Linux introduction: Difference between revisions

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This article is aimed at Windows and Mac users that do not have or have very little experience in UNIX environments. It should give you the necessary basics to access the compute servers and being quickly able to use them.  

Connections to the servers use the SSH protocol, in text mode. You do not use a graphical interface (GUI) but a ''console''. Note that Windows executables do not run on our servers, at least without using an emulator.

Generally, UNIX commands are documented in the reference manuals that are available on the servers. To access those from a terminal:

{{Command|man command}}

<tt>man</tt> uses <tt>less</tt> (see the section [[#Viewing and editing files|Viewing and editing files]]), and you must press <tt>q</tt> to exit this program.

To find manual pages pertaining to a certain subject or keyword (for example "sujet"), please enter:

{{Command|apropos sujet}}

By convention, the executables themselves contain some help on how to use them.

Generally, you invoke this help using the command line argument <tt>-h</tt> or <tt>--help</tt>, or in certain cases,  <tt>-help</tt>.

Following your connection, you are directed to your <tt>$HOME</tt> directory (the UNIX word for "folder") for your user account.

When your account is created, your <tt>$HOME</tt> only contains a few hidden configuration files that start with a ".", and nothing else.

On a Linux system, you are strongly discouraged to create files or directories that contain names with spaces or special characters, including accents.

To list all files in a directory in a terminal, use the <tt>ls</tt> (list) command:

{{Command|ls}}

The option <tt>-h</tt> gives the file sizes in human readable format.

You can combine options, for example:

{{Command|ls -alth}}

To move about in the file system, use the <tt>cd</tt> command (change directory).

So, to change to <tt>my_directory</tt> you type:

{{Command|cd my_directory}}

{{Command|cd}}

To create (make) a directory, use the <tt>mkdir</tt> command:

{{Command|mkdir my_directory}}

Deleting a directory like this only works if it is empty.

You can remove files using the <tt>rm</tt> command:

{{Command|rm my_file}}

The (potentially dangerous!) <tt>-f</tt> option can be useful to bypass confirmation prompts and to continue the operation after an error.

To copy a file use the <tt>cp</tt> command:

{{Command|cp source_file destination_file}}

This command also applies to directories. You should then replace <tt>source_file</tt> with <tt>source_directory</tt> and <tt>destination_file</tt> with <tt>destionation_directory</tt>.

UNIX systems support 3 types of permissions : read (<tt>r</tt>), write (<tt>w</tt>) and execute (<tt>x</tt>). For files, a file should be readable to be read, writable to be modified, and executable to be run (if it's a binary executable or a script). For a directory, read permissions are necessary to list its contents, write permissions enable modification (adding or removing a file) and execute permissions enable changing to it.

Permissions apply to 3 different classes of users, the owner (<tt>u</tt>), the group (<tt>g</tt>), and all others or "the world" (<tt>o</tt>). To know which permissions are associated to files and subdirectories of the current directory, use the following command:

{{Command|ls -la}}

The 10 characters at the beginning of each line show the permissions.

The first character indicates the file type :

* <tt>l</tt>: a symbolic link

Then, from left to right, this command shows read, write and execute permissions of the owner, the group and other users. Here are some examples :

* <tt>drwxrwxrwx</tt>: a world-readable and world-writable directory

* <tt>drwx-wx-wx</tt>: a directory that everybody can enter and modify but where only the owner can list its contents

Important note: to be able to read or write in a directory, you need to have execute permissions (<tt>x</tt>) set in all parent directories, all the way up to the file system's root ('''<tt>/</tt>'''). So if your home directory has <tt>drwx------</tt> permissions and contains a subdirectory with <tt>drwxr-xr-x</tt> permissions, other users cannot read the contents of this subdirectory because they do not have access (by the executable bit) to its parent directory.

After listing the permissions, <tt>ls -la</tt> command gives a number, followed by the file owner's name, the file group's name, its size, last modification date, and name.

The <tt>chmod</tt> command allows you to change file permissions. The simple way to use it is to specify which permissions you wish to add or remove to which type of user. To do this, you specify the list of users (<tt>u</tt> for the owner, <tt>g</tt> for the group, <tt>o</tt> for other users, <tt>a</tt> for all), followed by a <tt>+</tt> to add permissions or <tt>-</tt> to remove permissions, which is then followed by a list of permissions to modify (<tt>r</tt> for read, <tt>w</tt> for write, <tt>x</tt> for execute). Non-specified permissions are not affected. Here are a few examples:

* Prevent group members and all others to read or modify the file <tt>secret.txt</tt>: {{Command|chmod go-rwx secret.txt}}

* Allow everybody to read the file <tt>public.txt</tt>: {{Command|chmod a+r public.txt}}

* Prevent other users to read or modify your home directory: {{Command|chmod go-rw ~}}

=== Viewing a file ===

To view a file read-only, use the <tt>less</tt> command:

You can quit <tt>less</tt> by pressing the <tt>q</tt> key.

The <tt>diff</tt> command allows you to compare two files:

{{Command|diff file1 file2}}

The <tt>-y</tt> option shows both files side by side.

The <tt>grep</tt> command allows you to look for a given expression in one

{{Command|grep 'tata' file1}}

Note that, in Linux, the "<tt>*</tt>" wildcard matches zero or more characters. The "<tt>?</tt>" wilcard matches exactly one character.

The text to be searched for can also be variable. For example, to look for

the text "number 10 » or "number 11", etc. with any number between 10 and 29,