Ubuntu – Add lines from file to another file

Your script should work if fixed like so:

while
 last_line=$(tail -1 "./file.txt")
 [[ "$last_line" =~ ^$ ]] || [[ "$last_line" =~ ^[[:space:]]+$ ]]
do
 sed -i '$d' "./file.txt"
done

Your script had two main problems: (1) you never updated $last_line, so the loop's guard would always evaluate the same thing; (2) your [[ "$last_line" =~ $ ]] test matched any line, since any line has an end. (This is the reason why your script emptied your file completely.) You probably want to match against ^$ instead, which matches only empty lines. Additionally, I simplified the sed command to delete the last line in the loop's body (simply $d does the job).

However, this script is unnecessarily complicated. sed is there for just that kind of thing! This one-liner will do the same thing as the above script:

sed -i ':a;/^[ \n]*$/{$d;N;ba}' ./file.txt

Roughly,

1. Match current line against ^[ \n]*$. (i.e, can only contain whitespaces and newlines)
2. If it doesn't match, just print it. Read in next line and continue with step 1.
3. If it does match,
   • If we are at the end of the file, delete it.
   • If we are not at the end of the file, append the next line to the current line, inserting a newline character between the two, and go back to step 1 with this new, longer line.

There are lots of awesome sed tutorials on the Internet. For example, I can recommend this one. Happy learning! :-)

Update: And of course, if you additionally want to remove the last (non-blank) line of the file after having truncated the trailing blank lines, you can just use another sed -i '$d' ./file.txt after either your script or the above one-liner. I intentionally did not want to include that in the sed one-liner since I thought that removing trailing blank lines is quite a reusable piece of code that may be interesting for other people; but removing the last non-blank line is really specific to your use case, and trivial anyway once you removed the trailing blank lines.

Here's a sed approach:

$ sed -nE '1s/.{11}(.{8}).*/\1/p; 3s/.{3}(.{4}).*/\1/p' file  
Ethernet
t6 a

Explanation

The -n suppresses normal output (normal is to print every input line) so that it only prints when told to. The -E enables extended regular expressions.

The sed script has two commands, both using the substitution operator (s/original/replacement/). The 1s/.{11}(.{8}).*/\1/p will only run on the 1st line (that's what the 1s does), and will match the 1st 11 characters of the line (.{11}), then it captures the next 8 ((.{8}), the parentheses are a "capture group") and then everything else until the end of the line (.*). All this is replaced with whatever was in the capture group (\1; if there were a second capture group, it would be \2 etc.). Finally, the p at the end (s/foo/bar/p) causes the line to be printed after the substitution has been made. This results in only the target 8 characters being output.

The second command is the same general idea except that it will only run on the 3rd line (3s) and will keep the 4 characters starting from the 4th.

You could also do the same thing with perl:

$ perl -ne 'if($.==1){s/.{11}(.{8}).*/\1/}
            elsif($.==3){s/.{3}(.{4}).*/\1/}
            else{next}; print; ' file 
Ethernet
t6 a

Explanation

The -ne means "read the input file line by line and apply the script given by -e to each line. The script is the same basic idea as before. The $. variable holds the current line number so we check if the line number is either 1 or 3 and, if so, run the substitution, else skip. Therefore the print will only be run for those two lines since all others will be skipped.

Of course, this is Perl, so TIMTOWTDI:

$ perl -F"" -lane '$. == 1 && print @F[11..19]; $.==3 && print @F[3..6]' file 
Ethernet 
t6 a

Explanation

Here, the -a means "split each input line on the character given by -F and save as the array @F. Since the character given is empty, this will save each character of the input line as an element in @F. Then, we print elements 11-19 (arrays start counting at 0) for the 1st line and 3-7 for the 3rd.