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June
30, 2003 -
A Brief Introduction to awk
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awk
is a programmable, pattern-matching, and
processing tool that works equally well
with text and numbers. It derives
its name from the first letter of the last
name of its three authors (Alfred V. Aho,
Peter J. Weinberger, and Brian W.
Kernighan).
When used on the command line, the syntax
for awk is:
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awk 'pattern
{ action }' [filename]
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A few things
you should notice about the command line
syntax are:
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- since awk
can take its input from its standard
input, providing the name of a file is
optional
- single quotes are used to protect the
pattern and action from the shell
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Although awk can
be invoked with a script file (e.g. awk -f
script-file filename), this overview will
only demonstrate its command line usage.
The two key components of the command line
format are the pattern (a regular
expression), and the action. awk will
search its input for lines that match the
specified pattern. If it finds a match
it will perform the specified action, which
could be writing the entire line (record) or
individual fields* from the line to standard
output.
If a pattern is not specified, the action
will be performed on every line. If
the action is absent, then the default
action of printing the line to standard
output will be performed.
Our first example will demonstrate how awk
can be used to select and manipulate
specific records from a file, named unixfile,
that contains the following data:
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unix
training
learn unix
unix class
learning unix
unix course
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The following
awk statement will select records from this
file that contain the word "learn"
in them, and then will print the two fields
for each matching record in reverse order
with a space in-between them:
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# awk '/learn/
{ print $2 " " $1 }' unixfile
unix learn
unix learning
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Similar to shell
script arguments, each field in a record can
be referenced using the dollar sign followed
by the number indicating its position in the
record. $0 (dollar sign and zero)
references the entire record, and is often
used as an argument for various awk
functions.
This week's second example will illustrate how awk
can extract various pieces of information
from lines of data that are piped to awk's
standard input. In this example,
output from the df (disk free) utility will
be parsed:
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# df -k | grep -v used | awk '{ print $6 "\t" $5 }'
/ 27%
/usr 79%
/boot 17%
/proc 0%
/dev/fd 0%
/etc/mnttab 0%
/var 3%
/var/run 1%
/tmp 1%
/opt 2%
/export/home 1%
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This statement
extracts the filesystem mount point and
percentage of available space (by filesystem)
for every
record awk receives from df. The grep
command is used to discard df's column
headers,
and the "\t" in the print
statement inserts a tab between the two
fields. You may have noticed that a
pattern was not specified, which resulted in
the action being performed on every record.
This command, combined with an existing file
containing your system's mount points and
the available space thresholds for each
filesystem, could be used to build a shell
script to monitor your system's filesystem
space usage and take a pre-defined action if
a threshold was met. Scheduling
the script to run at fixed intervals using
the cron utility would provide you with an
automated filesystem space usage monitoring
solution.
* Record fields
are delimited by one or more consecutive
spaces or tabs.
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Learn
more...
If you are new to the UNIX or Linux
operating system and would like to learn
more about other frequently-used operating
system commands, you may want to consider
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