Manipulate files and permissions

Display content file

Two commands make it possible to do this:

• cat already seen (display the whole file). ‘-n’ for the line number.
• less display the file page by page. Here are some useful shortcuts:
  • Space displays the next part of the file.
  • Entry displays the next line.
  • b goes back to the last part.
  • q stops the reading.

Display the beginning and the end of a file

Two commands make it possible to do this:

• head retrieves the first lines. Option ‘n’ indicates the number of lines we want to display.
• tail for the end of a file. Same option ‘n’. Option ‘f’ to follow the evolution of a file.

Copy and displacement

• cp copies a file. If you want to do a copy of “testfile”:

$ cp testfile testcopie

The option ‘-R’ enables to copy a whole folder. Let’s see a powerful application using the wildcard (‘joker’) *:

$ cp jeu* jeuFolder

makes it possible to copy all the files beginning by jeu to the folder “jeuFolder”.

Mini Exercise (easy): copy all the jpeg images to another folder.

• `` mv ``` is useful to move a file or rename it.

$ mv file Folder

Mini Exercise (easy): move all the csv files to another folder.

You can use ‘mv’ to rename a file:

$ mv firstName secondName

Supress files

We use the command rm.

$ rm firstFile secondFile

The option ‘-i’ enables to ask confirmation for each file. The option ‘-r’ can supress all a folder:

$ rm -r folder

Create links between files

There are two kinds of links (shortcuts) in Linux created with ln:

• Physical links
• Symbolic ones

To understand their differences, we have to present some elements on the way Linux handles files.

Files storage

On the hard drive, each file is split into two parts:

• the name
• the permissions
• the content

The names and the contents are not stored at the same place! Each file has an identification number called the inode: file-> inode -> content.

Create symbolic links

A physical link makes it possible to have two names that refer to the same content (inode).

$ touch file1
$ ln file1 file2
$ ls -l

If you change ‘file1’ or ‘file2’, you change the same content.

Create symbolic links

Let’s type:

$ ln -s file1 file2
$ ls -l

“file2” points towards the name of ‘file1’ and not to its content.

The users and their permissions

In Linux, several users can be connected at the same time. Each user has its own account and its own persimissions (rights to do something).

Organization of users

Among the users, there is the root (the super-user) with all the rights. Users can be gather into groups.

Users and groups

sudo: become root in an instant

This require your password in order to execute the command with the root permission:

$ sudo commande

sudo su: become root definitely

$ sudo su 

To quit this mode, type exit or Ctrl+D.

Add users

If you want to add the user ‘yanis’:

$ sudo adduser yanis 

If you want to change his password lately, you can use:

$ sudo passwd yanis  

To supress its account, use:

$ sudo deluser yanis  

To also suppress his home, add the following option:

$ sudo deluser --remove-home yanis

Add groups

To create a new group, use:

$ sudo addgroup family

But anyone is present in this group.

To edit an user, we use usermod. If you want to add ‘yanis’ to the group ‘family’:

$ sudo usermod family yanis

If you want to suppress the group:

$ sudo delgroup family

Handle owners

Only the root can modify the owner of a file. If you want to modify the owner of a file (make yanis the owner of ‘filtest’):

$ sudo chown yanis testFile

You can use chgrp to do the same thing with a group:

$ sudo chgrp family testFile

The same thing could be done with chown by separating the new user and group by a ::

$ sudo chown yanis:family testFile

One can use the option ‘-R’ to do this recursively:

$ sudo chown -R yanis:family Folder

Change permissions

The permissions

When you type:

$ ls -l

The first column indicates five different permissions:

• d indicates if the element is a folder
• l indicates if the element is a link
• r indicates if you can read the element
• w indicates if you can write/modify the element
• x for a file indicates that you can run it. For a folder, you can see the different sub-folders.

Permissions/Rights

Modify the permissions

Here we will use chmod (sudo is not necessary: we just need to be the owner of the file). We can give permissions with two methods: using numbers or letters.

chmod numbers

To combine the permissions, we have to add up these numbers. If you want to give reading and writing rights, this corresponds to \(4+2=6\):

$ chmod 640 fileTest

The previous command gives:

• reading and writing rights for the owner
• reading right for the group
• nothing for the others.

Another way exists with letter. We need to know that:

• u=user
• g=grou
• o=other
• ‘+’ means adding the permission
• ‘-’ means removing the permission
• ‘=’ to affect permissions

If you want to affect all the rights to the user and the reading permission to the group:

$ chmod u=rwx, g+r fileTest

Software installation

In this part, we explain how to install a sofware on Debian (this depends on the Linux distributions). In Linux, we have packages: a zip folder that contains all the files of the program: it is a file .deb which contains all the instructions to install a program. We have to focus on two specific points of Linux:

• administration of the dependences: very often, a sofware depends on others softwares called the dependencies.
• All the packages are gathered at the same place, called the repository.

To download the last list of packages, use:

$ sudo apt-get update

To dowload a package, use (install openarena for instance):

$ sudo apt-get install nomPackage 

To supress a package and all its useless dependencies, use:

$ sudo apt-get autoremove nomPackage

To update all the packages:

$ sudo apt-get upgrade 

Commands documentation (important for your autonomy)

In this short part, we explain how to use the manual to have all the ways to use a command.

If you want to know everything on a command (say mkdir), use:

$ sudo man mkdir

To move on this page:

• Espace to go to the next page.
• / to research a word in this documentation
• Q to quit the page.

Each man usage displays the following sections:

• NAME and a short description
• SYNOPSIS lists all the ways to use the command.
• DESCRIPTION is a deeper description of the commands with all the options.
• AUTHOR of the program.
• REPORTING BUGS the people to contact for a bug.
• COPYRIGHT user license.
• SEE ALSO others related commands.

man is useful if you know the name of the command. Otherwhise, use apropos. For instance, if you look for a command related to the sound of your machine (gives all the command which talks about sound in their description):

$ apropos sound

Looking for files

There is a lot of commands in this direction. The most popular and powerful command is find. It goes all over your hard drive. The command can use three kinds of arguments:

• where: it is the folder where we look for.
• what: it is the file we look for (by name, size, date,…).
• action: automatic actions on the selected files.

Basic usage

Find a file by name in the current directory:

$ find -name "research.png"

If you want to find the file called ‘research.png’ in the directory ‘folder’:

$ find folder -name "research.png"

Research with the size:

$ find ~ -size +10M

Research with the date of the last access (less than 7 days):

$ find ~ -name ".odt" -atime 6 

Advanced manipulation

You can add an action to the result of the search. For instance, if you want to delete the results:

$ find ~ -name ".odt" -delete 

With exec you can execute a command for all the files giving by find. Let’s say that you want to change the permissions of all the ‘.odt’ files:

$ find ~ -name ".odt" -exec chmod 640 {} \; 

How does it work? For each file found, the command after exec is executed. The curly brackets ‘{}’ will be replaced by the name of this file. The command must end with ‘;’.

Exercise: gather all your .jpeg files into a folder ‘images’.