Text Processing – How to Print File Content Only if the First Line Matches a Pattern

filesregular expressiontext processing

I am writing a script, I want to check if the first line of the file matches a certain pattern and if it does then print out the file. How can I achieve this?

How do I check for pattern? Is there a way to check for pattern and based on the output do something..

EDIT:
Please take a look at this question: https://stackoverflow.com/questions/5536018/how-to-get-match-regex-pattern-using-awk-from-file

I want something like this, but none of them worked for me. I basically want to check if the first line matches a regex pattern or not and based on that print the lines of file.

Best Answer

You could do that with ed:

ed -s infile <<\IN 2>/dev/null
1s/PATTERN/&/
,p
q
IN

the trick here is to try to replace PATTERN on 1st line with itself. ed will error out if it can't find the specified pattern so ,p (print whole file) will only be executed if 1s/PATTERN/&/ is successful.

Or with sed:

sed -n '1{
/PATTERN/!q
}
p' infile

this quits if the first line does not (!) match PATTERN, otherwise it prints all lines.
Or, as pointed out by Toby Speight, with GNU sed:

sed '1{/PATTERN/!Q}' infile

Q is the same as q but it does not print the pattern space.

Related Solutions

Use sed to Print Lines Until Last Occurrence of a Pattern

Try this:

$ tac infile | sed -n '/fish/,$p' |tac

In general case if you run below sed command, you will get all line from first matched pattern to end of input file.

$ sed -n '/fish/,$p' file

this is a some fish
this is a fishie
this is a fish
this is a lion
this is a cat

So my solution is: If we run tac command on input file your last matched pattern will change to as first pattern. see the result of tac infile:

$ tac infile

this is a cat
this is a lion
this is a fish
this is a fishie
this is a some fish
this is a goat
this is a cow

The tac command is the same as cat command, while tac prints files in reverse order.

Now if we run our first sed command, you will get all lines first matched pattern to end of input file. like:

$ tac infile | sed -n '/fish/,$p'
this is a fish
this is a fishie
this is a some fish
this is a goat
this is a cow

Ok, finished. We only need to run tac command again to reverse back the lines to original order:

$ tac infile | sed -n '/fish/,$p' |tac
this is a cow
this is a goat
this is a some fish
this is a fishie
this is a fish

Done!

Extracting subset of lines of a file based on regex for first & last line

With sed:

sed -n '/R/,$!d; /S/q; p'

Example:

$ seq 20 | sed -n '/6/,$!d; /1/q; p'
6
7
8
9

Best Answer

Related Solutions

Use sed to Print Lines Until Last Occurrence of a Pattern

Extracting subset of lines of a file based on regex for first & last line

Related Question