awk

Ref: The GNU Awk User’s Guide

awk 
    { '<program>' | -f '<program file>' }
    [<input file> [...]]

<program>

# A BEGIN rule is executed once only, before the first input record is read.
BEGIN {
    # <action>
}

# The BEGINFILE rules is executed just before gawk reads the first record from a file.
BEGINFILE {
    # <action>
}

# If the pattern is omitted, then the action is performed for every record.
<pattern> {
    # <action>
}

# The ENDFILE rule is called when gawk has finished processing the last record in an input file.
ENDFILE {
    # <action>
}

# An END rule is executed once only, after all the input is read. 
END {
    # <action>
}

Default <action> is print

<pattern> {
    print
}

# equal to

<pattern>

Built-in Variable

Name	Description	Default
ARGC	Argument Count
ARGV	Argument Value
$0	Current Record
$1, $2, ..., $NF	Field 1, Field 2, ..., Last Field
NF	Number of Fields
NR	Number of Records has processed
FS	Field Separator	" "
OFS	Output Field Separator	" "
IGNORECASE	Ignore Case
RS	Record Separator	"\n"
ORS	Output Record Separator	"\n"

Pattern

Regular expression

/<regular expression>/

Expression

<expression> == <expression>
<expression> != <expression>
<expression> < <expression>
<expression> <= <expression>
<expression> > <expression>
<expression> >= <expression>

# Match Reg Exp
<expression> ~ /<reg exp>/

# Not match Reg Exp
<expression> !~ /<reg exp>/

Range

<begin pattern>,<end pattern>

Emtpy

Match every record

Assignment Expression

<variable> = <value>

String

"<string>"

if

if (<condition>) {
    # <code>
    # ...
}

if (<condition>) {
    # <code>
    # ...
} else {
    # <code>
    # ...
}

RegExp

Match

<value> ~ /<reg exp>/

Not match

<value> !~ /<reg exp>/

Default <string> is $0

/<reg exp>/

# equal to

$0 ~ /<reg exp>/

Print

{
    print <item>[, ...]
}

Default <item> is $0

{
    print $0

    # equal to

    print
}