sed on OSX insert at a certain line

Strictly speaking, the POSIX specification for sed requires a newline after a\:

[1addr]a\
text

Write text to standard output as described previously.

This makes writing one-liners a bit of a pain, which is probably the reason for the following GNU extension to the a, i, and c commands:

As a GNU extension, if between the a and the newline there is other than a whitespace-\ sequence, then the text of this line, starting at the first non-whitespace character after the a, is taken as the first line of the text block. (This enables a simplification in scripting a one-line add.) This extension also works with the i and c commands.

Thus, to be portable with your sed syntax, you will need to include a newline after the a\ somehow. The easiest way is to just insert a quoted newline:

$ sed -e 'a\
> text'

(where $ and > are shell prompts). If you find that a pain, bash [1] has the $' ' quoting syntax for inserting C-style escapes, so just use

sed -e 'a\'$'\n''text'

[1] and mksh (r39b+) and some non-bash bourne shells (e.g., /bin/sh in FreeBSD 9+)

As described in this answer, it's helpful when using sed commands like i and a to use multiple -e "..." clauses. Each of these clauses will be understood to be separated by newlines. The i and a commands are hard to use in inline sed scripts otherwise (they're designed for use in a multiline sed script file invoked using sed -f file ...). It looks like you can't use the implicit newline introduced by the end of an -e clause to separate the a\ and the line of text that's to be appended. But you can use it to terminate the line of text that's to be appended.

In this specific case, what you're trying to do might in fact be accomplished with a single -e ... clause. You just have to use the a command correctly. By the POSIX standard, a needs to be followed by a \, then that needs to be followed by a newline, then the remainder of the next line will be inserted (until a newline or the end of the -e clause is encountered). So you could do:

sed -i "" -e $'4 a\\\n'"mode '0755'" file.txt

GNU sed seems to be a lot more commonly used than POSIX sed, based on examples/tutorials I've used anyway.

I've found that it's much easier just to install GNU sed on any OSX machine I use. If you're interested, the best way to do this is install it through Homebrew.

You can either just $ brew install gnu-sed, or get all of the common GNU utilities with $ brew install coreutils.

Then when you run into a syntax issue with date or some other program you can just use the GNU version. Eventually I just decided it was easier to always use the GNU versions and put them earlier than the system versions in my PATH.