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 exist. This is to prevent multiple newlines at the end of the file.

I'm hoping to use sed. Here are the issues I'm having with my current code:

sed -i -e '/^$/d;$G' /inputfile  echo file1 name1 name2  echo file2 name3 name4 (newline) 

when I run my code on to the files;

echo file1 name1 name2 (newline)  echo file2 name3 name4 

it adds a newline if it doesn't have one but removes it if it exists... this puzzles me.

Answer 1

sed

GNU:

sed -i '$a\' *.txt 

OS X:

sed -i '' '$a\' *.txt 

$ addresses the last line. a\ is the append function.

OS X's sed

sed -i '' -n p *.txt 

-n disables printing and p prints the pattern space. p adds a missing newline in OS X's sed but not in GNU sed, so this doesn't work with GNU sed.

awk

awk 1 

1 can be replaced with anything that evaluates to true. Modifying a file in place:

{ rm file;awk 1 >file; }<file 

bash

[[ $(tail -c1 file) && -f file ]]&&echo ''>>file 

Trailing newlines are removed from the result of the command substitution, so $(tail -c1 file) is empty only if file ends with a linefeed or is empty. -f file is false if file is empty. [[ $x ]] is equivalent to [[ -n $x ]] in bash.

Answer 2

Rather than processing the whole file with see just to add a newline at the end, just check the last character and if it's not a newline, append one. Testing for newline is slightly interesting, since the shell will generally trim them from the end of strings, so I append "x" to protect it:

if [ "$(tail -c1 "$inputfile"; echo x)" != $'\nx' ]; then     echo "" >>"$inputfile" fi 

Note that this will append newline to empty files, which might not be what you want. If you want to leave empty files alone, add another test:

if [ -s "$inputfile" ] && [ "$(tail -c1 "$inputfile"; echo x)" != $'\nx' ]; then     echo "" >>"$inputfile" fi