Tuesday, August 18, 2009

How To Use awk In Bash Scripting

SkyHi @ Tuesday, August 18, 2009
How do I use awk pattern scanning and processing language under bash scripts? Can you provide a few examples?

Awk is an excellent tool for building UNIX/Linux shell scripts. AWK is a programming language that is designed for processing text-based data, either in files or data streams, or using shell pipes. In other words you can combine awk with shell scripts or directly use at a shell prompt.
Print a Text File

awk '{ print }' /etc/passwd
awk '{ print $0 }' /etc/passwd
Print Specific Field

Use : as the input field separator and print first field only i.e. usernames (will print the the first field. all other fields are ignored):
awk -F':' '{ print $1 }' /etc/passwd
Send output to sort command using a shell pipe:
awk -F':' '{ print $1 }' /etc/passwd | sort
Pattern Matching

You can only print line of the file if pattern matched. For e.g. display all lines from Apache log file if HTTP error code is 500 (9th field logs status error code for each http request):
awk '$9 == 500 { print $0}' /var/log/httpd/access.log
The part outside the curly braces is called the "pattern", and the part inside is the "action". The comparison operators include the ones from C:

== != < > <= >= ?:

If no pattern is given, then the action applies to all lines. If no action is given, then the entire line is printed. If "print" is used all by itself, the entire line is printed. Thus, the following are equivalent:
awk '$9 == 500 ' /var/log/httpd/access.log
awk '$9 == 500 {print} ' /var/log/httpd/access.log
awk '$9 == 500 {print $0} ' /var/log/httpd/access.log
Print Lines Containing tom, jerry AND vivek

Print pattern possibly on separate lines:
awk '/tom|jerry|vivek/' /etc/passwd
Print 1st Line From File

awk "NR==1{print;exit}" /etc/resolv.conf
awk "NR==$line{print;exit}" /etc/resolv.conf
Simply Arithmetic

You get the sum of all the numbers in a column:
awk '{total += $1} END {print total}' earnings.txt
Shell cannot calculate with floating point numbers, but awk can:
awk 'BEGIN {printf "%.3f\n", 2005.50 / 3}'
Call AWK From Shell Script

A shell script to list all IP addresses that accessing your website. This script use awk for processing log file and verification is done using shell script commands.

[ $# -eq 0 ] && { echo "Usage: $0 domain-name"; exit 999; }
if [ -f $HTTPDLOG ];
awk '{print}' $HTTPDLOG >$OUT
awk '{ print $1}' $OUT | sort -n | uniq -c | sort -n
echo "$HTTPDLOG not found. Make sure domain exists and setup correctly."
/bin/rm -f $OUT

AWK and Shell Functions

Here is another example. chrootCpSupportFiles() find out the shared libraries required by each program (such as perl / php-cgi) or shared library specified on the command line and copy them to destination. This code calls awk to print selected fields from the ldd output:

chrootCpSupportFiles() {
# Set CHROOT directory name
local BASE="$1" # JAIL ROOT
local pFILE="$2" # copy bin file libs

[ ! -d $BASE ] && mkdir -p $BASE || :

FILES="$(ldd $pFILE | awk '{ print $3 }' |egrep -v ^'\(')"
for i in $FILES
dcc="$(dirname $i)"
[ ! -d $BASE$dcc ] && mkdir -p $BASE$dcc || :
/bin/cp $i $BASE$dcc

sldl="$(ldd $pFILE | grep 'ld-linux' | awk '{ print $1}')"
sldlsubdir="$(dirname $sldl)"
if [ ! -f $BASE$sldl ];
/bin/cp $sldl $BASE$sldlsubdir

This function can be called as follows:
chrootCpSupportFiles /lighttpd-jail /usr/local/bin/php-cgi
AWK and Shell Pipes

List your top 10 favorite commands:
history | awk '{print $2}' | sort | uniq -c | sort -rn | head
Sample Output:

172 ls
144 cd
69 vi
62 grep
41 dsu
36 yum
29 tail
28 netstat
21 mysql
20 cat

whois cyberciti.com | awk '/Domain Expiration Date:/ { print $6"-"$5"-"$9 }'
Awk Program File

You can put all awk commands in a file and call the same from a shell script using the following syntax:
awk -f mypgoram.awk input.txt
Awk in Shell Scripts - Passing Shell Variables TO Awk

You can pass shell variables to awk using the -v option:

echo | awk -v x=$n1 -v y=$n2 -f program.awk

Assign the value n1 to the variable x, before execution of the program begins. Such variable values are available to the BEGIN block of an AWK program:

{print ans}

some of my one liner awk tricks:

— To convert squid log timestamps to readable, sortable format:

gawk '{print strftime("%m/%d %H:%M:%S ",$1)" "substr($0,12,999)}' access.log > dated

— To avoid having to cut/paste the above, I have in my .profile file:

alias tim="gawk '{print strftime(\"%m/%d %H:%M:%S \",\$1)\" \"substr(\$0,12,999)}'"

— To use AWK to process comma separated data:

awk -F, '{print $1, "," $6}' excel-save.csv> extract.csv

— To count complex pattern occurrence

awk '{if (substr($2,1,4) == "2008" && $4 == "Exception") {print $1}}' test|grep -c