How to delete specific lines from a txt file

Question

I have a Bash script that writes data into a log file, then from that I take out stuff I want using the sed command. That gives me a file data.txt. What I want to know is how I go about deleting specific line inside that data.txt file.

For example:

123
456
789

I want to remove the 2nd line, containing 456 so that I only have

123

789

I tried with sed '2d' data.txt but it doesn't work.

I use this command to create data.txt from log.log:

sed -nE '1s/.{1}(.{2}).*/\1/p;' log.log >> data.txt

Answer 1

Try this. This should work.

sed -i '2d' data.txt

You have to add -i flag with sed command unless you are redirecting your output to a new file. If you don't add the -i flag, sed will print the pattern space to STDOUT and will not make any changes to the original file.

Automatic backup option

It is quite dangerous to modify a file without taking a proper backup. So sed has its native method to backup a file before editing which is -i.bak option.

So in the above example if we use the backup option, the command would be.

sed -i.bak '2d' data.txt

So it will remove the first line from the file data.txt and will also make a backup copy of the original file with .bak extension.

Answer 2

You know, besides specifying by line number, you can also do it by matching the content of the line, if that's more useful (which sounds like it might):

sed -i '/^456$/d' data.txt

Answer 3

Using awk:

awk 'NR!=2{ print }' input-file > output-file

That could be read as: if the number of record is not equal to 2 then print the record (line). If you want to delete the second and the third line:

awk 'NR!=2 && NR!=3 { print }' input-file

---

If you want to have an empty line as it is shown into the question, the sed command should look as this:

 sed '2 s/^.*$//' input-file

The first character 2 is the number of the line, followed by the command s, that will substitute all characters .* from the beginning ^ to the end $ of the line with empty string //. In this case the beginning and the end characters could be omitted.

Example:

$ cat input-file
123
456
789
654

$ sed '2,3 s/.*//' input-file
123


654

Answer 4

This command will delete lines 2-4 and 6

sed -e '2,4d;6d' file