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courses/level101/linux_basics/command_line_basics.md
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@@ -2,60 +2,52 @@
## Lab Environment Setup
One can use an online bash interpreter to run all the commands that are provided as examples in this course. This will also help you in getting a hands-on experience of various linux commands.
One can use an online Bash interpreter to run all the commands that are provided as examples in this course. This will also help you in getting a hands-on experience of various Linux commands.
[REPL](https://repl.it/languages/bash) is one of the popular online bash interpreters for running linux commands. We will be using it for running all the commands mentioned in this course.
[REPL](https://repl.it/languages/bash) is one of the popular online Bash interpreters for running Linux commands. We will be using it for running all the commands mentioned in this course.
## What is a Command
A command is a program that tells the operating system to perform
specific work. Programs are stored as files in linux. Therefore, a
specific work. Programs are stored as files in Linux. Therefore, a
command is also a file which is stored somewhere on the disk.
Commands may also take additional arguments as input from the user.
These arguments are called command line arguments. Knowing how to use
the commands is important and there are many ways to get help in Linux,
especially for commands. Almost every command will have some form of
documentation, most commands will have a command-line argument -h or
\--help that will display a reasonable amount of documentation. But the
most popular documentation system in Linux is called man pages - short
documentation, most commands will have a command-line argument `-h` or
`--help` that will display a reasonable amount of documentation. But the
most popular documentation system in Linux is called `man` pages—short
for manual pages.
Using \--help to show the documentation for ls command.
Using `--help` to show the documentation for `ls` command.
![](images/linux/commands/image19.png)
## File System Organization
The linux file system has a hierarchical (or tree-like) structure with
its highest level directory called root ( denoted by / ). Directories
present inside the root directory stores file related to the system.
The Linux file system has a hierarchical (or tree-like) structure with
its highest-level directory called `root` (denoted by `/`). Directories
present inside the root directory stores files related to the system.
These directories in turn can either store system files or application
files or user related files.
files or user-related files.
![](images/linux/commands/image17.png)
bin | The executable program of most commonly used commands reside in bin directory
dev | This directory contains files related to devices on the system
etc | This directory contains all the system configuration files
home | This directory contains user related files and directories.
lib | This directory contains all the library files
mnt | This directory contains files related to mounted devices on the system
proc | This directory contains files related to the running processes on the system
root | This directory contains root user related files and directories.
sbin | This directory contains programs used for system administration.
tmp | This directory is used to store temporary files on the system
usr | This directory is used to store application programs on the system
| Directory | Description |
|------------|--------------------------------------------------------------------------------|
| bin | The executable program of most commonly used commands reside in `bin` directory|
| dev | This directory contains files related to devices on the system |
| etc | This directory contains all the system configuration files |
| home | This directory contains user-related files and directories |
| lib | This directory contains all the library files |
| mnt | This directory contains files related to mounted devices on the system |
| proc | This directory contains files related to the running processes on the system |
| root | This directory contains root user-related files and directories |
| sbin | This directory contains programs used for system administration |
| tmp | This directory is used to store temporary files on the system |
| usr | This directory is used to store application programs on the system |
## Commands for Navigating the File System
@@ -70,42 +62,42 @@ file system:
We will now try to understand what each command does and how to use
these commands. You should also practice the given examples on the
online bash shell.
online Bash shell.
### pwd (print working directory)
At any given moment of time, we will be standing in a certain directory.
To get the name of the directory in which we are standing, we can use
the pwd command in linux.
the `pwd` command in Linux.
![](images/linux/commands/image2.png)
We will now use the cd command to move to a different directory and then
We will now use the `cd` command to move to a different directory and then
print the working directory.
![](images/linux/commands/image20.png)
### cd (change directory)
The cd command can be used to change the working directory. Using the
The `cd` command can be used to change the working directory. Using the
command, you can move from one directory to another.
In the below example, we are initially in the root directory. we have
then used the cd command to change the directory.
In the below example, we are initially in the `root` directory. We have
then used the `cd` command to change the directory.
![](images/linux/commands/image3.png)
### ls (list files and directories)**
The ls command is used to list the contents of a directory. It will list
The `ls` command is used to list the contents of a directory. It will list
down all the files and folders present in the given directory.
If we just type ls in the shell, it will list all the files and
If we just type `ls` in the shell, it will list all the files and
directories present in the current directory.
![](images/linux/commands/image7.png)
We can also provide the directory name as argument to ls command. It
We can also provide the directory name as argument to `ls` command. It
will then list all the files and directories inside the given directory.
![](images/linux/commands/image4.png)
@@ -127,17 +119,17 @@ files:
We will now try to understand what each command does and how to use
these commands. You should also practice the given examples on the
online bash shell.
online Bash shell.
### touch (create new file)
The touch command can be used to create an empty new file.
This command is very useful for many other purposes but we will discuss
The `touch` command can be used to create an empty new file.
This command is very useful for many other purposes, but we will discuss
the simplest use case of creating a new file.
General syntax of using touch command
General syntax of using `touch` command:
```
```shell
touch <file_name>
```
@@ -145,12 +137,12 @@ touch <file_name>
### mkdir (create new directories)
The mkdir command is used to create directories.You can use ls command
The `mkdir` command is used to create directories. You can use `ls` command
to verify that the new directory is created.
General syntax of using mkdir command
General syntax of using `mkdir` command:
```
```shell
mkdir <directory_name>
```
@@ -158,90 +150,90 @@ mkdir <directory_name>
### rm (delete files and directories)
The rm command can be used to delete files and directories. It is very
The `rm` command can be used to delete files and directories. It is very
important to note that this command permanently deletes the files and
directories. It's almost impossible to recover these files and
directories once you have executed rm command on them successfully. Do
directories once you have executed `rm` command on them successfully. Do
run this command with care.
General syntax of using rm command:
General syntax of using `rm` command:
```
```shell
rm <file_name>
```
Let's try to understand the rm command with an example. We will try to
delete the file and directory we created using touch and mkdir command
Let's try to understand the `rm` command with an example. We will try to
delete the file and directory we created using `touch` and `mkdir` command
respectively.
![](images/linux/commands/image18.png)
### cp (copy files and directories)
The cp command is used to copy files and directories from one location
to another. Do note that the cp command doesn't do any change to the
The `cp` command is used to copy files and directories from one location
to another. Do note that the `cp` command doesn't do any change to the
original files or directories. The original files or directories and
their copy both co-exist after running cp command successfully.
their copy both co-exist after running `cp` command successfully.
General syntax of using cp command:
General syntax of using `cp` command:
```
```shell
cp <source_path> <destination_path>
```
We are currently in the '/home/runner' directory. We will use the mkdir
command to create a new directory named "test_directory". We will now
try to copy the "\_test_runner.py" file to the directory we created just
We are currently in the `/home/runner` directory. We will use the `mkdir`
command to create a new directory named `test_directory`. We will now
try to copy the `_test_runner.py` file to the directory we created just
now.
![](images/linux/commands/image23.png)
Do note that nothing happened to the original "\_test_runner.py" file.
Do note that nothing happened to the original `_test_runner.py` file.
It's still there in the current directory. A new copy of it got created
inside the "test_directory".
inside the `test_directory`.
![](images/linux/commands/image14.png)
We can also use the cp command to copy the whole directory from one
We can also use the `cp` command to copy the whole directory from one
location to another. Let's try to understand this with an example.
![](images/linux/commands/image12.png)
We again used the mkdir command to create a new directory called
"another_directory". We then used the cp command along with an
additional argument '-r' to copy the "test_directory".
We again used the `mkdir` command to create a new directory called
`another_directory`. We then used the `cp` command along with an
additional argument `-r` to copy the `test_directory`.
**mv (move files and directories)**
The mv command can either be used to move files or directories from one
The `mv` command can either be used to move files or directories from one
location to another or it can be used to rename files or directories. Do
note that moving files and copying them are very different. When you
move the files or directories, the original copy is lost.
General syntax of using mv command:
General syntax of using `mv` command:
```
```shell
mv <source_path> <destination_path>
```
In this example, we will use the mv command to move the
"\_test_runner.py" file to "test_directory". In this case, this file
already exists in "test_directory". The mv command will just replace it.
In this example, we will use the `mv` command to move the
`_test_runner.py` file to `test_directory`. In this case, this file
already exists in `test_directory`. The `mv` command will just replace it.
**Do note that the original file doesn't exist in the current directory
after mv command ran successfully.**
after `mv` command ran successfully.**
![](images/linux/commands/image26.png)
We can also use the mv command to move a directory from one location to
another. In this case, we do not need to use the '-r' flag that we did
while using the cp command. Do note that the original directory will not
exist if we use mv command.
We can also use the `mv` command to move a directory from one location to
another. In this case, we do not need to use the `-r` flag that we did
while using the `cp` command. Do note that the original directory will not
exist if we use `mv` command.
One of the important uses of the mv command is to rename files and
One of the important uses of the `mv` command is to rename files and
directories. Let's see how we can use this command for renaming.
We have first changed our location to "test_directory". We then use the
mv command to rename the ""\_test_runner.py" file to "test.py".
We have first changed our location to `test_directory`. We then use the
`mv` command to rename the `_test_runner.py` file to `test.py`.
![](images/linux/commands/image29.png)
@@ -262,9 +254,9 @@ files:
We will now try to understand what each command does and how to use
these commands. You should also practice the given examples on the
online bash shell.
online Bash shell.
We will create a new file called "numbers.txt" and insert numbers from 1
We will create a new file called `numbers.txt` and insert numbers from 1
to 100 in this file. Each number will be in a separate line.
![](images/linux/commands/image21.png)
@@ -277,7 +269,7 @@ later sections.
### cat
The most simplest use of cat command is to print the contents of the file on
The most simplest use of `cat` command is to print the contents of the file on
your output screen. This command is very useful and can be used for many
other purposes. We will study about other use cases later.
@@ -289,63 +281,63 @@ all the numbers.
### head
The head command displays the first 10 lines of the file by default. We
The `head` command displays the first 10 lines of the file by default. We
can include additional arguments to display as many lines as we want
from the top.
In this example, we are only able to see the first 10 lines from the
file when we use the head command.
file when we use the `head` command.
![](images/linux/commands/image15.png)
By default, head command will only display the first 10 lines. If we
By default, `head` command will only display the first 10 lines. If we
want to specify the number of lines we want to see from start, use the
'-n' argument to provide the input.
`-n` argument to provide the input.
![](images/linux/commands/image16.png)
### tail
The tail command displays the last 10 lines of the file by default. We
The `tail` command displays the last 10 lines of the file by default. We
can include additional arguments to display as many lines as we want
from the end of the file.
![](images/linux/commands/image22.png)
By default, the tail command will only display the last 10 lines. If we
want to specify the number of lines we want to see from the end, use '-n'
By default, the `tail` command will only display the last 10 lines. If we
want to specify the number of lines we want to see from the end, use `-n`
argument to provide the input.
![](images/linux/commands/image10.png)
In this example, we are only able to see the last 5 lines from the file
when we use the tail command with explicit -n option.
when we use the `tail` command with explicit `-n` option.
### more
More command displays the contents of a file or a command output,
The `more` command displays the contents of a file or a command output,
displaying one screen at a time in case the file is large (Eg: log files).
It also allows forward navigation and limited backward navigation in the file.
![](images/linux/commands/image33.png)
More command displays as much as can fit on the current screen and waits for user input to advance. Forward navigation can be done by pressing Enter, which advances the output by one line and Space, which advances the output by one screen.
The `more` command displays as much as can fit on the current screen and waits for user input to advance. Forward navigation can be done by pressing `Enter`, which advances the output by one line and `Space`, which advances the output by one screen.
### less
Less command is an improved version of more. It displays the contents of a file or a command output, one page at a time.
It allows backward navigation as well as forward navigation in the file and also has search options. We can use arrow keys for advancing backward or forward by one line. For moving forward by one page, press Space and for moving backward by one page, press b on your keyboard.
The `less` command is an improved version of `more`. It displays the contents of a file or a command output, one page at a time.
It allows backward navigation as well as forward navigation in the file and also has search options. We can use `arrow keys` for advancing backward or forward by one line. For moving forward by one page, press `Space` and for moving backward by one page, press `b` on your keyboard.
You can go to the beginning and the end of a file instantly.
## Echo Command in Linux
The echo command is one of the simplest commands that is used in the
shell. This command is equivalent to what we have <print> in other
The `echo` command is one of the simplest commands that is used in the
shell. This command is equivalent to `print` in other
programming languages.
The echo command prints the given input string on the screen.
The `echo` command prints the given input string on the screen.
![](images/linux/commands/image34.png)
@@ -371,61 +363,61 @@ texts:
We will now try to understand what each command does and how to use
these commands. You should also practice the given examples on the
online bash shell.
online Bash shell.
We will create a new file called "numbers.txt" and insert numbers from 1
We will create a new file called `numbers.txt` and insert numbers from 1
to 10 in this file. Each number will be in a separate line.
![](images/linux/commands/image8.png)
### grep
The grep command in its simplest form can be used to search particular
The `grep` command in its simplest form can be used to search particular
words in a text file. It will display all the lines in a file that
contains a particular input. The word we want to search is provided as
an input to the grep command.
an input to the `grep` command.
General syntax of using grep command:
General syntax of using `grep` command:
```
```shell
grep <word_to_search> <file_name>
```
In this example, we are trying to search for a string "1" in this file.
The grep command outputs the lines where it found this string.
The `grep` command outputs the lines where it found this string.
![](images/linux/commands/image36.png)
### sed
The sed command in its simplest form can be used to replace a text in a
The `sed` command in its simplest form can be used to replace a text in a
file.
General syntax of using the sed command for replacement:
General syntax of using the `sed` command for replacement:
```
```shell
sed 's/<text_to_replace>/<replacement_text>/' <file_name>
```
Let's try to replace each occurrence of "1" in the file with "3" using
sed command.
`sed` command.
![](images/linux/commands/image31.png)
The content of the file will not change in the above
example. To do so, we have to use an extra argument '-i' so that the
example. To do so, we have to use an extra argument `-i` so that the
changes are reflected back in the file.
### sort
The sort command can be used to sort the input provided to it as an
The `sort` command can be used to sort the input provided to it as an
argument. By default, it will sort in increasing order.
Let's first see the content of the file before trying to sort it.
![](images/linux/commands/image27.png)
Now, we will try to sort the file using the sort command. The sort
Now, we will try to sort the file using the `sort` command. The `sort`
command sorts the content in lexicographical order.
![](images/linux/commands/image32.png)
@@ -437,11 +429,11 @@ example.
Each open file gets assigned a file descriptor. A file descriptor is an
unique identifier for open files in the system. There are always three
default files open, stdin (the keyboard), stdout (the screen), and
stderr (error messages output to the screen). These files can be
default files open, `stdin` (the keyboard), `stdout` (the screen), and
`stderr` (error messages output to the screen). These files can be
redirected.
Everything is a file in linux -
Everything is a file in Linux -
[https://unix.stackexchange.com/questions/225537/everything-is-a-file](https://unix.stackexchange.com/questions/225537/everything-is-a-file)
Till now, we have displayed all the output on the screen which is the
@@ -449,12 +441,12 @@ standard output. We can use some special operators to redirect the
output of the command to files or even to the input of other commands.
I/O redirection is a very powerful feature.
In the below example, we have used the '>' operator to redirect the
output of ls command to output.txt file.
In the below example, we have used the `>` operator to redirect the
output of `ls` command to `output.txt` file.
![](images/linux/commands/image30.png)
In the below example, we have redirected the output from echo command to
In the below example, we have redirected the output from `echo` command to
a file.
![](images/linux/commands/image13.png)
@@ -462,13 +454,13 @@ a file.
We can also redirect the output of a command as an input to another
command. This is possible with the help of pipes.
In the below example, we have passed the output of cat command as an
input to grep command using pipe(\|) operator.
In the below example, we have passed the output of `cat` command as an
input to `grep` command using pipe (`|`) operator.
![](images/linux/commands/image6.png)
In the below example, we have passed the output of sort command as an
input to uniq command using pipe(\|) operator. The uniq command only
In the below example, we have passed the output of `sort` command as an
input to `uniq` command using pipe (`|`) operator. The `uniq` command only
prints the unique numbers from the input.
![](images/linux/commands/image28.png)

12
courses/level101/linux_basics/conclusion.md
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@@ -1,6 +1,6 @@
# Conclusion
We have covered the basics of Linux operating systems and basic commands used in linux.
We have covered the basics of Linux operating systems and basic commands used in Linux.
We have also covered the Linux server administration commands.
We hope that this course will make it easier for you to operate on the command line.
@@ -13,12 +13,12 @@ We hope that this course will make it easier for you to operate on the command l
4. `tail` command is very useful to view the latest data in the log file.
5. Different users will have different permissions depending on their roles. We will also not want everyone in the company to access our servers for security reasons. Users permissions can be restricted with `chown`, `chmod` and `chgrp` commands.
6. `ssh` is one of the most frequently used commands for a SRE. Logging into servers and troubleshooting along with performing basic administration tasks will only be possible if we are able to login into the server.
7. What if we want to run an apache server or nginx on a server? We will first install it using the package manager. Package management commands become important here.
8. Managing services on servers is another critical responsibility of a SRE. Systemd related commands can help in troubleshooting issues. If a service goes down, we can start it using `systemctl start` command. We can also stop a service in case it is not needed.
9. Monitoring is another core responsibility of a SRE. Memory and CPU are two important system level metrics which should be monitored. Commands like `top` and `free` are quite helpful here.
10. If a service is throwing an error, how do we find out the root cause of the error ? We will certainly need to check logs to find out the whole stack trace of the error. The log file will also tell us the number of times the error has occurred along with time when it started.
7. What if we want to run an Apache server or NGINX on a server? We will first install it using the package manager. Package management commands become important here.
8. Managing services on servers is another critical responsibility of a SRE. `systemd`-related commands can help in troubleshooting issues. If a service goes down, we can start it using `systemctl start` command. We can also stop a service in case it is not needed.
9. Monitoring is another core responsibility of a SRE. Memory and CPU are two important system-level metrics which should be monitored. Commands like `top` and `free` are quite helpful here.
10. If a service throws an error, how do we find out the root cause of the error? We will certainly need to check logs to find out the whole stack trace of the error. The log file will also tell us the number of times the error has occurred along with time when it started.
## Useful Courses and tutorials
## Useful Courses and Tutorials
* [Edx basic linux commands course](https://courses.edx.org/courses/course-v1:LinuxFoundationX+LFS101x+1T2020/course/)
* [Edx Red Hat Enterprise Linux Course](https://courses.edx.org/courses/course-v1:RedHat+RH066x+2T2017/course/)

46
courses/level101/linux_basics/intro.md
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@@ -3,7 +3,7 @@
## Introduction
### Prerequisites
- Should be comfortable in using any operating systems like Windows, Linux or Mac
- Should be comfortable in using any operating systems like Windows, Linux or
- Expected to have fundamental knowledge of operating systems
## What to expect from this course
@@ -15,17 +15,17 @@ difference between GUI and CLI.
In the second part, we cover some basic commands used in Linux.
We will focus on commands used for navigating the file system, viewing and manipulating files,
I/O redirection etc.
I/O redirection, etc.
In the third part, we cover Linux system administration. This includes day to day tasks
In the third part, we cover Linux system administration. This includes day-to-day tasks
performed by Linux admins, like managing users/groups, managing file permissions,
monitoring system performance, log files etc.
In the second and third part, we will be taking examples to understand the concepts.
In the second and third part, we will be showing examples to understand the concepts.
## What is not covered under this course
We are not covering advanced Linux commands and bash scripting in this
We are not covering advanced Linux commands and Bash scripting in this
course. We will also not be covering Linux internals.
## Course Contents
@@ -69,32 +69,32 @@ Most of us are familiar with the Windows operating system used in more than
75% of the personal computers. The Windows operating systems
are based on Windows NT kernel.
A kernel is the most important part of
an operating system - it performs important functions like process
management, memory management, filesystem management etc.
A _kernel_ is the most important part of
an operating system&mdash;it performs important functions like process
management, memory management, filesystem management, etc.
Linux operating systems are based on the Linux kernel. A Linux based
Linux operating systems are based on the Linux kernel. A Linux-based
operating system will consist of Linux kernel, GUI/CLI, system libraries
and system utilities. The Linux kernel was independently developed and
released by Linus Torvalds. The Linux kernel is free and open-source -
[https://github.com/torvalds/linux](https://github.com/torvalds/linux)
released by Linus Torvalds. The Linux kernel is free and open-source (See
[https://github.com/torvalds/linux](https://github.com/torvalds/linux)).
Linux is a kernel and not a complete operating system. Linux kernel is combined with GNU system to make a complete operating system. Therefore, linux based operating systems are also called as GNU/Linux systems. GNU is an extensive collection of free softwares like compiler, debugger, C library etc.
[Linux and the GNU System](https://www.gnu.org/gnu/linux-and-gnu.en.html)
Linux is a kernel and not a complete operating system. Linux kernel is combined with GNU system to make a complete operating system. Therefore, Linux-based operating systems are also called as GNU/Linux systems. GNU is an extensive collection of free softwares like compiler, debugger, C library etc. (See
[Linux and the GNU System](https://www.gnu.org/gnu/linux-and-gnu.en.html))
History of Linux -
[https://en.wikipedia.org/wiki/History_of_Linux](https://en.wikipedia.org/wiki/History_of_Linux)
## What are popular Linux distributions
A Linux distribution(distro) is an operating system based on
A Linux distribution (_distro_) is an operating system based on
the Linux kernel and a package management system. A package management
system consists of tools that help in installing, upgrading,
configuring and removing softwares on the operating system.
Software are usually adopted to a distribution and are packaged in a
distro specific format. These packages are available through a distro
specific repository. Packages are installed and managed in the operating
distro-specific format. These packages are available through a distro-specific
repository. Packages are installed and managed in the operating
system by a package manager.
**List of popular Linux distributions:**
@@ -116,8 +116,8 @@ system by a package manager.
| Packaging systems | Distributions | Package manager
| ---------------------- | ------------------------------------------ | -----------------
| Debian style (.deb) | Debian, Ubuntu | APT
| Red Hat style (.rpm) | Fedora, CentOS, Red Hat Enterprise Linux | YUM
| Debian style (`.deb`) | Debian, Ubuntu | APT
| Red Hat style (`.rpm`) | Fedora, CentOS, Red Hat Enterprise Linux | YUM
## Linux Architecture
@@ -141,11 +141,11 @@ Operating system based on Linux kernel are widely used in:
- Mobile phones - Android is based on Linux operating system
- Embedded devices - watches, televisions, traffic lights etc
- Embedded devices - watches, televisions, traffic lights, etc.
- Satellites
- Network devices - routers, switches etc.
- Network devices - routers, switches, etc.
## Graphical user interface (GUI) vs Command line interface (CLI)
@@ -172,9 +172,9 @@ programs available on Linux servers. Other popular shell programs are
zsh, ksh and tcsh.
Terminal is a program that opens a window and lets you interact with the
shell. Some popular examples of terminals are gnome-terminal, xterm,
konsole etc.
shell. Some popular examples of terminals are GNOME-terminal, xterm,
Konsole, etc.
Linux users do use the terms shell, terminal, prompt, console etc.
Linux users do use the terms shell, terminal, prompt, console, etc.
interchangeably. In simple terms, these all refer to a way of taking
commands from the user.

279
courses/level101/linux_basics/linux_server_administration.md
View File

@@ -1,6 +1,6 @@
# Linux Server Administration
In this course will try to cover some of the common tasks that a linux
In this course, will try to cover some of the common tasks that a Linux
server administrator performs. We will first try to understand what a
particular command does and then try to understand the commands using
examples. Do keep in mind that it's very important to practice the Linux
@@ -8,7 +8,7 @@ commands on your own.
## Lab Environment Setup
- Install docker on your system - [https://docs.docker.com/engine/install/](https://docs.docker.com/engine/install/) OR you can used online [Docker playground](https://labs.play-with-docker.com/)
- Install docker on your system - [https://docs.docker.com/engine/install/](https://docs.docker.com/engine/install/) OR you can use online [Docker playground](https://labs.play-with-docker.com/)
- We will be running all the commands on Red Hat Enterprise Linux (RHEL) 8 system.
@@ -18,7 +18,7 @@ commands on your own.
## Multi-User Operating Systems
An operating system is considered as multi-user if it allows multiple people/users to use a computer and not affect each other's files and preferences. Linux based operating systems are multi-user in nature as it allows multiple users to access the system at the same time. A typical computer will only have one keyboard and monitor but multiple users can log in via SSH if the computer is connected to the network. We will cover more about SSH later.
An operating system is considered as multi-user if it allows multiple people/users to use a computer and not affect each other's files and preferences. Linux-based operating systems are multi-user in nature as it allows multiple users to access the system at the same time. A typical computer will only have one keyboard and monitor but multiple users can log in via SSH if the computer is connected to the network. We will cover more about SSH later.
As a server administrator, we are mostly concerned with the Linux servers which are physically present at a very large distance from us. We can connect to these servers with the help of remote login methods like SSH.
@@ -37,26 +37,28 @@ Since Linux supports multiple users, we need to have a method which can protect
### id command
`id` command can be used to find the uid and gid associated with an user.
`id` command can be used to find the `uid` and `gid` associated with an user.
It also lists down the groups to which the user belongs to.
The uid and gid associated with the root user is 0.
The `uid` and `gid` associated with the root user is 0.
![](images/linux/admin/image30.png)
A good way to find out the current user in Linux is to use the whoami
A good way to find out the current user in Linux is to use the `whoami`
command.
![](images/linux/admin/image35.png)
**"root" user or superuser is the most privileged user with**
**`root` user or superuser is the most privileged user with**
**unrestricted access to all the resources on the system. It has UID 0**
### Important files associated with users/groups
| /etc/passwd | Stores the user name, the uid, the gid, the home directory, the login shell etc |
| -------------| ---------------------------------------------------------------------------------
| /etc/shadow | Stores the password associated with the users |
| /etc/group | Stores information about different groups on the system |
| Files | Description |
|--------------|----------------------------------------------------------------------------------------|
| /etc/passwd | Stores the user name, the `uid`, the `gid`, the home directory, the login shell etc |
| /etc/shadow | Stores the password associated with the users |
| /etc/group | Stores information about different groups on the system |
![](images/linux/admin/image23.png)
@@ -64,7 +66,7 @@ command.
![](images/linux/admin/image9.png)
If you want to understand each filed discussed in the above outputs, you can go
If you want to understand each field discussed in the above outputs, you can go
through below links:
- [https://tldp.org/LDP/lame/LAME/linux-admin-made-easy/shadow-file-formats.html](https://tldp.org/LDP/lame/LAME/linux-admin-made-easy/shadow-file-formats.html)
@@ -77,21 +79,18 @@ Some of the commands which are used frequently to manage users/groups
on Linux are following:
- `useradd` - Creates a new user
- `passwd` - Adds or modifies passwords for a user
- `usermod` - Modifies attributes of an user
- `userdel` - Deletes an user
### useradd
The useradd command adds a new user in Linux.
The `useradd` command adds a new user in Linux.
We will create a new user 'shivam'. We will also verify that the user
has been created by tailing the /etc/passwd file. The uid and gid are
We will create a new user `shivam`. We will also verify that the user
has been created by tailing the `/etc/passwd` file. The `uid` and `gid` are
1000 for the newly created user. The home directory assigned to the user
is /home/shivam and the login shell assigned is /bin/bash. Do note that
is `/home/shivam` and the login shell assigned is `/bin/bash`. Do note that
the user home directory and login shell can be modified later on.
![](images/linux/admin/image41.png)
@@ -104,17 +103,17 @@ override these default values when creating a new user.
### passwd
The passwd command is used to create or modify passwords for a user.
The `passwd` command is used to create or modify passwords for a user.
In the above examples, we have not assigned any password for users
'shivam' or 'amit' while creating them.
`shivam` or `amit` while creating them.
"!!" in an account entry in shadow means the account of an user has
`!!` in an account entry in shadow means the account of an user has
been created, but not yet given a password.
![](images/linux/admin/image13.png)
Let's now try to create a password for user "shivam".
Let's now try to create a password for user `shivam`.
![](images/linux/admin/image55.png)
@@ -129,118 +128,118 @@ Also, when you login using root user, the password will be asked.
### usermod
The usermod command is used to modify the attributes of an user like the
The `usermod` command is used to modify the attributes of an user like the
home directory or the shell.
Let's try to modify the login shell of user "amit" to "/bin/bash".
Let's try to modify the login shell of user `amit` to `/bin/bash`.
![](images/linux/admin/image17.png)
In a similar way, you can also modify many other attributes for a user.
Try 'usermod -h' for a list of attributes you can modify.
Try `usermod -h` for a list of attributes you can modify.
### userdel
The userdel command is used to remove a user on Linux. Once we remove a
The `userdel` command is used to remove a user on Linux. Once we remove a
user, all the information related to that user will be removed.
Let's try to delete the user "amit". After deleting the user, you will
not find the entry for that user in "/etc/passwd" or "/etc/shadow" file.
Let's try to delete the user `amit`. After deleting the user, you will
not find the entry for that user in `/etc/passwd` or `/etc/shadow` file.
![](images/linux/admin/image34.png)
## Important commands for managing groups
Commands for managing groups are quite similar to the commands used for managing users. Each command is not explained in detail here as they are quite similar. You can try running these commands on your system.
Commands for managing groups are quite similar to the commands used for managing users. Each command is not explained in detail here as they are quite similar. You can try running these commands on your system.
| groupadd \<group_name\> | Creates a new group |
| ------------------------ | ------------------------------- |
| groupmod \<group_name\> | Modifies attributes of a group |
| groupdel \<group_name\> | Deletes a group |
| gpasswd \<group_name\> | Modifies password for group |
| Command | Description |
| -----------------------| ------------------------------- |
| groupadd <group_name\> | Creates a new group |
| groupmod <group_name\> | Modifies attributes of a group |
| groupdel <group_name\> | Deletes a group |
| gpasswd <group_name\> | Modifies password for group |
![](images/linux/admin/image52.png)
We will now try to add user "shivam" to the group we have created above.
We will now try to add user `shivam` to the group we have created above.
![](images/linux/admin/image33.png)
## Becoming a Superuser
**Before running the below commands, do make sure that you have set up a
password for user "shivam" and user "root" using the passwd command
password for user `shivam` and user `root` using the `passwd` command
described in the above section.**
The su command can be used to switch users in Linux. Let's now try to
switch to user "shivam".
The `su` command can be used to switch users in Linux. Let's now try to
switch to user `shivam`.
![](images/linux/admin/image37.png)
Let's now try to open the "/etc/shadow" file.
Let's now try to open the `/etc/shadow` file.
![](images/linux/admin/image29.png)
The operating system didn't allow the user "shivam" to read the content
of the "/etc/shadow" file. This is an important file in Linux which
stores the passwords of users. This file can only be accessed by root or
users who have the superuser privileges.
The operating system didn't allow the user `shivam` to read the content
of the `/etc/shadow` file. This is an important file in Linux which
stores the passwords of users. This file can only be accessed by `root` or
users who have the `superuser` privileges.
**The sudo command allows a** **user to run commands with the security
**The `sudo` command allows a** **user to run commands with the security
privileges of the root user.** Do remember that the root user has all
the privileges on a system. We can also use su command to switch to the
the privileges on a system. We can also use `su` command to switch to the
root user and open the above file but doing that will require the
password of the root user. An alternative way which is preferred on most
modern operating systems is to use sudo command for becoming a
modern operating systems is to use `sudo` command for becoming a
superuser. Using this way, a user has to enter his/her password and they
need to be a part of the sudo group.
need to be a part of the `sudo` group.
**How to provide superpriveleges to other users ?**
Let's first switch to the root user using su command. Do note that using
Let's first switch to the root user using `su` command. Do note that using
the below command will need you to enter the password for the root user.
![](images/linux/admin/image44.png)
In case, you forgot to set a password for the root user, type "exit" and
In case, you forgot to set a password for the root user, type `exit` and
you will be back as the root user. Now, set up a password using the
passwd command.
`passwd` command.
**The file /etc/sudoers holds the names of users permitted to invoke
sudo**. In redhat operating systems, this file is not present by
default. We will need to install sudo.
**The file `/etc/sudoers` holds the names of users permitted to invoke
`sudo`**. In Red Hat operating systems, this file is not present by
default. We will need to install `sudo`.
![](images/linux/admin/image3.png)
We will discuss the yum command in detail in later sections.
We will discuss the `yum` command in detail in later sections.
Try to open the "/etc/sudoers" file on the system. The file has a lot of
Try to open the `/etc/sudoers` file on the system. The file has a lot of
information. This file stores the rules that users must follow when
running the sudo command. For example, root is allowed to run any
running the `sudo` command. For example, `root` is allowed to run any
commands from anywhere.
![](images/linux/admin/image8.png)
One easy way of providing root access to users is to add them to a group
which has permissions to run all the commands. "wheel" is a group in
redhat Linux with such privileges.
which has permissions to run all the commands. `wheel` is a group in
Red Hat Linux with such privileges.
![](images/linux/admin/image25.png)
Let's add the user "shivam" to this group so that it also has sudo
Let's add the user `shivam` to this group so that it also has `sudo`
privileges.
![](images/linux/admin/image48.png)
Let's now switch back to user "shivam" and try to access the
"/etc/shadow" file.
Let's now switch back to user `shivam` and try to access the
`/etc/shadow` file.
![](images/linux/admin/image56.png)
We need to use sudo before running the command since it can only be
accessed with the sudo privileges. We have already given sudo privileges
to user “shivam” by adding him to the group “wheel”.
We need to use `sudo` before running the command since it can only be
accessed with the `sudo` privileges. We have already given `sudo` privileges
to user `shivam` by adding him to the group `wheel`.
## File Permissions
@@ -250,8 +249,8 @@ permissions for the owner of the file, the members of a group of related
users and everybody else. This is to make sure that one user is not
allowed to access the files and resources of another user.
To see the permissions of a file, we can use the ls command. Let's look
at the permissions of /etc/passwd file.
To see the permissions of a file, we can use the `ls` command. Let's look
at the permissions of `/etc/passwd` file.
![](images/linux/admin/image40.png)
@@ -265,10 +264,10 @@ related to file permissions.
### Chmod command
The chmod command is used to modify files and directories permissions in
The `chmod` command is used to modify files and directories permissions in
Linux.
The chmod command accepts permissions in as a numerical argument. We can
The `chmod` command accepts permissions in as a numerical argument. We can
think of permission as a series of bits with 1 representing True or
allowed and 0 representing False or not allowed.
@@ -288,26 +287,26 @@ We will now create a new file and check the permission of the file.
![](images/linux/admin/image15.png)
The group owner doesn't have the permission to write to this file. Let's
give the group owner or root the permission to write to it using chmod
give the group owner or root the permission to write to it using `chmod`
command.
![](images/linux/admin/image26.png)
Chmod command can be also used to change the permissions of a directory
`chmod` command can be also used to change the permissions of a directory
in the similar way.
### Chown command
The chown command is used to change the owner of files or
The `chown` command is used to change the owner of files or
directories in Linux.
Command syntax: chown \<new_owner\> \<file_name\>
Command syntax: `chown \<new_owner\> \<file_name\>`
![](images/linux/admin/image6.png)
**In case, we do not have sudo privileges, we need to use sudo
command**. Let's switch to user 'shivam' and try changing the owner. We
have also changed the owner of the file to root before running the below
**In case, we do not have `sudo` privileges, we need to use `sudo`
command**. Let's switch to user `shivam` and try changing the owner. We
have also changed the owner of the file to `root` before running the below
command.
![](images/linux/admin/image12.png)
@@ -317,53 +316,53 @@ similar way.
### Chgrp command
The chgrp command can be used to change the group ownership of files or
directories in Linux. The syntax is very similar to that of chown
The `chgrp` command can be used to change the group ownership of files or
directories in Linux. The syntax is very similar to that of `chown`
command.
![](images/linux/admin/image27.png)
Chgrp command can also be used to change the owner of a directory in the
`chgrp` command can also be used to change the owner of a directory in the
similar way.
## SSH Command
The ssh command is used for logging into the remote systems, transfer files between systems and for executing commands on a remote machine. SSH stands for secure shell and is used to provide an encrypted secured connection between two hosts over an insecure network like the internet.
The `ssh` command is used for logging into the remote systems, transfer files between systems and for executing commands on a remote machine. `SSH` stands for secure shell and is used to provide an encrypted secured connection between two hosts over an insecure network like the internet.
Reference:
[https://www.ssh.com/ssh/command/](https://www.ssh.com/ssh/command/)
We will now discuss passwordless authentication which is secure and most
commonly used for ssh authentication.
commonly used for `ssh` authentication.
### Passwordless Authentication Using SSH
Using this method, we can ssh into hosts without entering the password.
Using this method, we can `ssh` into hosts without entering the password.
This method is also useful when we want some scripts to perform
ssh-related tasks.
Passwordless authentication requires the use of a public and private key pair. As the name implies, the public key can be shared with anyone but the private key should be kept private.
Lets not get into the details of how this authentication works. You can read more about it
Let's not get into the details of how this authentication works. You can read more about it
[here](https://www.digitalocean.com/community/tutorials/understanding-the-ssh-encryption-and-connection-process)
Steps for setting up a passwordless authentication with a remote host:
1. Generating public-private key pair
**If we already have a key pair stored in \~/.ssh directory, we will not need to generate keys again.**
**If we already have a key pair stored in `~/.ssh` directory, we will not need to generate keys again.**
Install openssh package which contains all the commands related to ssh.
Install `openssh` package which contains all the commands related to `ssh`.
![](images/linux/admin/image49.png)
Generate a key pair using the ssh-keygen command. One can choose the
Generate a key pair using the `ssh-keygen` command. One can choose the
default values for all prompts.
![](images/linux/admin/image47.png)
After running the ssh-keygen command successfully, we should see two
keys present in the \~/.ssh directory. Id_rsa is the private key and
id_rsa.pub is the public key. Do note that the private key can only be
After running the `ssh-keygen` command successfully, we should see two
keys present in the `~/.ssh` directory. `id_rsa` is the private key and
`id_rsa.pub` is the public key. Do note that the private key can only be
read and modified by you.
![](images/linux/admin/image7.png)
@@ -372,40 +371,48 @@ Steps for setting up a passwordless authentication with a remote host:
There are multiple ways to transfer the public key to the remote server.
We will look at one of the most common ways of doing it using the
ssh-copy-id command.
`ssh-copy-id` command.
![](images/linux/admin/image11.png)
Install the openssh-clients package to use ssh-copy-id command.
Install the `openssh-clients` package to use `ssh-copy-id` command.
![](images/linux/admin/image46.png)
Use the ssh-copy-id command to copy your public key to the remote host.
Use the `ssh-copy-id` command to copy your public key to the remote host.
![](images/linux/admin/image50.png)
Now, ssh into the remote host using the password authentication.
Now, `ssh` into the remote host using the password authentication.
![](images/linux/admin/image51.png)
Our public key should be there in \~/.ssh/authorized_keys now.
Our public key should be there in `~/.ssh/authorized_keys` now.
![](images/linux/admin/image4.png)
\~/.ssh/authorized_key contains a list of public keys. The users
associated with these public keys have the ssh access into the remote
`~/.ssh/authorized_key` contains a list of public keys. The users
associated with these public keys have the `ssh` access into the remote
host.
### How to run commands on a remote host ?
General syntax: ssh \<user\>@\<hostname/hostip\> \<command\>
General syntax:
```shell
ssh \<user\>@\<hostname/hostip\> \<command\>
```
![](images/linux/admin/image14.png)
### How to transfer files from one host to another host ?
General syntax: scp \<source\> \<destination\>
General syntax:
```shell
scp \<source\> \<destination\>
```
![](images/linux/admin/image32.png)
@@ -418,32 +425,32 @@ systems.
| Packaging systems | Distributions |
| ---------------------- | ------------------------------------------ |
| Debian style (.deb) | Debian, Ubuntu |
| Red Hat style (.rpm) | Fedora, CentOS, Red Hat Enterprise Linux |
| Debian style (`.deb`) | Debian, Ubuntu |
| Red Hat style (`.rpm`) | Fedora, CentOS, Red Hat Enterprise Linux |
**Popular Packaging Systems in Linux**
|Command | Description |
| ----------------------------- | --------------------------------------------------- |
| yum install \<package_name\> | Installs a package on your system |
| yum update \<package_name\> | Updates a package to it's latest available version |
| yum remove \<package_name\> | Removes a package from your system |
| yum search \<keyword\> | Searches for a particular keyword |
| yum install <package_name\> | Installs a package on your system |
| yum update <package_name\> | Updates a package to its latest available version |
| yum remove <package_name\> | Removes a package from your system |
| yum search <keyword\> | Searches for a particular keyword |
[DNF](https://docs.fedoraproject.org/en-US/quick-docs/dnf/) is
the successor to YUM which is now used in Fedora for installing and
managing packages. DNF may replace YUM in the future on all RPM based
managing packages. DNF may replace YUM in the future on all RPM-based
Linux distributions.
![](images/linux/admin/image20.png)
We did find an exact match for the keyword httpd when we searched using
yum search command. Let's now install the httpd package.
We did find an exact match for the keyword `httpd` when we searched using
`yum search` command. Let's now install the `httpd` package.
![](images/linux/admin/image28.png)
After httpd is installed, we will use the yum remove command to remove
httpd package.
After `httpd` is installed, we will use the `yum remove` command to remove
`httpd` package.
![](images/linux/admin/image43.png)
@@ -454,15 +461,15 @@ used to monitor the processes on Linux systems.
### ps (process status)
The ps command is used to know the information of a process or list of
The `ps` command is used to know the information of a process or list of
processes.
![](images/linux/admin/image24.png)
If you get an error "ps command not found" while running ps command, do
install **procps** package.
If you get an error "ps command not found" while running `ps` command, do
install `procps` package.
ps without any arguments is not very useful. Let's try to list all the
`ps` without any arguments is not very useful. Let's try to list all the
processes on the system by using the below command.
Reference:
@@ -470,28 +477,28 @@ Reference:
![](images/linux/admin/image42.png)
We can use an additional argument with ps command to list the
information about the process with a specific process ID.
We can use an additional argument with `ps` command to list the
information about the process with a specific process ID (PID).
![](images/linux/admin/image2.png)
We can use grep in combination with ps command to list only specific
We can use `grep` in combination with `ps` command to list only specific
processes.
![](images/linux/admin/image1.png)
### top
The top command is used to show information about Linux processes
The `top` command is used to show information about Linux processes
running on the system in real time. It also shows a summary of the
system information.
![](images/linux/admin/image53.png)
For each process, top lists down the process ID, owner, priority, state,
cpu utilization, memory utilization and much more information. It also
lists down the memory utilization and cpu utilization of the system as a
whole along with system uptime and cpu load average.
For each process, `top` lists down the process ID, owner, priority, state,
CPU utilization, memory utilization and much more information. It also
lists down the memory utilization and CPU utilization of the system as a
whole along with system uptime and CPU load average.
## Memory Management
@@ -500,21 +507,21 @@ used to view information about the system memory.
### free
The free command is used to display the memory usage of the system. The
The `free` command is used to display the memory usage of the system. The
command displays the total free and used space available in the RAM
along with space occupied by the caches/buffers.
![](images/linux/admin/image22.png)
free command by default shows the memory usage in kilobytes. We can use
`free` command by default shows the memory usage in kilobytes. We can use
an additional argument to get the data in human-readable format.
![](images/linux/admin/image5.png)
### vmstat
The vmstat command can be used to display the memory usage along with
additional information about io and cpu usage.
The `vmstat` command can be used to display the memory usage along with
additional information about IO and CPU usage.
![](images/linux/admin/image38.png)
@@ -525,27 +532,27 @@ used to view disk space on Linux.
### df (disk free)
The df command is used to display the free and available space for each
The `df` command is used to display the free and available space for each
mounted file system.
![](images/linux/admin/image36.png)
### du (disk usage)
The du command is used to display disk usage of files and directories on
The `du` command is used to display disk usage of files and directories on
the system.
![](images/linux/admin/image10.png)
The below command can be used to display the top 5 largest directories
in the root directory.
in the `root` directory.
![](images/linux/admin/image18.png)
## Daemons
A computer program that runs as a background process is called a daemon.
Traditionally, the name of daemon processes ended with d - sshd, httpd
A computer program that runs as a background process is called a _daemon_.
Traditionally, the name of daemon processes ends with `d` - `sshd`, `httpd`,
etc. We cannot interact with a daemon process as they run in the
background.
@@ -553,12 +560,12 @@ Services and daemons are used interchangeably most of the time.
## Systemd
Systemd is a system and service manager for Linux operating systems.
Systemd units are the building blocks of systemd. These units are
`systemd` is a system and service manager for Linux operating systems.
`systemd` units are the building blocks of `systemd`. These units are
represented by unit configuration files.
The below examples shows the unit configuration files available at
/usr/lib/systemd/system which are distributed by installed RPM packages.
`/usr/lib/systemd/system` which are distributed by installed RPM packages.
We are more interested in the configuration file that ends with service
as these are service units.
@@ -566,8 +573,8 @@ as these are service units.
### Managing System Services
Service units end with .service file extension. Systemctl command can be
used to start/stop/restart the services managed by systemd.
Service units end with `.service` file extension. `systemctl` command can be
used to start/stop/restart the services managed by `systemd`.
| Command | Description |
| ------------------------------- | -------------------------------------- |