How do I delete the first n lines of an ascii file using shell commands?

As long as the file is not a symlink or hardlink, you can use sed, tail, or awk. Example below.

$ cat t.txt
12
34
56
78
90

sed

$ sed -e '1,3d' < t.txt
78
90

You can also use sed in-place without a temp file: sed -i -e 1,3d yourfile. This won't echo anything, it will just modify the file in-place. If you don't need to pipe the result to another command, this is easier.

tail

$ tail -n +4 t.txt
78
90

awk

$ awk 'NR > 3 { print }' < t.txt
78
90

sed -i '1,3d' file.txt

This deletes first 3 line from file.txt.

If the tabulated lines are the ones that have a tab character:

grep '␉' <input_file >output_file

(␉ being a literal tab character) or equivalently

sed -n '/␉/p' <input_file >output_file

In a bash/ksh/zsh script, you can write $'\t' for a tab, e.g. grep $'\t' or sed -n $'/\t/p'.

If you want to eliminate 10 lines at the beginning of the file:

tail -n +11 <input_file >output_file

(note that it's +11 to eliminate 10 lines, because +11 means “start from line 11” and tail numbers lines from 1) or

sed '1,10d' <input_file >output_file

On Linux, you can take advantage of GNU sed's -i option to modify files in place:

sed -i -n '/\t/p' *.txt

Or you can use a shell loop and temporary files:

for x in *.txt; do
  tail -n +11 <"$x" >"$x.tmp"
  mv "$x.tmp" "$x"
done

Or if you don't want to modify the files in place, but instead give them a different name:

for x in *.txt; do
  tail -n +11 <"$x" >"${x%.txt}.data"
done