The awk Language

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The awk language is actually a UNIX filter. An awk program consistss of lines that look like this.

pattern {  action}

The awk program runs on each line. If a pattern matches its action is carried out.

There are two special patterns BEGIN, which matches before any lines are processed, and END which matches after all lines are procesed.

Run these. Why is the second one "stuck?"

awk BEGIN {print "foo"}
awk END {print "goo"}

Built-in and User-Defined Variables

Here are the most commonly-used ones. The first four have default values.

• FS field separator, default is a space
• OFS output field separator, default is a space`
• RS row separator, default is \n
• ORS output row separator, default is \n
• FILENAME name of file we ar awking.
• NF number of fields
• NR line number (record number)

Variables just "spring into existence." They have an initial value of 0 or "" and learn their type when first assigned. No variable declarations are needed.

Arithmetic Awk has arithmetic operations: +, -, *, /, and %. The ^ operator exponentiates.

Patterns

Relations among the fields such as $1 == $2, $1 == 45, $3 > 20
/regexes/

Puzzler Print out every other line of a file.

Treasure Hunt What is ~? What is !~

Boolean Operations Are you suprised to know that ||, &&, and ! have their expected meanings?

Functions, User-Defined and Builtin

Math

• atan2(y, x)
• cos(x)
• sin(x)
• exp(x)
• log(x)
• int(x)
• sqrt(x)
• rand(x)
• srand(x) This has a default value

String

• length(str)
• tolower(str) lowercases
• toupper(str) uppercases
• substr(str, start, len)
• strtonum(str) converts a numerical string to a number
• printf(format_string, expression_list)

Arrays

Sorting by Columns