UNIX Disk Usage

• By Dave Taylor
• Jan 17, 2003
• Sample Chapter is provided courtesy of Sams

This chapter is from the book
Sams Teach Yourself UNIX System Administration in 24 Hours

Simplifying Analysis with sort

The output of du has been very informative, but it's difficult to scan a listing to ascertain the four or five largest directories, particularly as more and more directories and files are included in the output. The good news is that the Unix sort utility is just the tool we need to sidestep this problem.

Task 3.3: Piping Output to sort

Why should we have to go through all the work of eyeballing page after page of listings when there are Unix tools to easily let us ascertain the biggest and smallest? One of the great analysis tools in Unix is sort, even though you rarely see it mentioned in other Unix system administration books.

1. At its most obvious, sort alphabetizes output:

   # cat names
   Linda
   Ashley
   Gareth
   Jasmine
   Karma
   # sort names
   Ashley
   Gareth
   Jasmine
   Karma
   Linda

   No rocket science about that! However, what happens if the output of du is fed to sort?

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

   Sure enough, it's sorted. But probably not as you expected—it's sorted by the ASCII digit characters! Not good.

2. That's where the -n flag is a vital addition: With -n specified, sort will assume that the lines contain numeric information and sort them numerically:

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

   A much more useful result, if I say so myself!

3. The only thing I'd like to change in the sorting here is that I'd like to have the largest directory listed first, and the smallest listed last.

   The order of a sort can be reversed with the -r flag, and that's the magic needed:

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

   One final concept and we're ready to move along. If you want to only see the five largest files or directories in a specific directory, all that you'd need to do is pipe the command sequence to head:

   # du -s * | sort -nr | head -5
   13984  Lynx
   10464  IBM
   3092   Gator
   412    bin
   196    DEMO

   This sequence of sort|head will prove very useful later in this hour.

A key concept with Unix is understanding how the commands are all essentially Lego pieces, and that you can combine them in any number of ways to get exactly the results you seek. In this vein, sort -rn is a terrific piece, and you'll find yourself using it again and again as you learn more about system administration.