Introduction to UNIX:  Lecture Four

This lecture covers:

• The concept of a process.
• Passing output from one process as input to another using pipes.
• Redirecting process input and output.
• Controlling processes associated with the current shell.
• Controlling other processes.

A process is a program in execution. Every time you invoke a system utility or an application program from a shell, one or more "child" processes are created by the shell in response to your command. All UNIX processes are identified by a unique process identifier or PID. An important process that is always present is the init process. This is the first process to be created when a UNIX system starts up and usually has a PID of 1. All other processes are said to be "descendants" of init.

The pipe ('|') operator is used to create concurrently executing processes that pass data directly to one another. It is useful for combining system utilities to perform more complex functions. For example:

    $ cat hello.txt | sort | uniq

creates three processes (corresponding to cat, sort and uniq) which execute concurrently. As they execute, the output of the who process is passed on to the sort process which is in turn passed on to the uniq process. uniq displays its output on the screen (a sorted list of users with duplicate lines removed). Similarly:

    $ cat hello.txt | grep "dog" | grep -v "cat"

finds all lines in hello.txt that contain the string "dog" but do not contain the string "cat".

The output from programs is usually written to the screen, while their input usually comes from the keyboard (if no file arguments are given). In technical terms, we say that processes usually write to standard output (the screen) and take their input from standard input (the keyboard). There is in fact another output channel called standard error, where processes write their error messages; by default error messages are also sent to the screen.

To redirect standard output to a file instead of the screen, we use the > operator:

    $ echo hello
    hello
    $ echo hello > output
    $ cat output
    hello

In this case, the contents of the file output will be destroyed if the file already exists. If instead we want to append the output of the echo command to the file, we can use the >> operator:

    $ echo bye >> output
    $ cat output
    hello
    bye

To capture standard error, prefix the > operator with a 2 (in UNIX the file numbers 0, 1 and 2 are assigned to standard input, standard output and standard error respectively), e.g.:

    $ cat nonexistent 2>errors
    $ cat errors
    cat: nonexistent: No such file or directory
    $

You can redirect standard error and standard output to two different files:

    $ find . -print 1>errors 2>files

or to the same file:

    $ find . -print 1>output 2>output
or
    $ find . -print >& output

Standard input can also be redirected using the < operator, so that input is read from a file instead of the keyboard:

    $ cat < output
    hello
    bye

You can combine input redirection with output redirection, but be careful not to use the same filename in both places. For example:

    $ cat < output > output

will destroy the contents of the file output. This is because the first thing the shell does when it sees the > operator is to create an empty file ready for the output.

One last point to note is that we can pass standard output to system utilities that require filenames as "-":

    $ cat package.tar.gz | gzip -d | tar tvf -

Here the output of the gzip -d command is used as the input file to the tar command.

Most shells provide sophisticated job control facilities that let you control many running jobs (i.e. processes) at the same time. This is useful if, for example, you are editing a text file and want ot interrupt your editing to do something else. With job control, you can suspend the editor, go back to the shell prompt, and start work on something else. When you are finished, you can switch back to the editor and continue as if you hadn't left.

Jobs can either be in the foreground or the background. There can be only one job in the foreground at any time. The foreground job has control of the shell with which you interact - it receives input from the keyboard and sends output to the screen. Jobs in the background do not receive input from the terminal, generally running along quietly without the need for interaction (and drawing it to your attention if they do).

The foreground job may be suspended, i.e. temporarily stopped, by pressing the Ctrl-Z key. A suspended job can be made to continue running in the foreground or background as needed by typing "fg" or "bg" respectively. Note that suspending a job is very different from interrupting a job (by pressing the interrupt key, usually Ctrl-C); interrupted jobs are killed off permanently and cannot be resumed.

Background jobs can also be run directly from the command line, by appending a '&' character to the command line. For example:

    $ find / -print 1>output 2>errors &
    [1] 27501
    $

Here the [1] returned by the shell represents the job number of the background process, and the 27501 is the PID of the process. To see a list of all the jobs associated with the current shell, type jobs:

    $ jobs
    [1]+  Running  find / -print 1>output 2>errors &
    $

Note that if you have more than one job you can refer to the job as %n where n is the job number. So for example fg %3 resumes job number 3 in the foreground.

To find out the process ID's of the underlying processes associated with the shell and its jobs, use ps (process show):

    $ ps
      PID TTY          TIME CMD
    17717 pts/10   00:00:00 bash
    27501 pts/10   00:00:01 find
    27502 pts/10   00:00:00 ps

So here the PID of the shell (bash) is 17717, the PID of find is 27501 and the PID of ps is 27502.

To terminate a process or job abrubtly, use the kill command. kill allows  jobs to referred to in two ways - by their PID or by their job number. So
    $ kill %1
or
    $ kill 27501

would terminate the find process. Actually kill only sends the process a signal requesting it shutdown and exit gracefully (the SIGTERM signal), so this may not always work. To force a process to terminate abruptly (and with a higher probability of sucess), use a -9 option (the SIGKILL signal):

     $ kill -9 27501

kill can be used to send many other types of signals to running processes. For example a -19 option (SIGSTOP) will suspend a running process. To see a list of such signals, run kill -l.

You can also use ps to show all processes running on the machine (not just the processes in your current shell):

    $ ps -fae(or ps -aux on BSD machines)

 ps -aeH displays a full process hierarchy (including the init process).

Many UNIX versions have a system utility called top that provides an interactive way to monitor system activity. Detailed statistics about currently running processes are displayed and constantly refreshed. Processes are displayed in order of CPU utilization. Useful keys in top are:

    s - set update frequency                  k - kill process (by PID)
    u - display processes of one user     q - quit

On some systems, the utility w is a non-interactive substitute for top.

One other useful process control utility that can be found on most UNIX systems is the pkill command. You can use pkill to kill processes by name instead of PID or job number. So another way to kill off our background find process (along with any another find processes we are running) would be:

    $ pkill find
    [1]+  Terminated find / -print 1>output 2>errors
    $

Note that, for obvious security reasons, you can only kill processes that belong to you (unless you are the superuser).

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