Vtyjkyuk

Example of a file

line 1
line 2
line 3
line 4

line 4 should be after line 1

line 1
line 4
line 2
line 3

Important: It could be more than 4 lines somehow the script has to understand that it has to read the last line, not line 4.

An alternative, slower, longer version.

#!/bin/sh

if [ ! -f "$1" ]
then
 printf "No input filen"
 exit
fi

FIRST=`head -n 1 "$1"`
LAST=`tail -n 1 "$1"`
MID=`sed -n '$d; 2,$p' "$1"`
printf "$FIRSTn$LASTn$MIDn"

exit

Slower because using head, tail and sed to access the file each time. On a large file (or many files) this could be very noticeable.

$ printf '4m1n,pn' | ed -s file
line 1
line 4
line 2
line 3

The short ed script

4m1
,p

will move line four to after line one, and then display the contents of the editing buffer in the terminal.

To save the result to a new file, you may use a redirection like

printf '4m1n,pn' | ed -s file >newfile

or you may tell ed to save the file with

printf '4m1nw newfilen' | ed -s file

The command w newfile would save the edited buffer into the named file. The command w with no filename would save the file back to the original filename.

Since the line editor ed reads the file into memory, it is okay to use it for this kind of thing if the file is of reasonable but not too large size (less than gigabytes).

To move the last line, change 4 to $ in the commands above.