3.4.3. Managing Processes
The
ps aux
command lists the processes currently running and helps to identify them by showing
their PID. Once you know the
PID of a process, the
kill -
signal pid
command allows you to send
it a signal (if you own the process). Several signals exist; most commonly used are TERM (a request
to terminate gracefully) and KILL (a forced kill).
The command interpreter can also run programs in the background if the command is followed
by “
&
”. By using the ampersand, you resume control of the shell immediately even though the
command is still running (hidden from view as a background process). The
jobs
command lists
the processes running in the background; running
fg %
job-number
(for
foreground) restores a
job to the foreground. When a command is running in the foreground (either because it was
started normally, or brought back to the foreground with
fg
), the Control+Z key combination
pauses the process and resumes control of the command line. The process can then be restarted
in the background with
bg %
job-number
(for
background).
3.4.4. Managing Rights
Linux is a multi-user system so it is necessary to provide a permissions system to control the set of
authorized operations on files and directories, which includes all the system resources and devices
(on a Unix system, any device is represented by a file or directory). This principle is common to
all Unix-like systems.
Each file or directory has specific permissions for three categories of users:
• Its owner (symbolized by u, as in User)
• Its owner group (symbolized by g, as in Group), representing all the members of the group
• The others (symbolized by o, as in Other)
Three types of rights can be combined:
• Reading (symbolized by r, as in Read);
• Writing (or modifying, symbolized by w, as in Write);
• Executing (symbolized by x, as in eXecute).
In the case of a file, these rights are easily understood: read access allows reading the content
(including copying), write access allows changing it, and execute access allows running it (which
will only work if it is a program).
