mjhjmtu

From useful one-line scripts for sed.

# Delete all trailing blank lines at end of file (only).
sed -e :a -e '/^n*$/$d;N;;/n$/ba' file

Since you already have answers with the more suitable tools sed and awk; you could take advantage of the fact that $(< file) strips off trailing blank lines.

a=$(<file); printf '%sn' "$a" > file

That cheap hack wouldn't work to remove trailing blank lines which may contain spaces or other non-printing characters, only to remove trailing empty lines. It also won't work if the file contains null bytes.

In shells other than bash and zsh, use $(cat file) instead of $(<file).

You can use this trick with cat & printf:

$ printf '%sn' "`cat file`"

For example

$ printf '%sn' "`cat ifile`" > ofile
$ cat -e ofile
1$
$
2$
$
$
3$

The $ denotes the end of a line.

References

• Removing trailing blank lines

Here's a Perl solution that doesn't require reading more than one line into memory at a time:

my $n = 0;
while (<>) 
 if (/./) 
 print "n" x $n, $_;
 $n = 0;
 else 
 $n++;
 

or, as a one-liner:

perl -ne 'if (/./) print "n" x $n, $_; $n = 0 else $n++ '

This reads the file a line at a time and checks each line to see if contains a non-newline character. If it doesn't, it increments a counter; if it does, it prints the number of newlines indicated by the counter, followed by the line itself, and then resets the counter.

Technically, even buffering a single line in memory is unnecessary; it would be possible to solve this problem using a constant amount of memory by reading the file in fixed-length chunks and processing it character by character using a state machine. However, I suspect that would be needlessly complicated for the typical use case.

This question is tagged with ed, but nobody has proposed an ed solution (I wonder why?).

Here's one:

ed file <<ED_END
a

.
?^..*?+1,.d
w
ED_END

ed will place you at the last line of the editing buffer by default upon startup.

The first command (a) adds an empty line to the end of the buffer (the empty line in the editing script is this line, and the dot (.) is just for coming back into command mode).

The second command (?) looks for the nearest previous line that contains something (even white-space characters), and then deletes everything to the end of the buffer from the next line on.

The third command (w) writes the file back to disk.

If your file is small enough to slurp into memory, you can use this

perl -e 'local($/);$f=<>; $f=~s/n*$/n/;print $f;' file

In python (I know it is not what you want, but it is much better as it is optimized, and a prelude to the bash version) without rewriting the file and without reading all the file (which is a good thing if the file is very large):

#!/bin/python
import sys
infile = open(sys.argv[1], 'r+')
infile.seek(-1, 2)
while infile.read(1) == 'n':
 infile.seek(-2, 1)
infile.seek(1, 1)
infile.truncate()
infile.close()

Note that it does not work on files where the EOL character is not 'n'.

A bash version, implementing the python algorithm, but less efficient as it needs many processes:

#!/bin/bash
n=1
while test "$(tail -n $n "$1")" == ""; do
 ((n++))
done
((n--))
truncate -s $(($(stat -c "%s" "$1") - $n)) "$1"

This one is quick to type, and, if you know sed, easy to remember:

tac < file | sed '/[^[:blank:]]/,$!d' | tac

It uses the sed script to delete leading blank lines from useful one line scripts for sed, referenced by Alexey, above, and tac (reverse cat).

In a quick test, on an 18MB, 64,000 line file, Alexey's approach was faster, (0.036 vs 0.046 seconds).