Basic commands

By now you have some basic knowledge about directories and files and you can interact with the command line interface.  We can learn some of the commands you'll be using many times each day.

ls

The first thing you likely need to know before you can start creating and making changes to files is what's already there?  With a graphical interface you'd do this by opening a folder and inspecting its contents. From the command line you use the program ls instead to list a folder's contents.

$ ls
Desktop  Documents  Music  Photos

By default, ls will use a very compact output format. Many terminals show the files and subdirectories in different colors that represent different file types.  Regular files don't have special coloring applied to their names.  Some file types, like JPEG or PNG images, or tar and ZIP files, are usually colored differently, and the same is true for programs that you can run and for directories.  Try ls for yourself and compare the icons and emblems your graphical file manager uses with the colors that ls applies on the command line.  If the output isn't colored, you can call ls with the option --color:

$ ls --color

man, info & apropos

You can learn about options and arguments using another program called man (man is short for manual) like this:

$ man ls

Here, man is being asked to bring up the manual page for ls. You can use the arrow keys to scroll up and down in the screen that appears and you can close it using the q key (for quit).

An alternative to obtain a comprehensive user documentation for a given program is to invoke info instead of man:

$ info ls

This is particularly effective to learn how to use complex GNU programs.  You can also browse the info documentation inside the editor Emacs, which greatly improves its readability.  But you should be ready to take your first step into the larger world of Emacs.  You may do so by invoking:

$ emacs -f info-standalone

that should display the Info main menu inside Emacs (if this does not work, try invoking emacs without arguments and then type Alt + x info, i.e. by depressing the Alt key, then pressing the x key, then releasing both keys and finally typing info followed by the Return or Enter key).  If you type then m ls, the interactive Info documentation for ls will be loaded inside Emacs.  In the standalone mode, the q key will quit the documentation, as usual with man and info.

Ok, now you know how to learn about using programs yourself.  If you don't know what something is or how to use it, the first place to look is its manual and information pages.  If you don't know the name of what you want to do, the apropos command can help.  Let's say you want to rename files but you don't know what command does that.  Try apropos with some word that is related to what you want, like this:

$ apropos rename
...
mv (1)               - move (rename) files
prename (1)          - renames multiple files
rename (2)           - change the name or location of a file
...

Here, apropos searches the manual pages that man knows about and prints commands it thinks are related to renaming.  On your computer this command might (and probably will) display more information but it's very likely to include the entries shown.

Note how the program names include a number besides them.  That number is called their section, and most programs that you can use from the command line will be in section 1.  You can pass apropos an option to display results from section 1 manuals only, like this:

$ apropos -s 1 rename
...
mv (1)               - move (rename) files
prename (1)          - renames multiple files
...

At this stage, the section number isn't terribly important.  Just know that section 1 manual pages are the ones that apply to programs you use on the command line.  To see a list of the other sections, look up the manual page for man using man man.

mv

Looking at the results from apropos, that mv program looks interesting.  You can use it like this:

$ mv oldname newname

Depending on your system configuration, you may not be warned when renaming a file will overwrite an existing file whose name happens to be newname.  So, as a safe-guard, always use `-i' option when issuing mv like this:

$ mv -i oldname newname

Just as the description provided by apropos suggests, this program moves files.  If the last argument happens to be an existing directory, mv will move the file to that directory instead of renaming it. Because of this, you can provide mv more than two arguments:

$ mv one_file another_file a_third_file ~/stuff

If ~/stuff exists, then mv will move the files there.  If it doesn't exist, it will produce an error message, like this:

$ mv one_file another_file a_third_file stuff
mv: target 'stuff' is not a directory

mkdir

How do you create a directory, anyway?  Use the mkdir command:

$ mkdir ~/stuff

And how do you remove it?  With the rmdir command:

$ rmdir ~/stuff

If you wish to create a subdirectory (say the directory bar) inside another directory (say the directory foo) but you are not sure whether this one exists or not, you can ensure to create the subdirectory and (if needed) its parent directory without raising errors by typing:

$ mkdir -p ~/foo/bar

This will work even for nested sub-sub-...-directories.

If the directory you wish to remove is not empty, rmdir will produce an error message and will not remove it.  If you want to remove a directory that contains files, you have to empty it first.  To see how this is done, we will need to create a directory and put some files in it first.  These files we can remove safely later.  Let's start by creating a directory called practice in your home and change the current working directory there:

$ mkdir ~/practice
$ cd ~/practice

cp, rm & rmdir

Now let's copy some files there using the program cp.  We are going to use some files that are very likely to exist on your computer, so the following commands should work for you:

$ cp /etc/fstab /etc/hosts /etc/issue /etc/motd .
$ ls
fstab  hosts  issue  motd

Don't forget the dot at the end of the line!  Remember it means "this directory" and being the last argument passed to cp after a list of files, it represents the directory in which to copy them.  If that list is very long, you'd better learn using globbing (expanding file name patterns containing wildcard characters into sets of existing file names) or some other tricky ways to avoid wasting your time in typing file names.  One trick can help when dealing with the copy of an entire directory content.  Passing to cp the option -R you can recursively copy all the files and subdirectories from a given directory to the destination:

$ cp -R . ~/foo
$ ls ~/foo
bar  fstab  hosts  issue  motd
$ cp -R . ~/foo/bar
$ ls -R ~/
~/foo:
bar  fstab  hosts  issue  motd

~/foo/bar:
fstab  hosts  issue  motd

In this case the current directory has no subdirectories so only files were copied.  As you can see, the option -R can be passed even to ls to list recursively the content of a given directory and of its subdirectories.

Now, if you go back to your home and try to remove the directory called practice, rmdir will produce an error message:

$ cd ..
$ rmdir practice
rmdir: failed to remove 'practice': Directory not empty

You can use the program rm to remove the files first, like this:

$ rm practice/fstab practice/hosts practice/issue practice/motd

And now you can try removing the directory again:

$ rmdir practice 

And now it works, without showing any output.

But what happens if your directories have directories inside that also have files, you could be there for weeks making sure each folder is empty!  The rm command solves this problem through the amazing option -R, which as usual stands for "recursive".  In the following example, the command fails because foo is not a plain file:

$ rm ~/foo/
rm: cannot remove `~/foo/`: Is a directory

So maybe you try rmdir, but that fails because foo has something else under it:

$ rmdir ~/foo
rmdir: ~/foo: Directory not empty

So you use rm -R, which succeeds and does not produce a message.

$ rm -R ~/foo/

So when you have a big directory, you don't have to go and empty every subdirectory.

But be warned that -R is a very powerful argument and you may lose data you wanted to keep!

cat & less

You don't need an editor to view the contents of a file.  What you need is just to display it.  The cat program fits the bill here:

$ cat myspeech.txt
Friends, Romans, Countrymen! Lend me your ears!

Here, cat just opens the file myspeech.txt and prints the entire file to your screen, as fast as it can.   However if the file is really long, the contents will go by very quickly, and when cat is done, all you will see are the last few lines of the file.  To just view the contents of a long file (or any text file) you can use the less program:

$ less myspeech.txt

Just as with using man, use the arrow keys to navigate, and press q to quit.