Add a newline only if it doesn't exist

Question

I want to add a newline at the end of a file only if it doesn\'t exists, this is to prevent multiple newlines at the end of the file.

I\'m hoping to use sed. Here\'s the

Answer 1

Using Bash only

You can use Command Substitution (remove trailing newlines) with Here Strings (appends newline):

   Command Substitution
       Command substitution allows the output of a command to replace the command  name.   There  are  two
       forms:

          $(command)
       or
          `command`

       Bash  performs  the expansion by executing command in a subshell environment and replacing the com-
       mand substitution with the standard output of the command,  with  any  trailing  newlines  deleted.
       Embedded newlines are not deleted, but they may be removed during word splitting.  The command sub-
       stitution $(cat file) can be replaced by the equivalent but faster $(< file).



   Here Strings
       A variant of here documents, the format is:

          [n]<<

Here's how it works:

cat <<<"$(

Output to file:

cat <<<"$(outputfile

If you need inputfile and outputfile to be the same file name, you have a couple options - use sponge command, save to temporary variable with more command substitution, or save to temporary file.

---

Using Sed

Others have suggested using

sed '$a\' inputfile

which appends nothing to the last line. This is fine, but I think

sed '$q' inputfile

is a bit clearer, because it quits on the last line. Or you can do

sed -n 'p'

which uses -n to suppress output, but prints it back out with p.

In any of these cases, sed will fix up the line and add a newline, at least for GNU and BSD sed. However, I'm not sure if this functionality is defined by POSIX. A version of sed might just skip your line without a newline since a line is defined as

A sequence of zero or more non- characters plus a terminating character.