sed not parsing \t as a tab

Question

I am trying to insert a line into a coffee script using sed and it looks as if it is ignoring my first tab character.

The file I want to insert into has a line like this:

    karmaServer.start

and I want to insert a line after it like this:

    karmaServer.start
        flags: ['--no-sandbox']

So I wrote a sed script that I think should work

sed "/^\t\tkarmaServer.start/a\t\t\t\tflags: ['--no-sandbox']" build/unit-test.coffee 

although the result is quite odd. I get the following

    karmaServer.start
t       flags: ['--no-sandbox']

And I don't understand why it starts the line with a t character or how to get rid of it.

any ideas?

Answer 1

You must add a newline in your sed command (and then account for the shell's handling of escaped newlines):

 sed "/^\t\tkarmaServer.start/a\\
 \t\t\t\tflags: ['--no-sandbox']" build/unit-test.coffee 

As the GNU sed manual says:

a \

text Append text, which has each embedded newline preceded by a backslash.

This is not very clear, unformatted like this. (It isn't particularly clearer when viewed with the man command, alas.) What it is telling you is that the format of the a command includes a backslash, a newline, and then an argument denoted by text. Everything in that column on the left, in other words.

As noted in question comments, seds differ in how they handle malformed commands. Spinellis sed yields an error, telling you that you have something other than the required backslash followed by newline:

% sed "/^\t\tkarmaServer.start/a\t\t\t\tflags: ['--no-sandbox']" /dev/null
sed: 1: "/^\t\tkarmaServer.start ...": extra characters after \ at the end of a command
%

Your particular choice of sed appears to be simply treating the rest of the line as the text argument. But the first backslash there is still counting as the mandatory backslash that should follow a. Therefore, what you end up with as the text argument is

t\t\t\tflags: ['--no-sandbox']

Notice the initial t, and one fewer tabs than you expected.