How to replace rows starting with a certain character with emptly lines in text file with Linux/Unix-CodePudding

I'm using Ubuntu and have a large text file where certain rows start with < character what I'd like to replace each of these to an empty row. So from this:

eeeeee
<
<
aaaa
bbbb
cccc
<
dddddd
<
ff

I want this:

eeee

aaaa
bbbb
cccc

dddddd

ff

(In case of multiple consecutive < rows only one empty rows is needed ideally)

How to perform this in command line?

CodePudding user response：

You could use sed. In this case the command would look something like: sed '/</c\\' file.txt This would find a line with a '<' character at the beginning, and replace the line with nothing. It would not however replace multiple empty rows with a single row.

CodePudding user response：

This Perl one-liner should do what you're asking for:

perl -ne 'if (/^</) {print "\n" if !$f; $f=1} else {$f=0; print}' tmp.txt

Here it is in action:

# cat tmp.txt
eeeeee
<
<
aaaa
bbbb
cccc
<
dddddd
<
ff
# perl -ne 'if (/^</) {print "\n" if !$f; $f=1} else {$f=0; print}' tmp.txt
eeeeee

aaaa
bbbb
cccc

dddddd

ff

Commented code:

# Found a line starting with '<'
if (/^</) {
    # Print a blank line if $f is false
    print "\n" if !$f;
    # Set $f to true so subsequent lines starting with '<' are ignored
    $f=1;
} else {
    # Not a line starting with '<'; reset $f to false
    $f=0;
    # Print the current line
    print;
}