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Different Kinds of Processes
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It depends on the way you look at them, but you could say that Linux basically has two different kinds of processes: automatic and interactive. First are the services that are started automatically when you boot your server. These processes are known as daemons. The start process that is responsible for an important part of your server s boot procedure takes care that these processes are started properly. Daemons are service processes that run in the background; in other words, they do not write their output directly to the standard output. The
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second kind of process is the interactive process. These processes are started by users from a shell. Any command started by a user and producing output on the standard output is an interactive process. To start an interactive process, a user needs to type the corresponding command. The process then runs as a child process from the shell in which the user entered the command. The process will do its work and will terminate when it s finished. While terminating, it will write its exit status to its parent (which is the shell if the process was an interactive process). Only after a child process has told its parent that it has terminated can it be closed properly. In case the parent is no longer present (which is generally considered an error condition), the child process will become a so-called zombie process, and it won t be possible to perform any management on the process, except for trying to restart the parent process. In general, zombie processes are the result of bad programming. You should try to upgrade (or maybe rewrite) your software if you see too many of them. The concepts of parent and child processes are universal on your system. The init process is started by upstart (which we ll cover later) as the first process and from there, all other processes are started. You can get an overview of the hierarchical process structure by using the pstree command, which provides a result such as in Listing 6-1. Listing 6-1. The pstree Command Shows Relations Between Parent and Child Processes. init ----apache2----5*[apache2] |--atd |--cron |--dd |--dhclient3 |--events/0 |--5*[getty] |--khelper |--klogd Although interactive processes are important to users working on your machine, daemon processes are more important on a server that is providing services. Daemon processes typically run in the background and normally don't send any output to the terminal. To see what they are doing, you must check the log files to which the daemon processes write. Generally speaking, it s a good idea to start with /var/log/messages if you are looking for daemon output. From a perspective of process management, it doesn't really matter if you re working with daemon or interactive processes: both can be handled the same way using the same commands.
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When working with interactive processes, it can be useful to know that processes can run in the foreground and in the background. Before talking about the way you can start and manage processes that run in the background, let's talk about some process details so that we can understand what's happening. A process always works with three standard file handlers that determine where the process should send its output and accept its input. They are the standard input (STDIN), the standard output (STDOUT), and the standard error (STDERR). Normally, when a process is running in
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the foreground, the STDIN is your keyboard, the STDOUT is the terminal the process is working on, and the STDERR is also the terminal that the process is working on. As you learned in 2, you can change them all by using redirection. It can be a little confusing that the three file descriptors don t change when you decide to run a process in the background. When it starts, the STDIN, STDOUT, and STDERR for a process are set, and, once they are set, they stay like that no matter what you do to the process. Therefore, you can run a long command like find / -name "*" -exec grep -ls something {} \; as a background job, but you ll still see its output and errors on your screen. If you don t want that, you should use redirection to send STDOUT and STDERR somewhere else: by putting > /somewhere after the command, you are redirecting the standard output to a file called /somewhere and by using 2> /dev/null, you can arrange for all errors to be redirected to the null device.
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