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?

Solution

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.
- Note: The command is a no-op if the file ends in two or more newlines.

^{. 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.