CSC128: Introduction to UNIX

Process Management

UNIX Process Structure

UNIX is a multitasking system, which means that it can execute more than one program at a time.  The execution of a program is called a process.   The kernel keeps track of system resources and hands them out in a controlled fashion to all the programs that are running.  To do this, it creates a process structure.

The kernel makes the first process, the init process.  The init process's Process ID (pid) is 1.  All other processes are children of this process.  To create another process, the initiating process is forked.  

When a process is forked, a complete copy of the process is made in system memory, and then the new process takes over in the memory space of the copy.  Every process has a pointer to the parent that initiated it.  By mapping the parent/child relationships between processes, a tree-shape similar to the file structure emerges, with the init process at the root.

ps
This command lists the processes currently running, each with its pid. (See p. 826-9 in the textbook for more details on the ps command.)

Processes can be in one of many states.  The most typical states are foreground , background, and stopped.

Foreground processes execute while the shell waits for them to terminate.  When the program finishes, the shell gives another prompt.

Background processes execute the same as foreground processes, but the shell returns immediately with a prompt, an doesn't wait for the process to finish.

Stopped processes don't execute; they remain in memory, suspended and ready to execute if ever placed in the foreground or background.  Note that many programs don't take to being placed in the background, and stop themselves instead-- vi is an example.

jobs
This command will display a listing of the shell's current jobs.

fg
fg %
fg %n  (job number n)
This command will place the specified job (or the last job if none is specified) in the foreground.

bg
bg %
bg %n  (job number n)
This command will place the specified job (or the last job if none is specified) in the background. Or will start it in the background, leaving it there.

&
By appending an ampersand to a command, the shell will automatically start its process in the background, and return immediately with a prompt.

Ctrl-Z
This command stops most processes.  The stopped process can be started again with fg or bg.

Ctrl-C
This command cancels most processes.  The process is killed, gone forever.

Sometimes, processes can't be easily ended in a normal way.

kill PID
kill -9 PID
This first of these commands will ask a process to terminate.  The second will force a process to terminate, whether it wants to or not.

The kernel gives different processes different priorities, so that important processes can get quicker access to the hardware, and unimportant porcesses don't get in the way.

top
This command displays the current processes in a continually-updating display, sorted by resource usage.

nice command
nice -n niceness command (10 is default niceness; 19 is usually max)
This command executes with a lower priority, so that it doesn't get in the way of other, more important, processes.  The superuser (root) can also use this command to give processes higher priority, by passing a negative value.

Usually, when you log out of a shell, it hangs up on all remaining processes.  Some shells will warn you when this happens.  To keep a process from being killed when you log out, run it with the nohup (no hang- up) command.

nohup command

The process will run until it terminates normally or is killed.   stdout will be redirected to nohup.out for the duration of the process's execution.