awk [faeeed28]

1. Structure

Everything in awk is a pattern -> action

  Pattern1 { ACTIONS; }

  Pattern2 { ACTIONS; }

Each line will go through each of the patterns, one at a time

2. Data Types

awk only has strings and numbers
strings can be cast into numbers
both can be assigned to variables in ACTIONS with the = operator
variables can be declared anywhere, uninitialized variables have "" empty string value
awk has unidimensional associative arrays
- var[key] = value
can simulate multidimensional arrays, but not very good

3. Patterns

3.1. Regular and Boolean Expressions

supports non-pcre regex (gawk might)
patterns can't capture specific groups, only to match
boolean expressions have && || and !
note that == does fuzzy matching (like js)
booleans can be used alongside regex

  /admin/ || debug == true { ACTION }

specific string matching against regex can use ~ or !~
can also just use {ACTIONS} to have it run against every line

3.2. Special Patterns

BEGIN matches only before any line has been input to the file, can initiate variables and other state
END lets you do some final cleanup
Fields see below

3.2.1. Fields

  # According to the following line
  #
  # $1         $2    $3
  # 00:34:23   GET   /foo/bar.html
  # \_____________  _____________/
  #               $0

  # Hack attempt?
  /admin.html$/ && $2 == "DELETE" {
    print "Hacker Alert!";
  }

Fields are default separated by whitespace
You can modify the line by assigning to the field

4. Actions

Many possible actions, most useful ones:

  { print $0; }  # prints $0. In this case, equivalent to 'print' alone
  { exit; }      # ends the program
  { next; }      # skips to the next line of input
  { a=$1; b=$0 } # variable assignment
  { c[$1] = $2 } # variable assignment (array)

  { if (BOOLEAN) { ACTION }
    else if (BOOLEAN) { ACTION }
    else { ACTION }
  }
  { for (i=1; i<x; i++) { ACTION } }
  { for (item in c) { ACTION } }

ALL VARIABLES ARE GLOBAL

4.1. Functions

Functions can be called with typical syntax { somecall($2) }

Built-in functions: www.gnu.org/software/gawk/manual/html_node/Built_002din.html#Built_002din

User defined functions:

  # function arguments are call-by-value
  function name(parameter-list) {
    ACTIONS; # same actions as usual
  }

  # return is a valid keyword
  function add1(val) {
    return val+1;
  }

5. Special Variables

  BEGIN { # Can be modified by the user
    FS = ",";   # Field Separator
    RS = "\n";  # Record Separator (lines)
    OFS = " ";  # Output Field Separator
    ORS = "\n"; # Output Record Separator (lines)
  }
  { # Can't be modified by the user
    NF          # Number of Fields in the current Record (line)
    NR          # Number of Records seen so far
    ARGV / ARGC # Script Arguments
  }

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