When you execute a program on your UNIX system, the system creates a special environment for that program. This environment contains everything needed for the system to run the program as if no other program were running on the system.
Whenever you issue a command in UNIX, it creates, or starts, a new process. When you tried out the ls command to list directory contents, you started a process. A process, in simple terms, is an instance of a running program.
The operating system tracks processes through a five digit ID number known as the pid or process ID . Each process in the system has a unique pid.
Pids eventually repeat because all the possible numbers are used up and the next pid rolls or starts over. At any one time, no two processes with the same pid exist in the system because it is the pid that UNIX uses to track each process.
Starting a Process:
When you start a process (run a command), there are two ways you can run it:
Foreground Processes
Background Processes
Foreground Processes:
By default, every process that you start runs in the foreground. It gets its input from the keyboard and sends its output to the screen.
You can see this happen with the ls command. If I want to list all the files in my current directory, I can use the following command:
$ls ch*.doc
This would display all the files whose name start with ch and ends with .doc:
ch01-1.doc ch010.doc ch02.doc ch03-2.doc
ch04-1.doc ch040.doc ch05.doc ch06-2.doc
ch01-2.doc ch02-1.doc
The process runs in the foreground, the output is directed to my screen, and if the ls command wants any input (which it does not), it waits for it from the keyboard.
While a program is running in foreground and taking much time, we cannot run any other commands (start any other processes) because prompt would not be available until program finishes its processing and comes out.
Background Processes:
A background process runs without being connected to your keyboard. If the background process requires any keyboard input, it waits.
The advantage of running a process in the background is that you can run other commands; you do not have to wait until it completes to start another!
The simplest way to start a background process is to add an ampersand ( &) at the end of the command.
$ls ch*.doc &
This would also display all the files whose name start with ch and ends with .doc:
ch01-1.doc ch010.doc ch02.doc ch03-2.doc
ch04-1.doc ch040.doc ch05.doc ch06-2.doc
ch01-2.doc ch02-1.doc
Here if the ls command wants any input (which it does not), it goes into a stop state until I move it into the foreground and give it the data from the keyboard.
That first line contains information about the background process - the job number and process ID. You need to know the job number to manipulate it between background and foreground.
If you press the Enter key now, you see the following:
[1] + Done ls ch*.doc &
$
The first line tells you that the ls command background process finishes successfully. The second is a prompt for another command.
Listing Running Processes:
It is easy to see your own processes by running the ps (process status) command as follows:
$ps
PID TTY TIME CMD
18358 ttyp3 00:00:00 sh
18361 ttyp3 00:01:31 abiword
18789 ttyp3 00:00:00 ps
One of the most commonly used flags for ps is the -f ( f for full) option, which provides more information as shown in the following example:
$ps -f
UID PID PPID C STIME TTY TIME CMD
amrood 6738 3662 0 10:23:03 pts/6 0:00 first_one
amrood 6739 3662 0 10:22:54 pts/6 0:00 second_one
amrood 3662 3657 0 08:10:53 pts/6 0:00 -ksh
amrood 6892 3662 4 10:51:50 pts/6 0:00 ps -f
Whenever you issue a command in UNIX, it creates, or starts, a new process. When you tried out the ls command to list directory contents, you started a process. A process, in simple terms, is an instance of a running program.
The operating system tracks processes through a five digit ID number known as the pid or process ID . Each process in the system has a unique pid.
Pids eventually repeat because all the possible numbers are used up and the next pid rolls or starts over. At any one time, no two processes with the same pid exist in the system because it is the pid that UNIX uses to track each process.
Starting a Process:
When you start a process (run a command), there are two ways you can run it:
Foreground Processes
Background Processes
Foreground Processes:
By default, every process that you start runs in the foreground. It gets its input from the keyboard and sends its output to the screen.
You can see this happen with the ls command. If I want to list all the files in my current directory, I can use the following command:
$ls ch*.doc
This would display all the files whose name start with ch and ends with .doc:
ch01-1.doc ch010.doc ch02.doc ch03-2.doc
ch04-1.doc ch040.doc ch05.doc ch06-2.doc
ch01-2.doc ch02-1.doc
The process runs in the foreground, the output is directed to my screen, and if the ls command wants any input (which it does not), it waits for it from the keyboard.
While a program is running in foreground and taking much time, we cannot run any other commands (start any other processes) because prompt would not be available until program finishes its processing and comes out.
Background Processes:
A background process runs without being connected to your keyboard. If the background process requires any keyboard input, it waits.
The advantage of running a process in the background is that you can run other commands; you do not have to wait until it completes to start another!
The simplest way to start a background process is to add an ampersand ( &) at the end of the command.
$ls ch*.doc &
This would also display all the files whose name start with ch and ends with .doc:
ch01-1.doc ch010.doc ch02.doc ch03-2.doc
ch04-1.doc ch040.doc ch05.doc ch06-2.doc
ch01-2.doc ch02-1.doc
Here if the ls command wants any input (which it does not), it goes into a stop state until I move it into the foreground and give it the data from the keyboard.
That first line contains information about the background process - the job number and process ID. You need to know the job number to manipulate it between background and foreground.
If you press the Enter key now, you see the following:
[1] + Done ls ch*.doc &
$
The first line tells you that the ls command background process finishes successfully. The second is a prompt for another command.
Listing Running Processes:
It is easy to see your own processes by running the ps (process status) command as follows:
$ps
PID TTY TIME CMD
18358 ttyp3 00:00:00 sh
18361 ttyp3 00:01:31 abiword
18789 ttyp3 00:00:00 ps
One of the most commonly used flags for ps is the -f ( f for full) option, which provides more information as shown in the following example:
$ps -f
UID PID PPID C STIME TTY TIME CMD
amrood 6738 3662 0 10:23:03 pts/6 0:00 first_one
amrood 6739 3662 0 10:22:54 pts/6 0:00 second_one
amrood 3662 3657 0 08:10:53 pts/6 0:00 -ksh
amrood 6892 3662 4 10:51:50 pts/6 0:00 ps -f
