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Here you go:

sed -i -e '$a\' file

And alternatively for OS X sed:

sed -i '' -e '$a\' file

This adds \n at the end of the file only if it doesn’t already end with a newline. So if you run it twice, it will not add another newline:

$ cd "$(mktemp -d)"
$ printf foo > test.txt
$ sed -e '$a\' test.txt > test-with-eol.txt
$ diff test*
1c1
< foo
\ No newline at end of file
---
> foo
$ echo $?
1
$ sed -e '$a\' test-with-eol.txt > test-still-with-one-eol.txt
$ diff test-with-eol.txt test-still-with-one-eol.txt
$ echo $?
0

Here you go:

sed -i -e '$a\' file

And alternatively for OS X sed:

sed -i '' -e '$a\' file

This adds \n at the end of the file only if it doesn’t already end with a newline. So if you run it twice, it will not add another newline:

$ cd "$(mktemp -d)"
$ printf foo > test.txt
$ sed -e '$a\' test.txt > test-with-eol.txt
$ diff test*
1c1
< foo
\ No newline at end of file
---
> foo
$ echo $?
1
$ sed -e '$a\' test-with-eol.txt > test-still-with-one-eol.txt
$ diff test-with-eol.txt test-still-with-one-eol.txt
$ echo $?
0

How it works:

• $ denotes the end of file
• a\ appends the following text (which is nothing, in this case) on a new line

In other words, if the last line contains a character that is not newline, append a newline.

Here you go:

sed -i '' -e '$a\' file

This adds \n at the end of the file only if it doesn’t already end with a newline. So if you run it twice, it will not add another newline:

$ cd "$(mktemp -d)"
$ printf foo > test.txt
$ sed -e '$a\' test.txt > test-with-eol.txt
$ diff test*
1c1
< foo
\ No newline at end of file
---
> foo
$ echo $?
1
$ sed -e '$a\' test-with-eol.txt > test-still-with-one-eol.txt
$ diff test-with-eol.txt test-still-with-one-eol.txt
$ echo $?
0

Here you go:

sed -i -e '$a\' file

And alternatively for OS X sed:

sed -i '' -e '$a\' file

This adds \n at the end of the file only if it doesn’t already end with a newline. So if you run it twice, it will not add another newline:

$ cd "$(mktemp -d)"
$ printf foo > test.txt
$ sed -e '$a\' test.txt > test-with-eol.txt
$ diff test*
1c1
< foo
\ No newline at end of file
---
> foo
$ echo $?
1
$ sed -e '$a\' test-with-eol.txt > test-still-with-one-eol.txt
$ diff test-with-eol.txt test-still-with-one-eol.txt
$ echo $?
0

Here you go:

sed -i -e '$a\' file

This adds \n at the end of the file only if it doesn't already end in a newline. So if you run it twice it will not add another newline:

$ cd "$(mktemp -d)"
$ printf foo > test.txt
$ sed -e '$a\' test.txt > test-with-eol.txt
$ diff test*
1c1
< foo
\ No newline at end of file
---
> foo
$ echo $?
1
$ sed -e '$a\' test-with-eol.txt > test-still-with-one-eol.txt
$ diff test-with-eol.txt test-still-with-one-eol.txt
$ echo $?
0

Here you go:

sed -i '' -e '$a\' file

This adds \n at the end of the file only if it doesn’t already end with a newline. So if you run it twice, it will not add another newline:

$ cd "$(mktemp -d)"
$ printf foo > test.txt
$ sed -e '$a\' test.txt > test-with-eol.txt
$ diff test*
1c1
< foo
\ No newline at end of file
---
> foo
$ echo $?
1
$ sed -e '$a\' test-with-eol.txt > test-still-with-one-eol.txt
$ diff test-with-eol.txt test-still-with-one-eol.txt
$ echo $?
0