Command Line Basics
+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 +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 +for manual pages.
+Using --help to show the documentation for ls command.
+
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. +These directories in turn can either store system files or application +files or user related files.
+
bin | The executable program of most commonly used commands reside in bin directory
+ sbin | This directory contains programs used for system administration.
+ home | This directory contains user related files and directories.
+ lib | This directory contains all the library files
+ etc | This directory contains all the system configuration files
+ proc | This directory contains files related to the running processes on the system
+ dev | This directory contains files related to devices on the system
+ mnt | This directory contains files related to mounted devices on the system
+ 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
+There are three basic commands which are used frequently to navigate the +file system:
+-
+
-
+
ls
+
+ -
+
pwd
+
+ -
+
cd
+
+
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.
+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.
+
We will now use the cd command to move to a different directory and then +print the working directory.
+
cd (change directory)
+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.
+
ls (list files and directories)**
+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 +directories present in the current directory.
+
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.
+
Commands for Manipulating Files
+There are four basic commands which are used frequently to manipulate +files:
+-
+
-
+
touch
+
+ -
+
mkdir
+
+ -
+
cp
+
+ -
+
mv
+
+ -
+
rm
+
+
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.
+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 simplest use case of creating a new file.
+General syntax of using touch command
+touch <file_name>
+
mkdir (create new directories)
+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
+mkdir <directory_name>
+
rm (delete files and directories)
+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 +run this command with care.
+General syntax of using rm command:
+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 +respectively.
+
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 +original files or directories. The original files or directories and +their copy both co-exist after running cp command successfully.
+General syntax of using cp command:
+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 +now.
+
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".
+
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.
+
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 +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:
+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. +Do note that the original file doesn't exist in the current directory +after mv command ran successfully.
+
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 +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".
+
Commands for Viewing Files
+There are three basic commands which are used frequently to view the +files:
+-
+
-
+
cat
+
+ -
+
head
+
+ -
+
tail
+
+
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.
+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.
+
Do not worry about the above command now. It's an advanced command which +is used to generate numbers. We have then used a redirection operator to +push these numbers to the file. We will be discussing I/O redirection in the +later sections.
+cat
+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.
+
You can try to run the above command and you will see numbers being +printed from 1 to 100 on your screen. You will need to scroll up to view +all the numbers.
+head
+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.
+
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.
+
tail
+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.
+
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.
+
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.
+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
The echo command prints the given input string on the screen.
+
Text Processing Commands
+In the previous section, we learned how to view the content of a file. +In many cases, we will be interested in performing the below operations:
+-
+
-
+
Print only the lines which contain a particular word(s)
+
+ -
+
Replace a particular word with another word in a file
+
+ -
+
Sort the lines in a particular order
+
+
There are three basic commands which are used frequently to process +texts:
+-
+
-
+
grep
+
+ -
+
sed
+
+ -
+
sort
+
+
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.
+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.
+
grep
+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.
+General syntax of using grep command:
+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.
+
sed
+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:
+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.
+
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 +changes are reflected back in the file.
+sort
+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.
+
Now, we will try to sort the file using the sort command. The sort +command sorts the content in lexicographical order.
+
The content of the file will not change in the above +example.
+I/O Redirection
+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 +redirected.
+Everything is a file in linux - +https://unix.stackexchange.com/questions/225537/everything-is-a-file
+Till now, we have displayed all the output on the screen which is the +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 redirected the output from echo command to +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 sort command as an +input to uniq command using pipe(|) operator. The uniq command only +prints the unique numbers from the input.
+
I/O redirection - +https://tldp.org/LDP/abs/html/io-redirection.html
+Applications in SRE Role
+-
+
-
+
As a SRE, you will be required to perform some general tasks on these linux servers. You will also be using the command line when you are troubleshooting issues.
+
+ -
+
Moving from one location to another in the filesystem will require the help of ls, pwd and cd commands
+
+ -
+
You may need to search some specific information in the log files. Grep command would be very useful here. I/O redirection will become handy if you want to store the output in a file or pass it as an input to another command.
+
+ -
+
Tail command is very useful to view the latest data in the log file.
+
+
Useful courses and tutorials
+-
+
-
+
Edx linuxcourse - + This video course can be very helpful in developing the basics of linux command line. This course is provided + in both free and paidmodes by edX. If you take the free course, you will not be able to access the assignments.
+
+ - + + +
