How can I remove the last character of a file in unix?

truncate

truncate -s-1 file

Removes one (-1) character from the end of the same file. Exactly as a >> will append to the same file.

The problem with this approach is that it doesn't retain a trailing newline if it existed.

The solution is:

if     [ -n "$(tail -c1 file)" ]    # if the file has not a trailing new line.
then
       truncate -s-1 file           # remove one char as the question request.
else
       truncate -s-2 file           # remove the last two characters
       echo "" >> file              # add the trailing new line back
fi

This works because tail takes the last byte (not char).

It takes almost no time even with big files.

Why not sed

The problem with a sed solution like sed '$ s/.$//' file is that it reads the whole file first (taking a long time with large files), then you need a temporary file (of the same size as the original):

sed '$ s/.$//' file  > tempfile
rm file; mv tempfile file

And then move the tempfile to replace the file.

Here's another using ex, which I find not as cryptic as the sed solution:

 printf '%s\n' '$' 's/.$//' wq | ex somefile

The $ goes to the last line, the s deletes the last character, and wq is the well known (to vi users) write+quit.

After a whole bunch of playing around with different strategies (and avoiding sed -i or perl), the best way i found to do this was with:

sed '$! { P; D; }; s/.$//' somefile

If the goal is to remove the last character in the last line, this awk should do:

awk '{a[NR]=$0} END {for (i=1;i<NR;i++) print a[i];sub(/.$/,"",a[NR]);print a[NR]}' file
sometext
moretext
lastlin

It store all data into an array, then print it out and change last line.

Just a remark: sed will temporarily remove the file. So if you are tailing the file, you'll get a "No such file or directory" warning until you reissue the tail command.

EDITED ANSWER

I created a script and put your text inside on my Desktop. this test file is saved as "old_file.txt"

sometext
moretext
lastline

Afterwards I wrote a small script to take the old file and eliminate the last character in the last line

#!/bin/bash
no_of_new_line_characters=`wc  '/root/Desktop/old_file.txt'|cut -d ' ' -f2`
let "no_of_lines=no_of_new_line_characters+1"
sed -n 1,"$no_of_new_line_characters"p  '/root/Desktop/old_file.txt' > '/root/Desktop/my_new_file'
sed -n "$no_of_lines","$no_of_lines"p '/root/Desktop/old_file.txt'|sed 's/.$//g' >> '/root/Desktop/my_new_file'

opening the new_file I created, showed the output as follows:

sometext
moretext
lastlin

I apologize for my previous answer (wasn't reading carefully)

sed 's/.$//' filename | tee newFilename

This should do your job.

A couple perl solutions, for comparison/reference:

(echo 1a; echo 2b) | perl -e '$_=join("",<>); s/.$//; print'

(echo 1a; echo 2b) | perl -e 'while(<>){ if(eof) {s/.$//}; print }'

I find the first read-whole-file-into-memory approach can be generally quite useful (less so for this particular problem). You can now do regex's which span multiple lines, for example to combine every 3 lines of a certain format into 1 summary line.

For this problem, truncate would be faster and the sed version is shorter to type. Note that truncate requires a file to operate on, not a stream. Normally I find sed to lack the power of perl and I much prefer the extended-regex / perl-regex syntax. But this problem has a nice sed solution.

I think the most Unix solution is to use 'head'

head -c-1 infile > outfile