Flow of control

First rule of coding is that awk code has to be ecapsulatied in an outer brace set. Only the pattern filters may appear outside of the braces.

Conditionals

if statement.

if ( condition )
{
  action
  action
}
else
{
  action
}

The else and else block is optional.

awk does not support the else-if, but other if statements can be embedded in the action block areas.

relational operators - "returns" a true of false.

< - less than.
> - greater than.
<= - less than or equal.
>= - greater than or equal.
== - is equal. Note : = is assignment.
!= - not equal.
~ - match /regex/ e.g $1 ~ /^[0-9]/
!~ - not match /regex/.

These operator may be used on both numeric and string values.

if ( $1 >= "b" && $1 < "c" )
{
   print $1
}

This would print all instances of field 1 that started with the letter b.

Uses ASCII ordering to do evaluation.

The ~ and !~ can be use both in the pattern area and in the coding blocks. It allows the use of /regex/ as an alterative conditional match.

When doing a regular expression match inside an action block, you must use flow of control commands. if, while, etc.

if ( $3 ~ /Gasoline/ )
{
  actions
}

Another example

Quick conditional operator. awk provide the ? as a way to write a simple quick if/else statement.

grade = ( $1 > 65) ? "pass" : "fail"

will test field 1 for value greater than 65, if true, it will assign "pass" to variable grade. Else if false, it will assignm "fail".

Loop structures

while ( condition )
{
  actions
}

The while loop runs condition before entering loop. It is possible that actions are never taken if no true condition found.

do
{
  actions
} while (condition)

The do-while loop will always execute actions at least once.

for ( set-var; test-condition; adjustment )
{
  actions
}

The for loop does a controled loop sequence.

The three fields are all optional, but the semi-colons are required.

Given the awk script, pname.awk

{
  for ( i = 8; i <= NF; ++i )
  {
    printf ( "%s ",  $i );
  }
  printf ( "\n" );
}

which is invoked with ps -ef | awk -f pname.awk

This will print from 8th field on of the output of the ps -ef command. Because some programs listed were started with options, this will show both progam name and any other trailing arguments.