Bsrgvty

I have a text file, and I want extract the string from each line coming after "OS="

input file line
A0A0A9PBI3_ARUDO Uncharacterized protein OS=Arundo donax OX=35708 PE=4 SV=1
K3Y356_SETIT ATP-dependent DNA helicase OS=Setaria italica OX=4555 PE=3 SV=1

Output desired

OS=Arundo donax
OS=Setaria italica

OR

Arundo donax
Setaria italica

Use GNU grep (or compatible) with extended regex:

grep -Eo "OS=w+ w+" file

or basic regex (you need to escape +

grep -o "OS=w+ w+" file
# or
grep -o "OS=w* w*" file

To get everything from OS= up to OX= you can use grep with perl-compatible regex (PCRE) (-P option) if available and make lookahead:

grep -Po "OS=.*(?=OX=)" file

#to also leave out "OS="
#use lookbehind
grep -Po "(?<=OS=).*(?=OX=)" file
#or Keep-out K
grep -Po "OS=K.*(?=OX=)" file

or use grep including OX= and remove it with sed afterwards:

grep -o "OS=.*( OX=)" file | sed 's/ OX=$//'

Output:

OS=Arundo donax
OS=Setaria italica

In Perl, two non-whitespace "words":

$ perl -lne 'print $1 if /OS=(S+ S+)/' input

or everything up to OX=:

$ perl -lne 'print $1 if /OS=(.*?) OX=/' input 

or everything up to the next something=:

$ perl -lne 'print $1 if /OS=(.*?) (w+)=/' input

With your sample input, they all give the same output, but the output would be different with e.g. an input like this:

ABC=something here OS=foo bar doo PE=3 OX=1234

A more robust way is to use sed to parse the full value until the word containing the next = is found. That way it will work on any sized value (e.g. if you have a font with one word or three words).

sed 's/.*OS=([^=]*).*/1/;s/ [^ ]*$//'

The first block grabs everything up to OS=, the second block in the capture group (denoted by ()'s) matches upto the next = and can be referred to in the replacement as 1. The next substitution rids the last word which is a fragment from the next assignment.

Note: the ^ in []'s is to exclude match the character in this case everything that is not an = sign.

awk 'print $(NF-4), $(NF-3)' file

OS=Arundo donax
OS=Setaria italica 

or

awk -F= 'sub(/OX/,""); print $(NF-3)' file 

Arundo donax 
Setaria italica