6.2. awk s Format

6.2. awk 's Format

An awk program consists of the awk command, the program instructions enclosed in quotes (or in a file), and the name of the input file. If an input file is not specified, input comes from standard input ( stdin ), the keyboard.

Awk instructions consist of patterns, actions, or a combination of patterns and actions. A pattern is a statement consisting of an expression of some type. If you do not see the keyword if , but you think the word if when evaluating the expression, it is a pattern. Actions consist of one or more statements separated by semicolons or newlines and enclosed in curly braces. Patterns cannot be enclosed in curly braces, and consist of regular expressions enclosed in forward slashes or expressions consisting of one or more of the many operators provided by awk .

Awk commands can be typed at the command line or in awk script files. The input lines can come from files, pipes, or standard input.

6.2.1 Input from Files

In the following examples, the percent sign is a shell prompt. Please take note: In all examples where the command is nawk , use either awk or gawk if you are on an HP-UX flavor system or using Linux.

FORMAT

 % nawk 'pattern' filename % nawk '{action}' filename % nawk 'pattern {action}' filename 

Here is a sample file called employees :

Example 6.1.

 %  cat employees   Tom Jones             4424    5/12/66                543354   Mary Adams            5346    11/4/63                28765   Sally Chang           1654    7/22/54                650000   Billy Black           1683    9/23/44                336500  %  nawk '/Mary/'  employees   Mary Adams            5346    11/4/63                28765  

EXPLANATION

Nawk prints all lines that contain the pattern Mary .

Example 6.2.

 %  cat employees   Tom Jones             4424    5/12/66                543354   Mary Adams            5346    11/4/63                28765   Sally Chang           1654    7/22/54                650000   Billy Black           1683    9/23/44                336500  %  nawk '{print }' employees   Tom   Mary   Sally   Billy  

EXPLANATION

Nawk prints the first field of file employees , where the field starts at the left margin of the line and is delimited by whitespace.

Example 6.3.

 %  cat employees   Tom Jones             4424    5/12/66              543354   Mary Adams            5346    11/4/63              28765   Sally Chang           1654    7/22/54              650000   Billy Black           1683    9/23/44              336500  %  nawk '/Sally/{print , }' employees   Sally Chang  

EXPLANATION

Nawk prints the first and second fields of file employees , only if the line contains the pattern Sally . Remember, the field separator is whitespace.

6.2.2 Input from Commands

The output from a UNIX/Linux command or commands can be piped to awk for processing. Shell programs commonly use awk for manipulating commands.

FORMAT

 % command  nawk 'pattern' % command  nawk '{action}' % command  nawk 'pattern {action}' 

Example 6.4.

 1   %  df  nawk ' > 75000'   /oracle   (/dev/dsk/c0t0d057 ):390780 blocks     105756 files   /opt    (/dev/dsk/c0t0d058 ):1943994 blocks      49187 files  2   %  rusers   nawk '/root$/{print  }'   owl   crow   bluebird  

EXPLANATION

1. The df command reports the free disk space on file systems. The output of the df command is piped to nawk . If the fourth field is greater than 75,000 blocks, the line is printed.

2. The rusers command prints those logged on remote machines on the network. The output of the rusers command is piped to nawk as input. The first field is printed if the regular expression root is matched at the end of the line ( $ ); that is, all machine names are printed where root is logged on.