Bash-Beginners-Guide

Bash Guide for Beginners
6.3.6. The printf program
For more precise control over the output format than what is normally provided by print, use printf. The
printf command can be used to specify the field width to use for each item, as well as various formatting
choices for numbers (such as what output base to use, whether to print an exponent, whether to print a sign,
and how many digits to print after the decimal point). This is done by supplying a string, called the format
string, that controls how and where to print the other arguments.
The syntax is the same as for the C-language printf statement; see your C introduction guide. The gawk info
pages contain full explanations.
6.4. Summary
The gawk utility interprets a special-purpose programming language, handling simple data-reformatting jobs
with just a few lines of code. It is the free version of the general UNIX awk command.
This tools reads lines of input data and can easily recognize columned output. The print program is the most
common for filtering and formatting defined fields.
On-the-fly variable declaration is straightforward and allows for simple calculation of sums, statistics and
other operations on the processed input stream. Variables and commands can be put in awk scripts for
background processing.
Other things you should know about awk:
• The language remains well-known on UNIX and alikes, but for executing similar tasks, Perl is now
more commonly used. However, awk has a much steeper learning curve (meaning that you learn a lot
in a very short time). In other words, Perl is more difficult to learn.
• Both Perl and awk share the reputation of being incomprehensible, even to the actual authors of the
programs that use these languages. So document your code!
6.5. Exercises
These are some practical examples where awk can be useful.
1.
For the first exercise, your input is lines in the following form:
Username:Firstname:Lastname:Telephone number
Make an awk script that will convert such a line to an LDAP record in this format:
dn: uid=Username, dc=example, dc=com
cn: Firstname Lastname
sn: Lastname
telephoneNumber: Telephone number
Create a file containing a couple of test records and check.
2. Create a Bash script using awk and standard UNIX commands that will show the top three users of
disk space in the /home file system (if you don't have the directory holding the homes on a separate
partition, make the script for the / partition; this is present on every UNIX system). First, execute the
commands from the command line. Then put them in a script. The script should create sensible output
(sensible as in readable by the boss). If everything proves to work, have the script email its results to
you (use for instance mail -s Disk space usage < result).
Chapter 6. The GNU awk programming language 77