<< toolchain

awk

# Usage

awk is a interpreted programming language that processing text file line-by-line, which usually used in the shell to accomplish simple field-based text processing task. There are two common usages of awk command

awk 'program' [file]
awk -f [program-file] [file]

By providing the option -f, we provide the awk script through a file, which avoid some compatibility problems when using the first inline form, the details is provided in awk_inline_problems. ==TODO: Create this.==

Another common approach is processing the output of some shell command through piping:

cmd | awk 'program'

# Format

The general format of awk program is

[pattern] [action]

The [pattern] specifies which lines should be processed by the given [action]. One of them can be omitted. If the pattern is omitted, every line of file will be processed; if the action is omitted, then awk will simply print the line obeys pattern.

For multiple [pattern] [action] combinations given in one file, awk will run them sequentially and independently. For example, the awk program file with content

#! /bin/awk -f

/12/ {print $0}
/21/ {print $0}

will print all the lines contains 12 or 21, and those lines with both patterns will be printed twice.

# Field

Each line is contructed by many fields, seperated by space characters by default, and the seperator can be specified by assigning variable FS, or setting the -F/--file-separator option (which actually also change FS).

The special variables $1, $2, $3 represents the fields, numbering starts from the left. Specially, the variable $0 represents the entire line. Take an example,

ls -l | awk '{print $9 " " $3 " " $1}'

This will printed the 9th, 3rd, 1s field of the output of ls -l, which is the filename, owner’s name, and rwx mode respectively.

Besides the constant, the field number can be provided by variable, for example,

# print

The print statement print its following object.

awk '{print $0}' data

Since no pattern is provided, this print applied print $0 to entire line, that is, print entire line. Hence this command actually print the entire file.

# Variables

variable can be used directly without assigned: