How to get the first line of an input text file, while deleting that line from the text file

At least with GNU sed:

$ cat file
► put returns between paragraphs
► for linebreak add 2 spaces at end
► _italic_ or **bold**

$ sed -i '1{
w /dev/stdout
d}' file
► put returns between paragraphs

$ cat file
► for linebreak add 2 spaces at end
► _italic_ or **bold**

With GNU sed it can be written as a one-liner

sed -i -e '1 {w /dev/stdout' -e 'd}' file

Assuming you are asking for a shell script, this will do what you requested:

NAME=$1
head -n 1 $NAME
sed -i '1d' $NAME

sed -ne '1p' -e '1!s/^//w temp_file' yourfile && mv temp_file yourfile

You can use head , tail and mv:

Display the first line:

head -1 myPathToTheFile.txt

Keeping the last (+2) lines:

tail -n +2 myPathToTheFile.txt > file.tmp && mv file.tmp myPathToTheFile.txt

Using file descriptors and a little bit of python

{
    { 
    head -n1 >&3; 
    3>&- tail -n +1;
    3>&- python -c 'import sys; sys.stdout.truncate(sys.stdout.tell())';
    }<file 1<>file
} 3>&1

Using head and tail. File is target.txt

head -1 target.txt && tail -n+2 target.txt > tmp
mv tmp target.txt && rm tmp

Note: Ensure there is no existing file tmp in the current folder, else it'll be removed.

Explanation:

• "head -1" selects the first line
• "tail -n+2 target.txt > tmp" selects all lines from 2nd onwards( including) and puts them in tmp
• mv overwrites the original file with tmp
• rm tmp will remove the tmp file thus created