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

Say I have some arbitrary multi-line text file:

sometext
moretext
lastline

How can I remove only the last character (the e, not the newline or null) of the file without making the text file invalid?

A simpler approach (outputs to stdout, doesn't update the input file):

sed '$ s/.$//' somefile

• $ is a Sed address that matches the last input line only, thus causing the following function call (s/.$//) to be executed on the last line only.

• s/.$// replaces the last character on the (in this case last) line with an empty string; i.e., effectively removes the last char. (before the newline) on the line.
  ^{. matches any character on the line, and following it with $ anchors the match to the end of the line; note how the use of $ in this regular expression is conceptually related, but technically distinct from the previous use of $ as a Sed address.}

• Example with stdin input (assumes Bash, Ksh, or Zsh):

    $ sed '$ s/.$//' <<< $'line one\nline two'
    line one
    line tw
  

To update the input file too (do not use if the input file is a symlink):

sed -i '$ s/.$//' somefile

^Note:

• On macOS, you'd have to use -i '' instead of just -i; for an overview of the pitfalls associated with -i, see the bottom half of this answer.
• If you need to process very large input files and/or performance / disk usage are a concern and you're using GNU utilities (Linux), see ImHere's helpful answer.

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.