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UNIX Disk Usage

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This chapter is from the book

A Closer Look with du

The df command is one you'll use often as you get into the groove of system administration work. In fact, some sysadmins have df e-mailed to them every morning from cron so they can keep a close eye on things. Others have it as a command in their .login or .profile configuration file so they see the output every time they connect.

Once you're familiar with how the disks are being utilized in your Unix system, however, it's time to dig a bit deeper into the system and ascertain where the space is going.

Task 3.2: Using du to Ascertain Directory Sizes

The du command shows you disk usage, helpfully enough, and it has a variety of flags that are critical to using this tool effectively.

  1. There won't be a quiz on this, but see if you can figure out what the default output of du is here when I use the command while in my home directory:

    # du
    12     ./.kde/Autostart
    16     ./.kde
    412    ./bin
    36     ./CraigsList
    32     ./DEMO/Src
    196    ./DEMO
    48     ./elance
    16     ./Exchange
    1232   ./Gator/Lists
    4      ./Gator/Old-Stuff/Adverts
    8      ./Gator/Old-Stuff
    1848   ./Gator/Snapshots
    3092   ./Gator
    160    ./IBM/i
    136    ./IBM/images
    10464  ./IBM
    76     ./CBO_MAIL
    52     ./Lynx/WWW/Library/vms
    2792   ./Lynx/WWW/Library/Implementation
    24     ./Lynx/WWW/Library/djgpp
    2872   ./Lynx/WWW/Library
    2880   ./Lynx/WWW
    556    ./Lynx/docs
    184    ./Lynx/intl
    16     ./Lynx/lib
    140    ./Lynx/lynx_help/keystrokes
    360    ./Lynx/lynx_help
    196    ./Lynx/po
    88     ./Lynx/samples
    20     ./Lynx/scripts
    1112   ./Lynx/src/chrtrans
    6848   ./Lynx/src
    192    ./Lynx/test
    13984   ./Lynx
    28484  .

    If you guessed that it's the size of each directory, you're right! Notice that the sizes are cumulative because they sum up the size of all files and directories within a given directory. So the Lynx directory is 13,984 somethings, which includes the subdirectory Lynx/src (6,848), which itself contains Lynx/src/chrtrans (1112).

    The last line is a summary of the entire current directory (.), which has a combined size of 28484.

    And what is that pesky unit of measure? Unfortunately, it's different in different implementations of Unix so I always check the man page before answering this question. Within RHL7.2, the man page for du reveals that the unit of measure isn't specifically stated, frustratingly enough. However, it shows that there's a -k flag that forces the output to 1KB blocks, so a quick check

    # du -k | tail -1
    28484  .

    produces the same number as the preceding, so we can safely conclude that the unit in question is a 1KB block. Therefore, you can see that Lynx takes up 13.6MB of space, and that the entire contents of my home directory consume 27.8MB. A tiny fraction of the 15GB /home partition!


Of course, I can recall when I splurged and bought myself a 20MB external hard disk for an early computer. I couldn't imagine that I could even fill it, and it cost more than $200 too! But I'll try not to bore you with the reminiscence of an old-timer, okay?

  1. The recursive listing of subdirectories is useful information, but the higher up you go in the file system, the less helpful that information proves to be. Imagine if you were to type du / and wade through the output:

    # du / | wc -l

    That's a lot of output!

    Fortunately, one of the most useful flags to du is -s, which summarizes disk usage by only reporting the files and directories that are specified, or . if none are specified:

    # du -s
    28484  .
    # du -s *
    4      badjoke
    4      badjoke.rot13
    412    bin
    4      browse.sh
    4      buckaroo
    76     CBO_MAIL
    36     CraigsList
    196    DEMO
    48     elance
    84     etcpasswd
    16     Exchange
    3092   Gator
    4      getmodemdriver.sh
    4      getstocks.sh
    4      gettermsheet.sh
    0      gif.gif
    10464  IBM
    13984  Lynx

    Note in the latter case that because I used the * wildcard, it matched directories and files in my home directory. When given the name of a file, du dutifully reports the size of that file in 1KB blocks. You can force this behavior with the -a flag if you want.


Tip - The summary vanishes from the bottom of the du output when I specify directories as parameters, and that's too bad, because it's very helpful. To request a summary at the end, simply specify the -c flag.

  1. While we're looking at the allocation of disk space, don't forget to check the root level, too. The results are interesting:

    # du -s /
    1471202 /

    Oops! We don't want just a one-line summary, but rather all the directories contained at the topmost level of the file system. Oh, and do make sure that you're running these as root, or you'll see all sorts of odd errors. Indeed, even as root the /proc file system will sporadically generate errors as du tries to calculate the size of a fleeting process table entry or similar. You can ignore errors in /proc in any case.

    One more try:

    # du -s /*
    5529     /bin
    3683     /boot
    244      /dev
    4384     /etc
    29808    /home
    1        /initrd
    67107    /lib
    12       /lost+found
    1        /misc
    2        /mnt
    1        /opt
    1        /proc
    1468     /root
    8514     /sbin
    12619    /tmp
    1257652  /usr
    80175    /var
    0        /web

    That's what I seek. Here you can see that the largest directory by a significant margin is /usr, weighing in at 1,257,652KB.

    Rather than calculate sizes, I'm going to use another du flag (-h) to ask for human-readable output:

    # du -sh /*
    5.4M  /bin
    3.6M  /boot
    244k  /dev
    4.3M  /etc
    30M   /home
    1.0k  /initrd
    66M   /lib
    12k   /lost+found
    1.0k  /misc
    2.0k  /mnt
    1.0k  /opt
    1.0k  /proc
    1.5M  /root
    8.4M  /sbin
    13M   /tmp
    1.2G  /usr
    79M   /var
    0     /web

    Much easier. Now you can see that /usr is 1.2GB in size, which is quite a lot!

  2. Let's use du to dig into the /usr directory and see what's so amazingly big, shall we?

    # du -sh /usr/*
    121M  /usr/bin
    4.0k  /usr/dict
    4.0k  /usr/etc
    40k   /usr/games
    30M   /usr/include
    3.6M  /usr/kerberos
    427M  /usr/lib
    2.7M  /usr/libexec
    224k  /usr/local
    16k   /usr/lost+found
    13M   /usr/sbin
    531M  /usr/share
    52k   /usr/src
    0     /usr/tmp
    4.0k  /usr/web
    103M  /usr/X11R6

    It looks to me like /usr/share is responsible for more than half the disk space consumed in /usr, with /usr/bin and /usr/X11R6 the next largest directories.

    You can easily step into /usr/share and run du again to see what's inside, but before we do, it will prove quite useful to take a short break and talk about sort and how it can make the analysis of du output considerably easier.

  3. Before we leave this section to talk about sort, though, let's have a quick peek at du within the Darwin environment:

    # du -sk *
    5888   Desktop
    396760 Documents
    84688  Library
    0      Movies
    0      Music
    31648  Pictures
    0      Public
    32     Sites

    Notice that I've specified the -k flag here to force 1KB blocks (similar to df, the default for du is 512-byte blocks). Otherwise, it's identical to Linux.

    The du output on Solaris is reported in 512-byte blocks unless, like Darwin, you force 1KB blocks with the -k flag:

    # du -sk *
    1      bin
    1689   boot
    4      cdrom
    372    dev
    13     devices
    2363   etc
    10     export
    0      home
    8242   kernel
    1      lib
    8      lost+found
    1      mnt
    0      net
    155306 opt
    1771   platform
    245587 proc
    5777   sbin
    32     tmp
    25     TT_DB
    3206   users
    667265 usr
    9268   var
    0      vol
    9      xfn

This section has demonstrated the helpful du command, showing how -a, -s, and -h can be combined to produce a variety of different output. You've also seen how successive du commands can help you zero in on disk space hogs, foreshadowing the diskhogs shell script we'll be developing later in this hour.

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