Using the Command Line

Using the Command Line#

Landing in the Linux Shell#

When you open a new Linux terminal, you will see a prompt like

User@Machine:~$

In this prompt, User is the username, Machine is the machine name, and ~ stands for the home directory, which is the current working directory. The colon : is a separator between the machine name and the working directory.

The prompt character is $ for a regular user and # for a superuser.

Basic Commands#

To navigate the file system in the shell, you will most likely use the following commands.

  • cd: Change directory

  • ls: List files and directories

  • pwd: Print working directory

Change Directory#

The cd command is used to change the current working directory. When a single argument is passed, namely, the path to the directory, the current working directory is changed to the specified directory.

User@Machine:~$ cd /tmp
User@Machine:/tmp$ cd ..
User@Machine:/$ cd
User@Machine:~$

The first command cd /tmp specifies an absolute path /tmp, so the working directory is changed accordingly as you see in the second command.

The second command cd .. changes the directory to the parent directory. .. is a relative path to the parent directory. As a result, the directory changed from /tmp to /. In Linux, / is known as the root directory. Every directory has an absolute path that starts from the root directory.

The third command cd without any argument changes the directory to the home directory. The home directory is the directory where the user is located. The directory change can be confirmed by the pwd command.

Case Sensitivity

Linux is case-sensitive because UNIX is. It is case sensitive, meaning that all the command and file names are case sensitive.

In the beginning, this might feel a bit confusing when you are used to Windows.

List Files and Directories#

The ls command is used to list the files and directories in the current directory.

ls can accept a path to a directory as an argument. It will then list the items in the specified directory.

ls can take arguments. A few commonly used arguments are

  • -a: Show all files and directories, including hidden ones

  • -l: Show detailed information about the files and directories

  • -t: Sort the files and directories by time

  • -r: Reverse the order of the files and directories

These options can be combined. For example, ls -lrt is equivalent to ls -l -r -t, which will list the files and directories in the current directory in reverse order of time. The combination is agnostic to the option order.

To list files that match a pattern, one can use the wildcard *. For example,

User@Machine:~$ ls f*.txt
file.txt

This command lists all files that start with f and end with .txt.

Hidden files and directories

Hidden files and directories are named to start with a dot .. They are not displayed by ls but are shown by ls -a.

Exercise

Use the ls command to list files and directories in the /var directory. Include hidden files in your listing. Then, sort the files by modification time.

Make a Directory#

The mkdir command is used to make a directory. It can take a path to the directory as an argument.

Remove#

Two commands are used to remove files and directories. rm is used to remove files or directories.

rmdir is used to remove empty directories.

Create a directory and a file in it.

User@Machine:~$ mkdir test
User@Machine:~$ ls
test
User@Machine:~$ touch test/file.txt
User@Machine:~$ ls test
file.txt

The touch command is used to create a new & empty file.

Since the directory test is not empty, rmdir will not work.

User@Machine:~$ rmdir test
rmdir: test: Directory not empty

One can remove the directory test with rm -r test. The option -r stands for recursive, meaning that the folder and everything (files and subfolders) inside will be removed.

User@Machine:~$ rm -r test
User@Machine:~$ ls

No Recycle Bin on Linux

Linux bash does not have a built-in recycle bin. Files deleted with rm are gone forever.

If unsure, use mv to rename the file or move it to a temporary location.

Exercise

In your home directory, create a new directory named power_logs. Inside power_logs, create an empty file named session1.log.

Verify the directory and file were created correctly.

Copy and Move#

The commands for copy and move are cp and mv.

The format for calling both commands is command SOURCE DESTINATION.

The example below makes a copy of file.txt to file2.txt.

User@Machine:~$ cp file.txt file2.txt
User@Machine:~$ ls
file.txt  file2.txt

The example below moves file2.txt to the directory test.

User@Machine:~$ mv file2.txt test
User@Machine:~$ ls test
file2.txt
User@Machine:~$ ls
file.txt

Exercise

Back to the power_logs folder, copy session1.log to a new file named session1_backup.log in the same directory.

Move session1_backup.log to a new directory under power_logs named backup.

View File Content#

The cat command is used to view the content of a file.

User@Machine:~$ cat file.txt
Hello, World!

Find Files#

The find command is used to search for files in a directory.

User@Machine:~$ find . -iname "file.txt"
./file.txt

where -iname is the combination of -i for case-insensitive search and -name for specifying the file name.

Search in Files#

grep is used to search for a pattern in a plain-text file.

User@Machine:~$ grep -i "hello" file.txt
Hello, World!

You can specify a path to the file or use a wildcard to search in multiple files.

User@Machine:~$ grep -iR "hello" .
Hello, World!

It searches for the pattern hello in all files in the current directory (.), recursively (-R).

Exercise

Use the find command to locate all .log files within the power_logs directory.

Manual Pages#

Most commands support --help or -h to show a brief description of the command. The full usage and options are documented in the manual pages. man COMMAND shows the manual page for the command.

man rmdir

will open the manual page for the rmdir command. When browsing, press q to exit, and use the arrow keys to scroll, and use / to search.

Advanced Commands#

Echo / Display Text#

The echo command is used to display a line of text.

User@Machine:~$ echo "Hello, World!"
Hello, World!

The echo command is powerful when combined with other commands, such as the redirection operator >.

Redirection#

The redirection operator > redirects the output of a shell command to a file.

User@Machine:~$ echo "Hello, World!" > file.txt
User@Machine:~$ cat file.txt
Hello, World!

There are two types of redirection operators. > overwrites the file, while >> appends the output to the file.

Further Reading: Linux File Descriptors

In Linux, everything is a file. That includes the standard input, output, and standard error. They correspond to file descriptors 0, 1, and 2, respectively.

  • > redirects the standard output (file descriptor 1) to a file.

  • 2> redirects the standard error (file descriptor 2) to a file.

  • &> redirects both the standard output and standard error to a file.

Environment Variables#

Environment variables are used to store information that can be used by programs. They are set in the shell and are inherited by child processes. The env command is used to list the environment variables. You will likely see a long output with many variables.

The best known environment variable is PATH. It is a colon-separated list of directories where the shell looks for executable files. When you type a command, the shell looks for the command in the directories listed in the PATH variable. This is how you can run a command without specifying the full path.

To find out where a command is located, you can use the which command.

User@Machine:~$ which ls
/usr/bin/ls

This command shows that the ls command is located at /usr/bin/ls.

Changing Ownership#

There may be times you run into permission issues. This often happens when you use the root user (superuser) to run a command. For example, you probably have used sudo to install a package.

If you accidentally run a command as the superuser, all the files and directories created by the command will be owned by the superuser. A regular issue will then not be able to modify or remove the files.

The chown command is used to change the ownership of a file or directory. The format is chown OWNER:GROUP FILES. For example,

User@Machine:~$ chown User:staff file.txt

changes the ownership of file.txt to the User user and the staff group.

If you need to change the ownership from root to yourself, you will need to use sudo, because only the superuser can relinquish its ownership.

User@Machine:~$ sudo chown User:staff file.txt

There are options like -R to apply the change recursively.

Caution with sudo

Always be careful when using sudo. It can easily mess up the permissions.

Pipes#

Pipes | are used to connect the output of one command to the input of another command. A basic idea of UNIX is that you can chain commands together to do complex tasks. Pipes are what makes the shell powerful.

For example, ls -l lists the files and folders, By inspection of ls -l, you can tell that the first line is the heading that indicates the number of disk blocks used. The rest of the lines are the files and folders.

The command wc -l counts the number of lines. You can combine them to count the number of files and folders:

User@Machine:~$ ls -l | wc -l
25

The output of ls -l is piped to wc -l. The output is 25, so excluding the header, there are 24 files and folders.

Text-based editors#

Occasionally, you may need to edit a file in the shell. There are several editors available. The most common ones are nano, vim, and emacs.

Nano#

The nano editor is a simple text editor that is available on most Linux distributions. It is easy to use: to edit a file, do nano FILENAME. When nano opens, just type your text.

To exit nano, press Ctrl + X. If you have unsaved changes, nano will ask you confirm.

Vim#

vim is a powerful text editor with a steep learning curve. As an evidence, the question how do I exit vim on StackOverflow has over 3 million views.

Vim has two modes, normal mode and insert mode. In normal mode, you can navigate and execute commands. In insert mode, you can type text. You enter insert mode by pressing i, and you exit back to normal mode by pressing Esc.

Exiting Vim

To exit vim, press Esc to ensure you are in the normal mode. Then type :q

To save a file, you can type :w. To save and exit, you can type :wq.

With the very basics, you will at least not stuck in vim. If interested, there are plenty of materials to teach yourself.

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