Linux: Extract the first line of a file

Question

I am working with OpenWrt and a very small amount of space.

Trying to extract the first line from a file. The line needs to go into a variable and be removed from the file. I can use head to put it into a variable but can't use tail because as far as I understand I would have to do tail file > newFile and I do not have room for that second file.

Does some one know if a better technic?

Answer 1

Edit: you can't use my old answer (see below) with OpenWrt since OpenWrt doesn't ship with ed. What a shame. So here are two methods:

The vi way

vi is a genuine editor too, so the following will work:

vi -c ':1d' -c ':wq' file > /dev/null

We open the file with vi, and use the commands :1d to delete the first line and :wq to save and quit, redirecting all output to /dev/null. Cool, clean, short and simple.

Oh, you will of course run:

firstline=$(head -n1 file)

before running this vi command to get the first line of the file into the variable firstline.

Note. On a system with very little memory, and when file is huge, this method fails.

The dd way

dd is a cool tool for this. The dd methods given in other answers are really great, yet they rely on the truncate utility which does not ship with OpenWrt. Here's a workaround:

firstline=$(head -n1 file)
linelength=$(head -n1 file | wc -c)
newsize=$(( $(wc -c < file) - $linelength ))
dd if=file of=file bs=1 skip=$linelength conv=notrunc
dd if=/dev/null of=file bs=1 count=0 seek=$newsize

This will work even with huge files and very little memory! The last dd command plays the role of the truncate command given in the other answers.

---

Old answer was:

You can use ed for this:

firstline=$(printf '%s\n' 1p d wq | ed -s file.txt)

At each call, you'll get the first line of the file file.txt in the variable firstline, and this line is removed from the file.