows:

My text

Copying a File or Directory (cp)

The cp command in Linux is used for copying files and directories. Copying creates an exact copy of the file or directory to the specified address:

cp myfile myfile2

Check the file system:

ls

Output shows our new file:

myfile  myfile2  myjob  mywork  resize.log  snap

Check its content:

sudo nano myfile2

It is identical to the content of the original file:

My text

Copying directories requires specifying the recursive execution flag -r:

cp mywork mywork2 -r

Check the current directory:

ls

The directory was successfully copied:

myfile  myfile2  myjob  mywork  mywork2  resize.log  snap

Deleting a File or Directory (rm)

Let's delete the previously created file:

rm myfile2

To delete directories, specify the -r flag:

rm mywork2 -r

You can also use a special command to delete a directory:

rmdir mywork2

Moving a File or Directory (mv)

Moving a file is similar to copying it, but the original file is deleted:

mv myfile mywork

The file will be located in the specified directory.

Moving directories is no different from moving files:

mv myjob mywork

Quick File Content Viewing (cat)

Instead of using the nano editor, you can immediately display the content of a file in the console using the Linux cat command:

cat mywork/myfile

The console will display the following:

My text

System and Network Information

Basic commands to display system data.

System Clock (date)

You can request the system date and time through the console terminal:

date

Output:

Sun Jul 07 09:27:16 PM BST 2024

List of Active Users (w)

You can request a list of all users currently logged into the system:

w

The console output will be similar to this:

05:00:30 up 40 min,  1 user,  load average: 0.02, 0.01, 0.00
USER     TTY      FROM             LOGIN@   IDLE   JCPU   PCPU WHAT
root     pts/0    91.206.179.207   04:51    1.00s  0.09s  0.00s w

List of Active Processes (ps)

You can request a list of running processes:

ps

Output:

PID TTY          TIME CMD
11643 pts/1    00:00:00 
11671 pts/1    00:00:00 ps

Connecting to a Remote Server via SSH (ssh)

ssh is a frequently used Linux command for managing remote hosts through an SSH connection:

ssh [email protected]

The command above follows this structure:

ssh USER@IP

Replace USER with the remote user's name and IP with the remote host's IP address. The console terminal will then ask for the root password:

[email protected]'s password:

Downloading Files via URL (wget)

Often, some programs are manually downloaded from remote repositories as regular files:

wget https://github.com/nginx/nginx/archive/refs/tags/release-1.25.4.tar.gz

In this example, we download an archive with the Nginx web server from the official GitHub repository. After that, you can check the current directory's status with the ls command to see that the downloaded archive has appeared.

Extracting Archives (tar)

Next you will need to extract files from the downloaded archive:

tar -xvf release-1.25.4.tar.gz

The flags indicate how to perform the extraction:

  • -x means extracting compressed files from the archive;

  • -v means displaying detailed information about the extraction process in the console;

  • -f means that the passed parameters are archive file names.

After extraction, a new folder with the extracted archive's name will appear in the current directory.

After extraction, you can remove the archive.

rm release-1.25.4.tar.gz

Installing and Removing Packages

Commands for managing Linux packages.

Updating the Repository List

Linux distributions have a standard package manager, apt. It is used by default in Debian and Ubuntu distributions. Typically, before using it, update the list of available repositories:

sudo apt update

It also makes sense to update already installed packages in the system:

sudo apt upgrade

You can also view the list of already installed packages in the system:

sudo apt list --installed

The console will display something like this:

Powershell Hr Xj Iijh Lv

Installing a Package

Packages are installed as follows:

sudo apt install nginx

For example, this way we installed the Nginx web server. Very often, during installation, the package manager asks additional questions, which can be answered with yes or no using the console inputs y and n. To have APT automatically answer yes to all questions during installation, add the -y flag:

sudo apt install nginx -y

Removing a Package

To remove a Linux package:

sudo apt remove nginx

Console Management

Commands for managing the console terminal.

Command History (history)

You can view the history of commands entered in the console terminal:

history

Clearing the Console Terminal (clear)

You can periodically clear the commands entered in the terminal:

clear

This will return the command line to its initial (clean) state.

Command Help (man)

You can always get help on any Linux command:

man ls

The console will display information about the command's structure, its possible flags, and parameters.

You can also obtain a shorter and less extensive version of the manual for as follows:

help

Conclusion

In this guide, we discussed the most frequently used commands in Linux. You can use it as a Linux commands cheat sheet, as these are the most basic commands that any user needs to know when working with Linux.

The official Linux kernel project website provides a complete list of commands and official documentation (including command guides, installation, and configuration guides).

Linux
12.07.2024
Reading time: 9 min
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Bash Regular Expressions

One of the core principles of Unix systems is the extensive use of text data: configuration files, as well as input and output data in *nix systems, are often organized as plain text. Regular expressions are a powerful tool for manipulating text data. This guide delves into the intricacies of using regular expressions in Bash, helping you fully harness the power of the command line and scripts in Linux. What Are Regular Expressions? Regular expressions are specially formatted strings used to search for character patterns in text. They resemble shell wildcards in some ways, but their capabilities are much broader. Many text-processing utilities in Linux and programming languages include a regular expression engine. However, different programs and languages often employ different regular expression dialects. This article focuses on the POSIX standard to which most Linux utilities adhere. The grep Utrequires at least one match of theility The grep program is the primary tool for working with regular expressions. grep reads data from standard input, searches for matches to a specified pattern, and outputs all matching lines. grep is typically pre-installed on most distributions. You can try the commands in a virtual machine or a VPS to practice using regular expressions. The syntax of grep is as follows: grep [options] regular_expression [file...] The simplest use case for grep is finding lines that contain a fixed substring. In the example below, grep outputs all lines that contain the sequence nologin: grep nologin /etc/passwd Output: daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin bin:x:2:2:bin:/bin:/usr/sbin/nologin sys:x:3:3:sys:/dev:/usr/sbin/nologin games:x:5:60:games:/usr/games:/usr/sbin/nologin ... grep has many options, which are detailed in the documentation. Here are some useful options for working with regular expressions: -v — Inverts the match criteria. With this option, grep outputs lines that do not contain matches: ls /bin | grep -v zip# Output:411toppm 7z 7za 7zr ... -i — Ignores case. -o — Outputs only the matches, not the entire lines: ls /bin | grep -o zip# Output:zip zip zip zip ... -w — Searches for lines containing whole words matching the pattern. ls /bin | grep -w zip# Output:gpg-zipzip For comparison, the same command without the -w option also includes lines where the pattern appears as a substring within a word. ls /bin | grep zip# Output:bunzip2 bzip2 bzip2recover funzip Basic Regular Expressions (BRE) As previously mentioned, there are multiple dialects of regular expressions. The POSIX standard defines two main types of implementations: Basic Regular Expressions (BRE), which are supported by almost all POSIX-compliant programs, and Extended Regular Expressions (ERE), which allow for more complex patterns but aren't supported by all utilities. We'll start by exploring the features of BRE. Metacharacters and Literals We've already encountered simple regular expressions. For example, the expression “zip” represents a string with the following criteria: it must contain at least three characters; it includes the characters “z”, “i”, and “p” in that exact order; and there are no other characters in between. Characters that match themselves (like “z”, “i”, and “p”) are called literals. Another category is metacharacters, which are used to define various search criteria. Metacharacters in BRE include: ^ $ . [ ] * \ - To use a metacharacter as a literal, you need to escape it with a backslash (\). Note that some metacharacters have special meanings in the shell, so enclose it in quotes when passing a regular expression as a command argument. Any Character The dot (.) metacharacter matches any character in that position. 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Here’s an example of using a character class: ls ~ | grep '[[:upper:]].*' Output: Books Desktop Documents Downloads GNS3 GOG Games Learning Music ... Extended Regular Expressions (ERE) Most POSIX-compliant applications and those using BRE (such as grep and the stream editor sed) support the features discussed above. The POSIX ERE standard allows for more expressive regular expressions, though not all programs support it. The egrep program traditionally supported the ERE dialect, but the GNU version of grep also supports ERE when run with the -E option. In ERE, the set of metacharacters is expanded to include: ( ) { } ? + | Alternation Alternation allows for a match with one of multiple expressions. Similar to square brackets that allow a character to match one of several characters, alternation allows for matching one of multiple strings or regular expressions. 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01 November 2024 · 10 min to read
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How to Use DNF to Manage Packages on CentOS

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How to Use the diff Command in Linux

The diff command in Linux is a powerful tool that allows users to compare files and directories. With the help of this command, one can identify differences between files, and perform tasks like code reviews, configuration management, and version control.  This tutorial will guide users through what is the diff command, its possible methods, and practical examples. Introduction The diff command is used in Linux to compare the content of two files line by line. When executed, this command analyzes the two files and outputs the differences in a specific format. The output shows which lines need to be added, deleted, or changed to make the files identical. Basic Syntax and Options for diff The basic syntax for the Linux diff command is provided below: diff [options] file1 file2 Here, diff is the command itself. [options] are optional flags used to modify the behavior of the diff Linux command. file1 and file2 are the two files used for Linux file comparison. The following table describes a few options that can be used with diff: Option Description -a Process every file as a text file and perform a line-by-line comparison. -b Does not consider white space differences. -c Show differences with a few lines of context around them. -d Opt for a different algorithm to pinpoint a more concise set of changes. -e Output an ed script. -E Ignore changes due to tab expansion. --binary Compare files in binary mode. -i Ignore case differences in file contents. -l Paginate the output through pr. -N Treat absent files as empty. -q Report only when files differ. -s Report when files are identical. -u Display output in a unified format, showing differences more compactly. -w Ignore all white space. For more details and to explore more options, the users can get help by opening the diff manual using the following command: man diff Comparing Two Text Files Using diff There are two ways to compare files on Linux with diff. Basic Comparison of Two Text Files The basic way to use the diff in Linux is to compare two files line by line and display their differences. To compare two text files, file1.txt and file2.txt, one can use the following command: diff file1.txt file2.txt This command will output the differences between file1.txt and file2.txt. Display Differences in a Unified Format For a more readable format, the -u option can be used with diff. This option provides a unified format that includes a few lines of context around the differences. This makes it easier to understand the changes. Follow the command provided below: diff -u file1.txt file2.txt The unified format output includes line numbers, context lines, and change indicators. Lines starting with - indicate deletions, lines starting with + indicate additions and lines starting with a space are unchanged context lines. Using diff for Directory Comparisons The Linux command diff can also be used to compare directories, it can be done using the -r option. For example: diff -r dir1 dir2 The above command when executed will recursively compare all files and subdirectories within dir1 and dir2. Understanding diff Output and Symbols The diff output uses specific symbols to indicate changes, these are provided below: ---: Denotes the first file. +++: Denotes the second file.  @@ -1,4 +1,4 @@: This line is part of the unified diff format. It gives context about where the changes are happening in the files. @@ indicates the start of a change hunk. -1,4 means the chunk starts at line 1 in the first file and spans 4 lines. +1,4 means the chunk starts at line 1 in the second file and spans 4 lines. <: This marker signifies lines that exist in the first file but not in the second one. Such lines must be removed from the first file to match the second file exactly. >: This marker indicates lines that are in the second file but not in the first one. These lines should be added to the first file to make it identical to the second file. -: This marker shows lines that have been deleted from the first file. +: This marker indicates lines that have been inserted into the second file. Let’s look at an example to make it clearer. Suppose there are two files, file1.txt and file2.txt. Contents of file1.txt: applebananacherrydate Contents of file2.txt: applebananadateraspberry Running the command diff file1.txt file2.txt will produce the following output: Here’s how to interpret this output: 3d2: This means that line 3 in file1.txt (cherry) needs to be deleted to match file2.txt. The d stands for "delete". < cherry: This indicates that cherry is present in file1.txt but not in file2.txt. 4a4: This means that after line 4 in file1.txt, users need to add "raspberry" to match file2.txt. The a stands for "add". > raspberry: This indicates that raspberry is present in file2.txt but not in file1.txt. Creating Patch Files with diff To create a patch file, the -u (unified) option is used, which provides a more readable format by showing a few lines of context around the changes. The output is then redirected to a file, typically with a .patch extension. For example: diff -u file1.txt file2.txt > changes.patch diff -u: Compares file1.txt and file2.txt and generates a unified diff. >: Redirects the output to a file named changes.patch. To apply the patch, use the patch command like this: patch file1.txt < changes.patch Using diff with Various Output Formats The diff also supports multiple output formats, here are a few examples. Unified Format This format gives users a snapshot of the changes with a few lines of context before and after each change. It’s great for quickly seeing what was added or removed. diff -u file1.txt file2.txt Context Format This format shows more surrounding lines for each change and gives users a bigger picture of where the changes happened. diff -c file1.txt file2.txt Side-by-Side Format This format places the two files next to each other and makes it easy to compare them line by line. diff -y file1.txt file2.txt Brief Format This format gives a summary of whether the files differ but does not show the actual changes. diff -q file1.txt file2.txt Practical Examples of Using diff Here are some practical examples of using the diff command in Linux. Ignoring Case Differences When comparing files, sometimes the case of the letters might differ, but the content is essentially the same. The -i option is used to ignore case differences. For example: diff -i file3.txt file4.txt In this example, diff will treat "Hello" and "hello" as identical, ignoring the case difference. Ignoring White Space White space differences, such as extra spaces or tabs, can be ignored using the -w option. This is useful when formatting changes have been made but the content remains the same. For example: diff -w file1.txt file2.txt Here, diff will ignore all white spaces, treating "Hello   World" and "Hello World" as identical. Comparing Binary Files The diff in Linux can also be used to compare binary files using the --binary option. This is helpful when users need to check if two binary files are identical or not. For example: diff --binary file1.bin file2.bin In this case, diff will compare the binary data of file1.bin and file2.bin and report any differences. Ignoring Blank Lines To ignore blank lines when comparing files, simply use the -B option, which is useful when blank lines have been added or removed. diff -B file1.txt file2.txt Conclusion The diff is a versatile command in Linux for comparing files and directories. By understanding its syntax, options, and output formats, users can efficiently identify differences and manage changes. Whether for code reviews, configuration management, or version control, the diff command is an essential part of any Linux user’s toolkit. On Hostman, you can try Linux VPS hosting for your projects. 
17 October 2024 · 7 min to read

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