How to truncate file by lines?

The sed approach is fine, but to loop over all lines is not. If you know how many lines you want to keep (to have an example, I use 99 here), you can do it like this:

sed -i '100,$ d' myfile.txt

Explanation: sed is a regular expression processor. With the option -i given, it processes a file directly ("inline") -- instead of just reading it and writing the results to the standard output. 100,$ just means "from line 100 to the end of the file" -- and is followed by the command d, which you probably guessed correctly to stand for "delete". So in short, the command means: "Delete all lines from line 100 to the end of the file from myfile.txt". 100 is the first line to be deleted, as you want to keep 99 lines.

Edit: If, on the other hand, there are log files where you want to keep e.g. the last 100 lines:

[ $(wc -l myfile.txt) -gt 100 ] && sed -i "1,$(($(wc -l myfile.txt|awk '{print $1}') - 100)) d" myfile.txt

What is going on here:

• [ $(wc -l myfile.txt) -gt 100 ]: do the following only if the file has more than 100 lines
• $((100 - $(wc -l myfile.txt|awk '{print $1}'))): calculate number of lines to delete (i.e. all lines of the file except the (last) 100 to keep)
• 1, $((..)) d: remove all lines from the first to the calculated line

EDIT: as the question was just edited to give more details, I will include this additional information with my answer as well. Added facts are:

• a specific size shall remain with the file (10,000 bytes)
• each line has a specific size in bytes (300 bytes in the example)

From these data it is possible to calculate the number of lines to remain as " / ", which with the example would mean 33 lines. The shell term for the calculation: $((size_to_remain / linesize)) (at least on Linux using Bash, the result is an integer). The adjusted command now would read:

# keep the start of the file (OPs question)
sed -i '34,$ d' myfile.txt
# keep the end of the file (my second example)
[ $(wc -l myfile.txt) -gt 33 ] && sed -i "1,33 d" myfile.txt

As the sizes are known in advance, there's no longer any need for a calculation embedded to the sed command. But for flexibility, inside some shell script one can use variables.

For conditional processing based on the file size, one can use th following "test"-construct:

[ "$(ls -lk $file | awk ' {print $5}')" -gt 100 ] &&

which means: "if the size of $file exceeds 100kB, do..." (ls -lk lists the file size in kB at position 5, hence awk is used to extract exactly this).