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

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?

linuxunixtext


Solution 1:[1]

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;
}

Solution 2:[2]

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.

Sources

This article follows the attribution requirements of Stack Overflow and is licensed under CC BY-SA 3.0.

Source: Stack Overflow

Solution Source
Solution 1 plentyofcoffee
Solution 2 jjbmiller

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