awk

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

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

Patterns

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

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

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

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

Functions

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

Built-in functions: https://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;
}

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
}