File Permissions

In Linux files are associated with an owner (user) and with a group that has permissions on the file (read, write, execute)

Commands to change this are:

• chown to change ownership. THe owner can manipulate the file, for example delete it
• chgroup to change group ownership
• chmod to change the permissions for a file (can be done for owner, group and other)

Changing permissions

Permissions in a file can be of three types:

• write(w)
• read(r)
• execute(x)

These 3 types fall into three categories:

• user/owner (u)
• group (g)
• others (o)

WIth this logic, permissions can be given in particular

ls -l somefile
-rw-rw-r-- 1 student student 1601 Mar 9 15:04 somefile
chmod uo+x,g-w somefile #User and others execute, remove write from group
ls -l somefile
-rwxr--r-x 1 student student 1601 Mar 9 15:04 somefile

The command can be simplified using a simple replacement for numbers:

• 4 for read permissions
• 2 for write permissions
• 1 for execute

For example chmod 755 file gives the owner all permissions, and the other two read and execute