sed - how to remove all lines that do not match

Question

I have a html file. I want to remove all lines that do not start with <tr>.

I tried:

cat my_file | sed $'
s/^[^tr].*//
' | sed '/^$/d'

but it deleted all the lines.

Answer 1

Try this with GNU sed:

sed -n '/^<tr>/p' file

or

sed '/^<tr>/!d' file

Answer 2

sed -e '/^<tr>/d'

The part between / is a regex. The d command deletes matching lines.

Update: oops, sorry I saw you said NOT. So

sed -e '/^<tr>/!d'

Where ! negates the sense of the match.

Answer 3

If it has to be sed:

sed -ni '/^<tr>/p' file

-i edits the file in-place, -n prevents sed to print all lines, the regular expression means to match all line that start (^) with <tr> and those lines will be printed (p).

With grep:

grep -E '^<tr>' file

With -E grep interprets extended regular expressions.

With awk:

awk '/^<tr>/' file

Or pure bash:

while IFS= read -r l; do [[ "$l" =~ ^\<tr\> ]] && echo $l; done <file

The [[ is bashs internal conditional expression. We compare $l against the regular expression and if it succeded (&&) we print the line with echo.

Answer 4

Easiest and simplest answer would be:

grep '^<tr>' path/to/file 

This will print out the file with only the lines that start with which could be good if you don't want to modify the file directly (like with sed).

Then, if you like what you see in the output you can just print out to a file with > file

In this case you save some time backing up your file before trying some commands.