What is this sed doing with the script: `$a\`?

Question

I've come upon this sed command and I cannot figure out what it is doing. I understand that it is changing a file in place using -i, that it is using a script -e and that the script is $a\, but what is this script doing?

sed -i -e '$a\' filename

Answer 1

As others have said, e.g. in How to add a newline to the end of a file?, with GNU sed (and some other implementations), $a\ adds a newline to the end of a file if it doesn’t have one.

Why it does this isn’t so clear, and the documentation doesn’t explain it. However, examining the source code does...

Let’s start with the documentation. $a\ is a variant of a, exploiting a special case of a GNU extension:

As a GNU extension, the a command and text can be separated into two -e parameters, enabling easier scripting:

$ seq 3 | sed -e '2a\' -e hello
1
2
hello
3
$ sed -e '2a' -e "$VAR"

The way a is implemented in sed is with a queue of text to append, tracked in an append_queue. When the time comes to process this queue, in a function called dump_append_queue, the first step is

output_missing_newline (&output_file);

which adds a missing newline if necessary — to ensure that the appended text will be added to separate lines, not to the end of the current line. Then the contents of the queue are appended.

With sed -i '$a\', the missing newline is added if necessary, and then the queue is processed — but the queue is empty, so no additional change is made.

Answer 2

That adds a newline to the last line, if it is absent:

Create a file with no trailing newline

$ printf 'line1\nline2' > file
$ od -c file
0000000   l   i   n   e   1  \n   l   i   n   e   2
0000013

Let's see what that sed does:

$ sed -i '$a\' file
$ od -c file
0000000   l   i   n   e   1  \n   l   i   n   e   2  \n
0000014

What happens if we run it again? Does it add another newline?

$ sed -i '$a\' file
$ od -c file
0000000   l   i   n   e   1  \n   l   i   n   e   2  \n
0000014

Nope.