sed: -e expression #1, char 62: unknown command: `\' - sed

Question

I am trying to add
-
paths:
- /var/log/consumer.log
document_type: consumer
input_type: log
after prospectors: in my file. I am using command:
sed -i '/prospectors:/a\ \ \ \ \-
\ \ \ \ \ \ paths:\
\ \ \ \ \ \ \- \/var\/log\/consumer.log
\ \ \ \ \ \ document_type: consumer
\ \ \ \ \ \ input_type: log' new.txt
But the above command gives following error:
sed: -e expression #1, char 62: unknown command: `\'
How can I achieve the desired?

Answer 1

In classic (POSIX) sed, each line of data appended needs to be on its own line after the a command, and all lines except the last need a backslash at the end to indicate that the data continues. GNU sed allows some information on the same line as the a command, but otherwise follows the rules.
There's an additional wrinkle: sed removes leading blanks from the data. To get the leading blanks, you can use backslash-blank at the start.
Hence, you can end up with:
sed -i '/prospectors:/a \
\ -\
\ paths:\
\ - /var/log/consumer.log\
\ document_type: consumer\
\ input_type: log' new.txt
The leading blanks are ignored; the backslash is deleted; the following blanks are copied to the output. Thus given an input containing just a line containing prospectors:, the output is:
prospectors:
-
paths:
- /var/log/consumer.log
document_type: consumer
input_type: log
Obviously, you can adjust the spacing to suit yourself.
I note that BSD sed requires a suffix after the -i option; it can be -i '' to get an 'empty string' suffix. To be portable between GNU and BSD sed, use -i.bak (no space; GNU sed doesn't like the space; BSD sed accepts the attached suffix, but you can't attach an empty suffix). And the -i option is not mandated by POSIX, so it isn't available on all Unix-like systems. If you're only using GNU sed, you don't have to worry about this trivia.