The Linux free(1) utility shows your memory status. It simply reads the file /proc/meminfo and reformats the information. If you want an alias (Section 29.2) that simply shows how much memory is free, it's probably simpler to read the meminfo file directly. For example:

grep Section 13.1

$ cat /proc/meminfo
        total:    used:    free:  shared: buffers:  cached:
Mem:  263929856 253022208 10907648 79675392 30797824 57868288
Swap: 394784768 14585856 380198912
MemTotal:    257744 kB
MemFree:      10652 kB
MemShared:    77808 kB
Buffers:      30076 kB
Cached:       56512 kB
BigTotal:         0 kB
BigFree:          0 kB
SwapTotal:   385532 kB
SwapFree:    371288 kB
$ alias memfree='grep Free: /proc/meminfo'
$ memfree
MemFree:      10616 kB
BigFree:          0 kB
SwapFree:    371288 kB

(The free RAM decreased a bit while I was writing the alias.)

The /proc/stat file has statistics on the kernel and system. As with most of the rest of /proc, it's updated constantly. For example, we can grep for the CPU statistics. The four fields on the cpu line show the number of jiffies (hundredths of a second) since the system was last rebooted: time spent in normal-priority user mode, niced user mode (Section 26.5), system (kernel) mode, and the idle task, respectively. You might want to use this information from a script that monitors your system's utilization. Here's an example: grepping for the CPU ststistics, then the start of an awk (Section 20.10) script that could watch the CPU usage:

!! Section 30.8

$ grep cpu /proc/stat
cpu  14693561 48135949 638573 4031301
$ awk '/^cpu/ { print $5 / 100 " seconds idle" }' /proc/stat
40318.7 seconds idle
$ !!
awk '/^cpu/ { print $5 / 100 " seconds idle" }' /proc/stat
40323.8 seconds idle

The sections below describe per-process subdirectories in /proc. One special directory is /proc/self. It has the unusual property of giving a different answer for every process that examines it: information about the current process. (This "directory" is actually a symbolic link (Section 10.4) to the directory numbered for the process' PID.)

For instance, a process can check its /proc/self/fd directory to see which files its file descriptors (Section 36.15) are currently pointing to. This isn't just what type of file (disk file, tty (Section 2.7), pipe, etc.) but the actual full pathname of the file. If you're new to Unix, this may not seem too earth-shaking, but it's actually pretty amazing.

For a simple example, here's a shell script that lists its input and outputs. It then redirects its standard input (file descriptor 0) from /dev/null (Section 43.12) and lists again.

$ pwd
/tmp
$ tty
/dev/pts/5
$ cat showfds
#!/bin/sh
cd /proc/self/fd
ls -l
exec 0</dev/null
ls -l
$ ./showfds < somefile
total 0
lr-x------    1 jpeek    jpeek     64 Dec  2 09:03 0 -> /tmp/somefile
lrwx------    1 jpeek    jpeek     64 Dec  2 09:03 1 -> /dev/pts/5
lrwx------    1 jpeek    jpeek     64 Dec  2 09:03 2 -> /dev/pts/5
lr-x------    1 jpeek    jpeek     64 Dec  2 09:03 3 -> /tmp/showfds
total 0
lr-x------    1 jpeek    jpeek     64 Dec  2 09:03 0 -> /dev/null
lrwx------    1 jpeek    jpeek     64 Dec  2 09:03 1 -> /dev/pts/5
lrwx------    1 jpeek    jpeek     64 Dec  2 09:03 2 -> /dev/pts/5
lr-x------    1 jpeek    jpeek     64 Dec  2 09:03 3 -> /tmp/showfds

All versions of /proc that I've seen have subdirectories named for each process currently running on the system. Each subdirectory is named for the process PID (Section 24.3). Here are a series of examples of the useful info on my Linux system:

Go to http://examples.oreilly.com/upt3 for more information on: showenv

• You can use printenv or env (Section 35.3) to find the environment of your current process. How about the environment of another process? Here's a shell script called showenv that works like printenv:

  #!/bin/sh
  # showenv - show environment of a process, by PID
  # If second argument given, show just that one environment variable.
  
  f=/proc/$1/environ
  
  if [ ! -r "$f" ]; then
      echo "`basename $0`: can't access process $1" 1>&2
      exit 1
  fi
  
  case $# in
  1) tr '\000' '\012' < $f | sort ;;
  2) tr '\000' '\012' < $f | grep "^$2=" ;;
  *) echo "Usage: `basename $0` pid [envariable-name]" 1>&2; exit 1 ;;
  esac

  The tr (Section 21.11) command translates the NUL-separated entries from the environ file into newline-separated lines. With one argument, the whole environment is shown. With two arguments, the script greps for the environment variable named in the second argument. Maybe you'd like to know what the EXINIT (Section 17.27) environment variable was set to in a vi process with PID 8984:

  $ showenv 8984
  DISPLAY=:0.0
  ECIINI=/usr/lib/ViaVoiceTTS/eci.ini
  EDITOR=vi
  EXINIT=so ~/.lib/vi/exrc8
  HISTFILESIZE=1000
    ...
  $ showenv 8984 EXINIT
  EXINIT=so ~/.lib/vi/exrc8

• The status file gives status information about the process. A lot of this information is available in ps (Section 24.5) output, but it's broken out nicely here. For instance, maybe you're wondering what group access process 918 has, or what process started it (its parent PID (Section 24.3)):

  % cd /proc/918
  % grep PPid status
  PPid:   916
  % grep Groups status
  Groups: 1000 501 103

  The PPID is 916. The process has the group numbers (can access resources with the group permissions of) GIDs 1000, 501, and 103.

• The command-line arguments of a process are in the cmdline file, separated by NUL characters. Hmmm, what files is that tail -f job, process 861, watching? Let's see...using echo (Section 27.5) to add a final newline:

; Section 28.16

$ tr '\000' ' ' < /proc/861/cmdline; echo
tail -f /var/log/messages /var/log/maillog /u/jerry/tmp/startx.log

If you are curious about your system's hardware, a quick look at /proc/cpuinfo, /proc/interrupts, and /proc/ioports will help you size up the system. All the following examples came from a Red Hat Linux box, but you will find these proc files on most Linux and BSD systems. For instance, /proc/cpuinfo looks like this (on my system):

This file makes it easy to diagnosis hardware conflicts. However, if your system is working well, you probably won't be looking at any of these files much.