The df program displays the amount of free space, partition by partition. The drives/partitions must be mounted in order to get this information. NFS information can be gathered this way, as well. Some parameters for df are listed here; additional (rarely used) options are listed in the df manual page.

To show the free space in a human-readable format for the file system on which /tmp is located, type this command:

    [michael@workbox michael]$ df -h /tmp

Synchronizing Disks: sync

Like most other modern operating systems, Linux maintains a disk cache to improve efficiency. The drawback, of course, is that not everything you want written to disk will have been written to disk at any given moment. To schedule the disk cache to be written out to disk, you use the sync command. If sync detects that writing the cache out to disk has already been scheduled, the kernel is instructed to immediately flush the cache. This command takes no command-line parameters. Type this command to ensure the disk cache has been flushed:

    [root@workbox /root]# sync ; sync

Tracking and Terminating Processes

Under Linux (and UNIX in general), each running program comprises at least one process. From the operating system’s standpoint, each process is independent of the others. Unless it specifically asks to share resources with other processes, a process is confined to the memory and CPU allocation assigned to it. Processes that overstep their memory allocation (which could potentially corrupt another running program and make the system unstable) are immediately killed. This method of handling processes has been a major contributor to the stability of UNIX systems: User applications cannot corrupt other user programs or the operating system. This section describes the tools used to list and manipulate processes. They are very important elements of a system administrator’s daily work.

Listing Processes: ps

The ps command lists all the processes in a system, their state, size, name, owner, CPU time, wall clock time, and much more. There are many command-line parameters available;

The most common set of parameters used with the ps command is –auxww. These parameters show all the processes (regardless of whether they have a controlling terminal), each process’s owners, and all the processes’ command-line parameters. Let’s examine the output of an invocation of ps –auxww.

    USER PID %CPU %MEM VSZ RSS TTY STAT START TIME COMMAND
    root 1 0.0 0.3 1096 476 ? S Jun10 0:04 init
    root 2 0.0 0.0 0 0 ? SW Jun10 0:00 [kflushd]
    root 3 0.0 0.0 0 0 ? SW Jun10 0:00 [kpiod]
    root 4 0.0 0.0 0 0 ? SW Jun10 0:00 [kswapd]
    root 5 0.0 0.0 0 0 ? SW< Jun10 0:00 [mdrecoveryd]
    root 102 0.0 0.2 1068 380 ? S Jun10 0:00 /usr/sbin/
    apmd -p 10 -w 5
    bin 253 0.0 0.2 1088 288 ? S Jun10 0:00 portmap
    root 300 0.0 0.4 1272 548 ? S Jun10 0:00 syslogd -m 0
    root 311 0.0 0.5 1376 668 ? S Jun10 0:00 klogd
    daemon 325 0.0 0.2 1112 284 ? S Jun10 0:00 /usr/sbin/atd
    root 339 0.0 0.4 1284 532 ? S Jun10 0:00 crond
    root 357 0.0 0.3 1232 508 ? S Jun10 0:00 inetd
    root 371 0.0 1.1 2528 1424 ? S Jun10 0:00 named
    root 385 0.0 0.4 1284 516 ? S Jun10 0:00 lpd
    root 399 0.0 0.8 2384 1116 ? S Jun10 0:00 httpd
    xfs 429 0.0 0.7 1988 908 ? S Jun10 0:00 xfs
    root 467 0.0 0.2 1060 384 tty2 S Jun10 0:00 /sbin/
    mingetty tty2
    root 468 0.0 0.2 1060 384 tty3 S Jun10 0:00 /sbin/
    mingetty tty3
    root 469 0.0 0.2 1060 384 tty4 S Jun10 0:00 /sbin/
    mingetty tty4
    root 470 0.0 0.2 1060 384 tty5 S Jun10 0:00 /sbin/
    mingetty tty5
    root 471 0.0 0.2 1060 384 tty6 S Jun10 0:00 /sbin/
    mingetty tty6
    root 473 0.0 0.0 1052 116 ? S Jun10 0:01 update
    (bdflush)
    root 853 0.0 0.7 1708 940 pts/1 S Jun10 0:00 BASH
    root 1199 0.0 0.7 1940 1012 pts/2 S Jun10 0:00 su
    root 1203 0.0 0.7 1700 920 pts/2 S Jun10 0:00 BASH
    root 1726 0.0 1.3 2824 1760 ? S Jun10 0:00 xterm
    root 1728 0.0 0.7 1716 940 pts/8 S Jun10 0:00 BASH
    root 1953 0.0 1.3 2832 1780 ? S Jun11 0:05 xterm
    root 1955 0.0 0.7 1724 972 pts/10 S Jun11 0:00 BASH
    nobody 6436 0.0 0.7 2572 988 ? S Jun13 0:00 httpd
    nobody 6437 0.0 0.7 2560 972 ? S Jun13 0:00 httpd
    nobody 6438 0.0 0.7 2560 976 ? S Jun13 0:00 httpd
    nobody 6439 0.0 0.7 2560 976 ? S Jun13 0:00 httpd
    nobody 6440 0.0 0.7 2560 976 ? S Jun13 0:00 httpd
    nobody 6441 0.0 0.7 2560 976 ? S Jun13 0:00 httpd
    root 16673 0.0 0.6 1936 840 pts/10 S Jun14 0:00 su -michael
    michael 16675 0.0 0.8 1960 1112 pts/10 S Jun14 0:00 -zsh
    root 18243 0.0 0.9 2144 1216 tty1 S Jun14 0:00 login --
    rwhite
    rwhite 18244 0.0 0.8 1940 1080 tty1 S Jun14 0:00 -tcsh

The very first line of the output provides column headers for the listing, as follows:

- Column 1: USER Who owns what process.

- Column 2: PID Process identification number.

- Column 3: %CPU Percentage of the CPU taken up by a process. Note: For a system with multiple processors, this column will add up to more than 100 percent.

- Column 4: %MEM Percentage of memory taken up by a process.

- Column 5: VSZ The amount of virtual memory a process is taking.

- Column 6: RSS The amount of actual (resident) memory a process is taking.

- Column 7: TTY The controlling terminal for a process. A question mark in this column means the process is no longer connected to a controlling terminal.

- Column 8: STAT The state of the process:

- S Process is sleeping. All processes that are ready to run (that is, being multitasked, and the CPU is currently focused elsewhere) will be asleep.

- R Process is actually on the CPU.

- D Uninterruptible sleep (usually I/O related).

- T Process is being traced by a debugger or has been stopped.

- Z Process has gone zombie. This means either (1) the parent process has not acknowledged the death of its child using the wait system call; or (2) the parent was improperly killed, and until the parent is completely killed, the init process (see Linux tutorial 8) cannot reap the child itself. A zombied process usually indicates poorly written software. In addition, the STAT entry for each process can take one of the following modifiers: W = No resident pages in memory (it has been completely swapped out); < = High-priority process; N = Low-priority task; L = Pages in memory are locked there (usually signifying the need for real-time functionality).

- Column 9: START Date the process was started.

- Column 10: TIME Amount of time the process has spent on the CPU.

- Column 11: COMMAND Name of the process and its command-line parameters.