Awk

AWK Programming
Variables
Operators
Formatting
Print the Next Line
Common Examples

AWK Programming

awk is filter. awk performs the specified action on lines that match the pattern. Like 'sed', a regular expression pattern must be enclosed in a forward slashes. If no pattern is listed, the action is performed on every input line.

 awk 'pattern {action}' [files]

 $ awk '/^S/' phone.list
 $ awk '/^S/ {print $1}' phone.list

Variables

 $0 represents the whole current input record
 $1 represents the first field of the current input record
 $NF is number of fields for the current input record
 NR is number of Lines read

Operators

Relational Operators

 <  and >     Less than and Greater than
 <=  and >=   Less than or equl to AND  Greater than or equl to 
 ==  and !=   Equl to AND Not qual to 
 ~ and !~     Matches AND Doesn't match regular expressions

Boolean Operator:

 || 	Logical OR 
 && 	Logical AND 
 ! 	Logical NOT 

Assignment Operators

 ++ Add 1 to variable. 
 -- Subtract 1 from variable. 
 += Assign result of addition. 
 -= Assign result of subtraction. 
 *= Assign result of multiplication. 
 /= Assign result of division. 
 %= Assign result of modulo. 
 ^= Assign result of exponentiation. 
 **= Assign result of exponentiation.[6] 

Arithmetic Operator

 +  Addition
 - Subtraction
 * Multiplication
 / Division
 % Modulo Division (Remainder)
 = assignment

Formatting

printf fuction: printf statement can be used to specify the width and alignment of output fields. Unlike print function, it does not give any new line charater at the end of the line. The contents of the field will ne right justif ied by default. You musr specify "-" to get left justification. Thus "%-20s" out puts a string left justif ied in a fiels 20 character wide.

Format Specifiers used in printf

 c 	ASCII Character 
 d 	Decimal Integer 
 e 	Floating point format 
 s	String 
 %	Literal %

Example of formatting:

 $ ls -l 
 -rwxrwxr-x    1 c1jkaila usa          543 Jan   30 15:28 autopath
 drwxr-xr-x    2 c1jkaila usa          512 Mar   19 15:14 bin
 drwxr-x---    2 c1jkaila usa          512 Mar   19 09:23 odbc

Without formating:

 $ ls -l | awk '{print $9, $2}'
 autopath 1
 bin 2
 odbc 2

With formating:

 $ ls -l | awk '{printf "%-25s%-5s\n", $9, $2}'
 autopath          1
 bin               2
 odbc              2
 sdata             2
 shareware         2
 tmp               2

Printing the next line of a search Pattern

To search and print the next line use the following awk commands

 awk '/pattern/{getline;print}' 

 # ioscan –funC disks | awk '/ROM/{getline;print}'
 disk 4 0/0/2/1.2.0 sdisk CLAIMED DEVICE HP CD-ROM 305

Print the current line and nextline

 # ioscan -funC disk | awk '$1=="disk" { getline L2 ; print $0,L2 }' 

Same as above, but another way

 # ioscan -funNC disk | awk '{if ($1=="disk") {L1=$0;getline; sub (" *","");print L1,$0}}'

Common Examples

To find a patern "Dec" in 6th field

 ls -l | awk '$6 == "Dec"'

To display the first and second fields of each line

 awk '{print $1, $2}' <filename>

To display all the lines containing string1 in file called file1

 awk '/string1/' file1

To display the first and 3rd fields of /etc/passwd file

 awk -F: '{print $1, $3}'

To display the first and last field of /etc/passwd file

 awk -F: '{print $1, $NF}'

To display the previous field to the last filed of any file

 awk '{print $(NF-1)}' file_name

To sum all the fields using awk command:

 ls -l | awk '{sum += $5} {print sum}'
 ls -l | awk '{sum += $5} END {print sum}'

To add few numbers and find out the average

 echo "test 10 20 30" | awk '{total = $2 + $3 + $4} {ave = total / 3} {print ave}'

To add the value of Unix variable $var1 to the beginning of each line in file1

 cat file1 | awk -v name=$var1 '{print name"  " $0}'

Running Unix commands directly using awk

 # ps -ef | grep syncd | grep -v grep | awk '{system("kill -9 " $2)}'

The above command find the process ID of sysncd and kills that process

To replace DS with DSsmtp.comcast.net if the line has only single word 'DS'

  awk '{if ($0~/^DS$/) {print "DSsmtp.comcast.net"} \
    else {print}}' /etc/mail/sendmail.cf 

Another AWK Complex example

  awk '{if ($0!~/^\*/ && $0~/:$/ && $1~/^default:$/) {print $0"\n\tregistry = NIS"} \
     else if ($0!~/^\*/ && $0~/:$/ && $1!~/^default:$/) {print $0"\n\tregistry = files"} \
     else {print}}' /etc/security/user > /etc/security/user.new

To add # mark to the beginning of each line except for the lines starting with #, ftp, shell, and telnet:

  awk '{if ($1~/^#/ || $1~/^ftp/ \ 
  || $1~/^shell/ || $1~/^telnet/) \ 
  {print} else {print "#"$0}}' /etc/inetd.conf 

Some examples with sample output

 $ awk -F":" '{ print "username: " $1 "\t\tuid:" $3" }' /etc/passwd
 username: halt          uid:7
 username: operator      uid:11
 username: root          uid:0

http://www.ibm.com/developerworks/linux/library/l-awk1.html
http://www.ibm.com/developerworks/linux/library/l-awk2.html