Linux Command Line Basics

What is this "shell"?

The "shell" is another name for the command shell or command interpreter. This is the program that gives you a command prompt, accepts the commands you type there, and basically makes the computer do what you tell it to. In DOS the program that did this was command.com (unless you were a real technogeek and used 4dos or something). In Linux, the shell is the first program that starts when you log in, and it keeps running until you log out, waiting to do your bidding. Linux is able to use any of several different shells, but the default Linux shell is called bash and is the only one I will discuss.

Why It Is Called a Shell

The shell's job is to interpret your commands and run the programs you request. Linux was designed to be a multitasking operating system, which means you can run more than one program at one time. Linux was also designed as a multi-user OS, which means that you can have more than one shell running at the same time. (Each user gets his own shell at login.) As a user, you have access only to the programs you are running, not the ones other users are running (though you can run your own copy of the same program). The programs are kept separate because they are "enclosed" in a "shell".

Cool Command Line Tricks

Here is the stuff that you MUST know. The command line interface would be completely unbearable if not for a few simple tricks that make life okay.

Command History

The shell stores a list of the commands that you have issued. You can navigate up and down through this list using the Up and Down Arrow keys. To repeat the last command is just Up Arrow, Enter.

Command Completion

Linux can have some outrageously long file names, which is great when you're reading them in a list, but it sucks when you have to type it on the command line. At least it would suck if not for this nifty little feature. Type the first letter or two and then press the Tab key. The shell will try to complete your command. If you typed enough letters to uniquely identify the file name, the shell fills the rest in for you. If the letters match multiple file names, the shell will fill in everything up to the point where they differ and wait. Hit Tab again and it will give you a list of possibilities. The shell will also help you complete path names. Try it out. You will love this.

Multitasking

I'll try to expand this later, but here's the one minute version. To run a job (program) in the background, type an ampersand (&) at the end of the command line. To suspend the currently running job, Ctrl-Z. To force a suspended job to run in the background, type bg <job>. To bring a background job into the foreground, type fg <job>. To see a list of jobs currently running (or suspended) type jobs. If you exclude the [<job>] argument on these commands, it defaults to whatever job was last running in the foreground (not the one currently in the foreground).

Navigating the Command Line: ls, cd, and pwd

Navigating the Linux filesystem from the command line is a fundamental skill for any user. The three essential commands for moving around and understanding your location within the filesystem are ls, cd, and pwd.

Listing Directory Contents with ls

The ls command lists the contents of a directory. By default, it displays the files and directories in your current location:

ls

You can also list the contents of a specific directory by specifying its path:

ls /path/to/directory

Adding options to ls can provide more detailed information. For example, -l displays the contents in long format, which includes permissions, number of links, owner, group, size, and modification time:

ls -l

The -a option shows all files, including hidden ones (those starting with a dot .):

ls -a

Combining options gives you a comprehensive view:

ls -la

Changing Directories with cd

The cd (change directory) command is used to move around the filesystem. To go to a specific directory, type cd followed by the directory's path:

cd /path/to/directory

To move to your home directory, simply use:

cd

Or:

cd ~

To navigate to the parent directory (one level up), use:

cd ..

To move to the root directory, use:

cd /

You can also use relative paths. For example, if you are in /home/user and want to go to /home/user/documents, you can simply use:

cd documents

Printing the Working Directory with pwd

The pwd (print working directory) command displays the full path of the current directory. This is useful to confirm your current location within the filesystem:

pwd

Example Workflow

Here's a typical workflow using these commands:

1. List the contents of your current directory:

   ls
   

2. Change to a different directory (e.g., moving to a directory named projects):

   cd projects
   

3. Print the working directory to confirm your new location:

   pwd
   

4. List the contents of a specific subdirectory within projects:

   ls subdirectory
   

By mastering these commands, you can efficiently navigate the Linux filesystem and manage your files and directories with ease.

Viewing Text Files: cat, more or less

In Linux, viewing the contents of text files is a common task. There are several commands available to accomplish this, each with its own advantages. Here, we will cover cat, more, and less.

Viewing Files with cat

The cat (short for "concatenate") command is the simplest way to view the contents of a file. It displays the entire file content in the terminal.

For example, to view a file named example.txt, you would use:

cat example.txt

cat is best used for small files because it prints the entire content at once. For large files, it can be overwhelming as it will flood the terminal with text.

Paging Through Files with more

The more command is a pager, meaning it allows you to view the contents of a file one screen at a time. This is useful for larger files that don't fit in a single screen.

For example, to view example.txt one screen at a time:

more example.txt

While viewing the file, you can navigate with:

• Spacebar: Move to the next screen.
• Enter: Scroll down one line.
• b: Go back one screen.
• q: Quit and return to the command line.

more is a step up from cat, but it has limitations, especially with backward navigation.

Navigating Files with less

The less command is similar to more but with additional features and better navigation capabilities. It allows you to move forward and backward through the file seamlessly.

For example, to view example.txt with less:

less example.txt

While viewing the file with less, you can navigate with:

• Spacebar: Move to the next screen.
• Enter: Scroll down one line.
• b: Go back one screen.
• Up/Down Arrow Keys: Move one line up or down.
• Page Up/Page Down: Move one screen up or down.
• g: Go to the beginning of the file.
• G: Go to the end of the file.
• /search_term: Search for a term in the file.
• n: Go to the next occurrence of the search term.
• N: Go to the previous occurrence of the search term.
• q: Quit and return to the command line.

less is highly recommended for viewing large files due to its robust navigation and search capabilities.

By mastering these commands, you can efficiently view and navigate through text files of any size in the Linux environment.

Creating Files and Directories in Linux

Creating files and directories is a fundamental task in Linux. In this tutorial, we'll cover three essential commands: mkdir for creating directories, touch for creating empty files, and echo with redirection for creating and appending to files.

Creating Directories with mkdir

The mkdir (make directory) command is used to create new directories.

For example, to create a directory named new_directory:

mkdir new_directory

You can also create nested directories by using the -p option:

mkdir -p parent_directory/child_directory

This command creates both parent_directory and child_directory in one step. Without the -p you would get an error if parent_directory did not exist. Think of -p as "with parents".

Creating Empty Files with touch

The touch command is used to create empty files or update the timestamp of existing files.

For example, to create an empty file named new_file.txt:

touch new_file.txt

If new_file.txt already exists, touch updates its timestamp to the current time.

Creating and Appending to Files with echo and Redirection

The echo command outputs text to the terminal, but combined with redirection, it can be used to create or append text to files.

Creating a New File with echo

To create a new file and write text to it, use the > (greater than) operator:

echo "This is some text" > new_file.txt

This command creates new_file.txt (if it doesn't exist) and writes "This is some text" into it. WARNING: If new_file.txt already exists, it will be overwritten, and its previous contents will be lost. Check your targets!

Appending to an Existing File with echo

To append text to an existing file, use the >> (double greater than) operator:

echo "This is additional text" >> existing_file.txt

This command adds "This is additional text" to the end of existing_file.txt without overwriting its existing contents.

Example Workflow

Here's a typical workflow using these commands:

1. Create a new directory:

   mkdir my_directory
   

2. Navigate into the new directory:

   cd my_directory
   

3. Create an empty file:

   touch my_file.txt
   

4. Write text to a new file:

   echo "Hello, World!" > hello.txt
   

5. Append text to the existing file:

   echo "Welcome to my file." >> hello.txt
   

By mastering mkdir, touch, and echo with redirection, you can efficiently create and manage files and directories in Linux. These basic commands form the foundation for more advanced file manipulation and system administration tasks.

Copying, Moving, and Renaming Files and Directories in Linux

Managing files and directories often involves copying, moving, and renaming them. In this tutorial, we will cover the cp and mv commands for these tasks.

Copying Files and Directories with cp

The cp (copy) command is used to copy files and directories.

To copy a file, specify the source file and the destination:

cp file1.txt file2.txt

This command copies file1.txt to file2.txt. If file2.txt exists, it will be overwritten. Check your targets!

To copy directories, use the -r (recursive) option:

# copy the dir1 directory as the new dir2 directory
cp -r dir1 dir2

This command copies the contents of dir1 into dir2. If dir2 does not exist, it will be created. Note that although these are directories, we do NOT include the / at the end, because that would change the meaning of the command.

# copy the dir1 directory into an existing dir2 directory. Result: dir2/dir1
cp -r dir1 dir2/

ls dir1
  example.txt
# copy the *contents* of dir1 into an existing dir2 directory, but not dir1 itself
cp -r dir1/ dir2/
ls dir2
  example.txt

Watch for these subtle differences in the use of / when copying files!

Moving and Renaming Files and Directories with mv

The mv (move) command is used to move or rename files and directories. The basic syntax is:

mv source destination

To move a file into another directory, specify the source file and the destination directory:

mv file.txt /path/to/destination/

This command moves file.txt to the specified directory. Note that the destination ends in / showing it is a directory. This will keep the file name unchanged, so the file will now reside at /path/to/destination/file.txt.

To rename a file, specify the old name and the new name:

mv oldname.txt newname.txt

To move a directory, specify the source directory and the destination:

mv dir1 /path/to/destination/

This command moves dir1 to the specified destination directory. It now resides at /path/to/destination/dir1.

Using cp, mv, and mv commands, you can efficiently copy, move, and rename files and directories in Linux. These basic file manipulation commands are essential for effective file management and organization on your system.

Deleting Files and Directories in Linux

Managing the deletion of files and directories is an essential task in Linux. In this tutorial, we'll cover how to delete files and directories using the rm and rmdir commands, including how to remove non-empty directories with rm -r.

Deleting Files with rm

The rm (remove) command is used to delete files.

For example, to delete a file named example.txt:

rm example.txt

Using Options with rm

• Prompt Before Deleting: Use the -i option to prompt for confirmation before each file is deleted:

  rm -i filename
  

• Force Deletion: Use the -f option to force the deletion of files without prompting for confirmation:

  rm -f filename
  

Deleting Empty Directories with rmdir

The rmdir (remove directory) command is used to delete empty directories.

For example, to delete an empty directory named empty_dir:

rmdir empty_dir

Note that rmdir will only remove directories that are completely empty. If there are any files or subdirectories inside the target, you will get an error.

rmdir non_empty_dir
  rmdir: non_empty_dir: Directory not empty

Deleting Non-Empty Directories with rm -r

To delete directories that are not empty, use the rm command with the -r (recursive) option. This will delete the directory and all its contents:

rm -r non_empty_dir

Using Options with rm -r

• Prompt Before Deleting: Use the -i option with -r to prompt for confirmation before deleting each file and subdirectory:

  rm -r -i directory_name
  

• Force Deletion: Use the -f option with -r to force the deletion of directories and their contents without prompting:

  rm -r -f directory_name
  

This can also be combined as:

rm -rf directory_name

Using rm and rmdir commands, you can effectively manage the deletion of files and directories in Linux. Note that all of these commands are permanent -- there is no "undo". You will not find files deleted in this way in a recycle bin. They are gone for good. As always, check your targets!