beginning of each line are the permissions set (Web server)
beginning of each line are the permissions set for each file. (Permissions and configuring shell property files are described later in this chapter.) Other information in the listing includes the size of each file in bytes (column 4) and the date and time each file was most recently modified (column 5). Checking system activity In addition to being a multi-user operating system, Linux is also a multitasking system. Multitasking means that many programs can be running at the same time. An instance of a running program is referred to as a process. Linux provides tools for listing running processes, monitoring system usage, and stopping (or killing) processes when necessary. The most common utility for checking running processes is the ps command. With ps, you can see which programs are running, the resources they are using, and who is running them. The following is an example of the ps command: $ ps au USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND root 2146 0.0 0.8 1908 1100 ttyp0 S 14:50 0:00 login — jake jake 2147 0.0 0.7 1836 1020 ttyp0 S 14:50 0:00 -bash jake 2310 0.0 0.7 2592 912 ttyp0 R 18:22 0:00 ps au In this example, the -a option asks to show processes of all users that are associated with your current terminal, and the -u option asks that user names (as opposed to numeric user IDs) be shown, as well as the time the process started. The concept of terminal comes from the old days, when people worked exclusively from character terminals, so a terminal typically represented a single person. Now you can have many “terminals” on one screen by opening multiple Terminal windows. On this shell session, there isn t much happening. The first process shows that the user named jake logged in to the login process (which is controlled by the root user). The next process shows that jake is using a bash shell and has just run the ps au command. The terminal device ttyp0 is being used for the login session. The STAT column represents the state of the process, with R indicating a currently running process and S representing a sleeping process. The USER column shows the name of the user who started the process. Each process is represented by a unique ID number referred to as a process ID (PID). (You can use the PID if you ever need to kill a runaway process.) The %CPU and %MEM columns show the percentage of the processor and random access memory, respectively, that the process is consuming. VSZ (virtual set size) shows size of the image process (in kilobytes), and RSS (resident set size) shows the size of the program in memory. START shows the time the process began running, and TIME shows the cumulative system time used. Many processes running on a computer are not associated with a terminal. A normal Red Hat Linux system has many processes running in the background. Background system processes perform such tasks as logging system activity or listening for data coming in from the network. They are often started when Red Hat Linux boots up and runs continuously until it shuts down. To see and thereby monitor all the processes running on your Red Hat Linux system, type: $ ps aux | less I added the pipe ( | )and the less command to ps aux to allow you to page through the many processes that will appear on your screen. A pipe lets you direct the output of one command to be the input of the next command. Exiting the shell To exit the shell when you are done, either type exit or press Ctrl+D. If you are exiting from your login shell (the shell that started when you first logged in), type logout to exit the shell.
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