Syntax

awk '/Pattern1/ {Actions}

     /Pattern2/ {Actions}' File1 File2

• Pattern is a regular expression.

• Actions  are the statement(s) to be performed.

• several patterns and actions are allowed in Awk.

• File is the Input file. Usually you would use one input file but you can specify multiple files.

• Single quotes are used around the program to prevent shell interpreting any special characters.

• Awk reads the input files one line at a time.

• For each line, it matches with given pattern in the given order, if matches performs the corresponding action.

• If no pattern matches, no action will be performed.

• In the above syntax, either search pattern or action are optional, But not both.

• If the search pattern is not given, then Awk performs the given actions for each line of the input.

• If the action is not given, print all that lines that matches with the given patterns which is the default action.

• Empty braces with out any action does nothing. It wont perform default printing operation.

• Each statement in Actions should be delimited by semicolon.

You can also save your awk program to a file, which can make the syntax more readable. Run it like this...

awk -f program.awk File

Examples

Field Separators and substititions

To set APXPRT to the first Port the current listener is listening on...

APXPRT=$(lsnrctl stat $LISTENER_NAME | grep PORT | head -1 | awk -F"PORT=" '{sub(/ .*/,"",$2);print $2}' | sed 's/)//g')

Explanation...

lsnrctl stat $LISTENER_NAME | grep PORT | head -1

yeilds 

  (DESCRIPTION=(ADDRESS=(PROTOCOL=tcp)(HOST=myhost)(PORT=1523)))

the subsequent awk command then uses -F to set the field separator to PORT=, uses sub to remove any spaces, and prints the second field. sed is then used to remove trailing parentheses, giving the result...

1523

Print field using record separators

Line in file to process...

one-two-three-four

Command we run...

cat myfile | awk -F '-' '{print $3}'

Result...

three

Print field using two record separators

Line in file to process...

<one><two><three><four>

Command we run...

cat myfile | awk -F '[>=<]' -v OFS='>' '{print $4}'

Result...

two

Real world example...

Line in file to process...

    <property name="hibernate.connection.password">password</property>

Command we run...

cat confluence.cfg.xml | grep hibernate.connection.password | awk -F '[>=<]' -v OFS='>' '{print $4}'

Result...

password

Selecting all lines between two patterns

opatch lsinventory -all | awk '/List of Oracle Homes:/,/Installed Top-level Products/'

if.. then.. else

To compare two numbers and print something based on the result...

echo ${MYFSSZ} ${fre} | awk '{ if ($1 > $2) print "*** VG too small!"; else print "*** OK"; }'

For more complex examples see:

While 

To print a line containing 50 x's...

awk 'BEGIN { while (count++<50) string=string "x"; print string }'

Do.. While

awk 'BEGIN{count=1;do print "This gets printed at least once";while(count!=1)}'

For...

Break

This script prints "Iteration" ten times then stops...

awk 'BEGIN{x=1;while(1){print "Iteration";if ( x==10 ) break;x++;}}'

Continue

This script prints ten lines but skips the 5th one...

awk 'BEGIN{x=1;while(x<=10){if(x==5){x++;continue;}print "Value of x",x;x++;}}'

Exit

This script starts to print 10 lines but exits from the loop after the 5th line...

awk 'BEGIN{x=1;while(x<=10){if(x==5){exit;}print "Value of x",x;x++;}}'

Pruning

Shutdown...

adrci exec=show alert -p "originating_timestamp > systimestamp-1 and message_group like '%ddl%'" -term | \

   nawk 'c-->0;$0~s{if(b)for(c=b+1;c>1;c--)print r[(NR-c+1)%b];print;c=a}b{r[NR%b]=$0}' b=1 a=0 s="^ALTER DATABASE CLOSE"

Startup...

adrci exec=show alert -p "originating_timestamp > systimestamp-1 and message_group like '%ddl%'" -term | \

   nawk 'c-->0;$0~s{if(b)for(c=b+1;c>1;c--)print r[(NR-c+1)%b];print;c=a}b{r[NR%b]=$0}' b=1 a=0 s="^Completed: ALTER DATABASE OPEN"

Explanation

• The Patterns (c-->0;$0~s) are not enclosed in / but specify that the Action should only be performed if c-1 is greater than 0 and the line matches the string contained in variable s

• Variables b, a, and s are set outside the Action. 

• b is a variable to hold the number of lines to print before the matched line (s)

• a is a variable to hold the number of lines to print after the matched line (s)

• c-- means c=c-1

• NR=ordinal number of the current record

Counting fields in a file

awk -F ':' '{ total += NF }; END { print total }' /etc/passwd

Explanation

• awk steps through an input file line by line

• -F defines the field separator

• total is a user variable to hold the count

• NF is the Number of Files

• END is a special pattern which runs at the end of the script

Counting lines where last field matches a pattern

awk -F ':' '$NF ~ /\/bin\/sh/ { n++ }; END { print n }' /etc/passwd

Explanation

• awk steps through an input file line by line

• -F defines the field separator

• $NF is the field at the end of a line

• ~ is the regular expression "match" operator

• Regular expressions should be enclosed in / (which means any other / need to be escaped by \)

• n is a user variable to hold the count

• END is a special pattern which runs at the end of the script

Find line with the highest value for a field

awk -F ':'  '$3 > maxuid { maxuid=$3; maxline=$0 }; END { print maxuid, maxline }' /etc/passwd

Explanation

• awk steps through an input file line by line

• -F defines the field separator

• $3 is the third field on the current line (this is the field we want to find highest value for)

• $0 is the entire current line

• maxuid is a user variable to hold the highest value found so far

• maxline is a user variable to hold the entire line containing the highest value found so far

• if $3 is greater than maxuid then update maxuid and maxline with the values from the current line

• END is a special pattern which runs at the end of the script

Print even numbered lines

awk 'NR % 2 == 0' /etc/passwd

Explanation

• awk steps through an input file line by line and prints the lines where the pattern is matched

• NR is the line number

• if the remainder after dividing NR by 2 equals 0 the pattern is matched otherwise the line is ignored

Find lines where fields match

awk -F ':' '$3==$4' passwd.txt

Explanation

• awk steps through an input file line by line and prints the lines where the pattern is matched

• -F defines the field separator

• where field 3 equals field 4 the pattern is matched otherwise the line is ignored

Find lines where two conditions match

awk -F ':' '$3>=100 && $NF ~ /\/bin\/sh/' passwd.txt

Explanation

• -F defines the field separator

• where field 2 is greater than or equal to 100 and the last field on the line ($NF) matches (~) /bin/sh (/ escaped with \ as necessary) print line

Find lines with empty fields

awk -F ':' '$5 == "" ' passwd.txt

Explanation

• -F defines the field separator

• where field 5 equals null ("") then print line

Remove blank lines

awk 'NF' filename

Number of lines in a file as a percentage of the number of lines in another file

echo "$(cat file1|wc -l)|$(cat file2}|wc -l)" | awk 'BEGIN { FS = "|" } { $3 = $1 / $2 * 100 "%" } { print $3 }'

Using a program file

Create a file called etc_passwd.awk...

BEGIN{

FS=":";

print "Name\tUserID\tGroupID\tHomeDirectory";

}

{

  print $1"\t"$3"\t"$4"\t"$6;

}

END {

  print NR,"Records Processed";

}

Run it using...

awk -f etc_passwd.awk /etc/passwd

Explanation

• The file separator has bee set within the script. i.e. FS=":"; is equivalent to using -F ":" on the command line.

• \t prints tabs between the literal strings in the first print command

• The BEGIN action block is performed once before the file is processed (to give headings in this case)

• The END action block is performed once after the file is processed (to give a record count in this case, using the NR Built-In variable).

Generating HTML with awk

Bibliography