Learn-Bash-Shell-Scripting/Filter Commands: cut command - syntax | usage | -b | -c | -f | -d at main · Azad-02/Learn-Bash-Shell-Scripting · GitHub

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> cut command

- cut command is a powerful tool to extract parts of each line of a file

- is based on
	- Byte Position (-b)
	- Character Position (-c)
	- Fields based on Delimiter (by default delimiter is a tab) (-f)

- complement
	SYNTAX:
		--complement
		>> cut --complement -c 4 /etc/passwd		# gives characters other than 4th character, applies to -b, -c, & -f

USAGE -

>> cut -c 1 /etc/passwd		# -c is for character position. This command will show first character of each line in passwd file

>> cut -b 1 /etc/passwd		# -b is for byte position. This command will show first byte position i.e. nothing but first character (in this case)

>> cut -c 4 /etc/passwd		# gives 4th position character from passwd file

>> cut -c 3,9 /etc/passwd	# gives 3rd and 9th character

>> cut -c 1-8 /etc/passwd	# gives first to 8 characters

>> cut -c 1-5,9 /etc/passwd	# gives first to 5th and 9th character

>> cut -c -10 /etc/passwd	# gives first positioned char to 10th character

>> cut -c 4- /etc/passwd	# gives characters from 4th position to last position of each line


ADVANCE -

-f : fields, tabs are by default field separators (aka delimiter)
- mostly works with files having multiple columns, i.e. these columns are separated by some tabs or any other separators.

** Use -s alongwith -f to ignore the line that do not contain a delimiter

>> cut -f 1 file.txt 		# -f is for fields. This command will print the First column in file.txt

>> cut -f 2,4 file.txt		# gives second and 4th column

>> cut -f -5 file.txt		# gives first to 5th columns

>> cut -f 4- file.txt		# gives 4th to last columns

-- These (-f) commands are applicable only for file containing columns/words separated by tab
-- only '-f <num>' is not sufficient to filter lines containing symbols/spaces/special characters etc
-- For files having symbols/spaces, like /etc/passwd, we filter using '-d'

>> cut -d ":" -f 2 /etc/passwd		# prints words/characters after two colons (:), here, field separated is changed to ':', -f filter will work w.r.t. ':' instead of tabs

>> cut -d ":" -f 4- /etc/passwd		# gives 4th to last words/characters separated by ':'

-- For printing this output with different field separator (only for displaying output) we use delimiter

>> cut -d ":" -f 4- /etc/passwd --output-delimter="  "		# prints same output but instead of ':'s, they are separated by two spaces '  ' for displaying

LIMITATION of delimiter -

- When using delimter, we can separate fields using only one single character, no group or multiple characters are allowed for separation
- This limitation is overcome with 'awk'. We can separate using group of characters, patterns etc.

- for eg., file.txt contains:

advnkh	kwbnck	wkndnx	cjnsxmz
wdcdes	xnnow	xnsscs	efdc213
sxcd sc q3erfr q3wsdx vsdx
dc dwvcvx  vfcdef cd e rfdc  ef vfs defv324

-- here, words are separated by tabs on first 2 lines and by spaces on next 2 lines

>> cut -f 4 file.txt		# gives 4th column of first 2 lines , because for -f by default separation field is tab (delimiter as a tab)

>> cut -f 1 file.txt		# gives first column for first 2 lines and prints last two complete lines, because for -f, 'sxcd sc q3erfr q3wsdx vsdx' is a single unit as there are no tabs

>> cut -sf 1 file.txt		# gives only first column of first 2 lines. As only first 2 lines have delimter (separator=tab) and other two don't
--
>> cut -d ' ' -f 1 file.txt	# prints first two complete lines and then first word of next two lines as delimiter is changed to ' ', which is not present in first 2 lines

>> cut -d ' ' -sf 1 file.txt	# -s removes the lines where delimiter is not present, here, first 2 lines won't print as our delimiter is ' ' and not tab

SYNTAX to be used in real-life scenarios -

>> cat file.txt | cut -f 1			# cut command is applied on output of cat file.txt, i.e. output of cat is input for cut command

>> cat /etc/passwd | cut -d ':' -f 4-

Example:

** Combination of grep and cut command is a powerfull tool

>> apache2 -v
Server version: Apache/2.4.54 (Debian)
Server built:   2022-07-05T13:49:58

-- print only version, 2.4.54 --

>> apache2 -v | cut -d '/' -f 2
2.4.54 (Debian)
Server built:   2022-07-05T13:49:58								# printed another line too

>> apache2 -v | grep "version" | cut -d '/' -f 2
2.4.54 (Debian)

>> apache2 -v | grep "version" | cut -d '/' -f 2 | cut -d ' ' -f 1		# to remove '(Debian)'
2.4.54

>> apache_version=$(apache2 -v | grep "version" | cut -d '/' -f 2 | cut -d ' ' -f 1)		# stores complete set of commands in a variable 'apache_version'
>> echo "$apache_version"
2.4.54