Process:
Any application that runs on a Linux system is assigned a process ID or PID. This is a numerical representation of the instance of the application on the system. In most situations this information is only relevant to the system administrator who may have to debug or terminate processes by referencing the PID. Process Management is the series of tasks a System Administrator completes to monitor, manage, and maintain instances of running applications.
Type of Process:
1. Interactive Process
2. System Process (daemon)
Interactive Process:
The processes that are invoked by a user and can interact with the user is called Interactive processes. Interactive processes can be classified into foreground and background processes. Foreground process is the process that you are currently interacting with, and is using the terminal as its stdin (standard input) and stdout (standard output). Background process is not interacting with the user and can be in one of two states – paused or running.
Example of Interactive process:
The following example will show how foreground and background processes are running.
1. Logon as root.
# su -
2. Run [cd \]
# cd \
3. Run [vi]
# vi data.txt
4. Press [ctrl + z]. This will pause vi
Ctrl + z
5. Type [jobs]
# job
Notice: vi is running in the background
7. Type [fg %1]. This will bring the first background process to the foreground.
# fg %1
8. Close vi
:q
System Process:
The process that runs on Linux is a system process or Daemon (day-mon). Daemon is the term used to refer to process’ that are running on the computer and provide services but do not interact with the console. Most server software is implemented as a daemon. Apache, Samba, and inn are all examples of daemons.
Any process can become a daemon as long as it is run in the background, and does not interact with the user. A simple example using the [gedit] command. This will list all text editor window separately on the computer. This command can be set to run in the background by typing [gedit&], and although technically you have control over the shell prompt, you will be able to do little work as the screen displays the output of the process that you have running in the background.
Notice: The standard pause (ctrl+z) and kill (ctrl+c) commands.
1. View linux process
Show process launched for the current linux user
$ ps
Show process launched for a linux user
$ ps -u username
View all the running linux process
$ ps aux
To see a process tree
$ pstree
To see the processes in real time
2. Stop temporarily processes
$ bigjob
^Z (CRTL+Z)
To see the stopped linux processes
$ jobs
[1]+ Stopped bigjob
Kill an stopped process
$ kill %1
[1]+ Killed bigjob
Resume the linux process
$ fg 1
3. Foreground and background linux processes
To start a process in background mode
$ bigjob &
See process
$jobs
[1]+ Running bigjob
Put it in foreground mode
$fg 1
ps command:
The ps command is the used to manage running processes and can be used for many things including viewing the status of your computer and knowing how well the computer is performing.
Some common ps commands:
View display currently running process on this terminal.
# ps
All current processes on this terminal, by all users.
# ps –a
All processes not assigned to a terminal (daemons).
# ps –x
To get information about all running process
# ps –ag
To kill all process except your shell
# kill 0
Start a process in background
# linux-command &
To get all the details regarding the running process.
# ps aux
To check a particular process.
# ps ax | grep process-name
To see currently running processes and there memory usage.
# top
To see currently running processes in a tree structure.
# pstree
4 killing linux processes
· kill -1 pid. Restart the process
· kill -9 pid . kill the process
· kill -15 pid End the process
System calls used for Process management:
- Fork () :- Used to create a new process
- Exec() :- Execute a new program
- Wait():- wait until the process finishes execution
- Exit():- Exit from the process
- Getpid():- get the unique process id of the process
- Getppid():- get the parent process unique id
- Nice():- to bias the existing property of process
Signals:
Various Signals are:
| Signal Name | Number | Description |
| SIGHUP | 1 | Hangup (POSIX) |
| SIGINT | 2 | Terminal interrupt (ANSI) |
| SIGQUIT | 3 | Terminal quit (POSIX) |
| SIGILL | 4 | Illegal instruction (ANSI) |
| SIGTRAP | 5 | Trace trap (POSIX) |
| SIGIOT | 6 | IOT Trap (4.2 BSD) |
| SIGBUS | 7 | BUS error (4.2 BSD) |
| SIGFPE | 8 | Floating point exception (ANSI) |
| SIGKILL | 9 | Kill(can't be caught or ignored) (POSIX) |
| SIGUSR1 | 10 | User defined signal 1 (POSIX) |
| SIGSEGV | 11 | Invalid memory segment access (ANSI) |
| SIGUSR2 | 12 | User defined signal 2 (POSIX) |
| SIGPIPE | 13 | Write on a pipe with no reader, Broken pipe (POSIX) |
| SIGALRM | 14 | Alarm clock (POSIX) |
| SIGTERM | 15 | Termination (ANSI) |
| SIGSTKFLT | 16 | Stack fault |
| SIGCHLD | 17 | Child process has stopped or exited, changed (POSIX) |
| SIGCONT | 18 | Continue executing, if stopped (POSIX) |
| SIGSTOP | 19 | Stop executing(can't be caught or ignored) (POSIX) |
| SIGTSTP | 20 | Terminal stop signal (POSIX) |
| SIGTTIN | 21 | Background process trying to read, from TTY (POSIX) |
| SIGTTOU | 22 | Background process trying to write, to TTY (POSIX) |
| SIGURG | 23 | Urgent condition on socket (4.2 BSD) |
| SIGXCPU | 24 | CPU limit exceeded (4.2 BSD) |
| SIGXFSZ | 25 | File size limit exceeded (4.2 BSD) |
| SIGVTALRM | 26 | Virtual alarm clock (4.2 BSD) |
| SIGPROF | 27 | Profiling alarm clock (4.2 BSD) |
| SIGWINCH | 28 | Window size change (4.3 BSD, Sun) |
| SIGIO | 29 | I/O now possible (4.2 BSD) |
| SIGPWR | 30 | Power failure restart (System V) |
