Filter Commands: basic awk command - syntax | options | variables

> Basic awk command

- awk command is a powerful method for processing or analyzing text or data files, which are organized by lines (rows or records) and columns (fields)
- We can use awk as a linux command  and also as a  Scripting language like Bash Shell Scripting (Advance)

SYNTAX --
	>> awk [options] '[selection_criteria] {action}' input_file
		- options - are always optional
		- selection_criteria - can be condition, pattern/search string and also can be optional
		- action - logic to perform on each row/record (print (mostly used))
	>> cat input_file | awk [options] '[selection_criteria] {action}'
		- mostly used in CLI

OPTIONS --
	-F : fs : To specify a Field Separator (default field separator in awk is space (tab is nothing but multiple spaces))
	-f : file : To specify a file that contains awk script
	-v : var=value : To declare a Variable

SIMPLE awk command SYNTAX --
	>> awk '{action}' input_file
		- eg. awk {print $1} file.txt		# '$1' prints first field from each line
	
	- Default Variables for awk
		$0 : Entire File
			>> awk '{print $0}' file.txt				# prints entire file content
			>> awk '{print}' file.txt				# also prints entire file content
			>> awk '{print $1,$3}' file.txt			# 1st and 3rd columns/fields of each line/row/record. Output is by default separated by ' '.
			>> awk 'BEGIN {OFS=":"} {print $1,$3}' file.txt	# output is separated by ':'. 
			>> awk '{print $3,$1}' file.txt			# reverses order of printing 1st and 3rd column to 3rd and 1st column
			>> awk '{print NR}' file.txt				# prints only Line(Record) Number on each line 
			>> awk '{print NR,$0}' file.txt			# prints Line Number on each line along with their content
			>> awk '{print NR,$0,NF}' file.txt			# prints Line Number alongwith their content(all fields) and in the end Number of Fields present in that record
			>> awk '{print $NF}' file.txt				# prints last field of each row
			>> awk '{print $NR}' file.txt				# prints 1st field from 1st record, 2nd field from 2nd record, 3rd from 3rd and so on. 
			
- grep with cut vs awk 

>> apache2 -v | grep "version" | cut -d '/' -f 2 | cut -d ' ' -f 1
2.4.54

>> apache2 -v | awk -F '[ /]' '/version/{print $4}'		# ' ' & '/' are separators inside [], '/version/' is search string (field) &  'print $4' inside {} is an action. 
2.4.54
# both the commands give same output

>> apache2 -v | awk -F '[ /]' 'NR==1{print $4}'			# NR is a condition, i.e. Record Number == 1, record is line and the condition means 'in line No. 1'
2.4.54

ADVANTAGE --

- Single awk command works instead of using grep alongwith cut, especially for fields and patterns.

LIMITATION --

- In awk command, we will get only the fields and not the characters. For characters only cut command works.



** ADVANCED AWK COMMAND FOR SHELL SCRIPTING WILL BE DISCUSSED LATER **