Escape slashes in shell script variables with sed

sed is a tool of wonder, the in-place replacement (or deletion) of lines in files matching some regexp being my favorite use of it. But some basic functions are quite under-documented (the same old problem of powerful commands having man pages just too long to be read thoroughly).

What I wanted to do is to drop all lines in a file containing a path stored in a variable in my bash script. Problem is, slashes must be escaped for sed to understand that it must treat them as literal, but I have a reference to a variable so I just can’t do something like

sed -i \/path\/to\/delete/d /path/to/file

I found out (on this thread on stackoverflow) that you can use any character you want as separator, and one thing that is usually not known is that you must escape the first occurrence of the separator to have sed use it, unless you are using the s command (replacing patterns, as in sed -i 's/abc/def/' file, while I’m only deleting matches).

So, this is it:

PATH_TO_BE_DELETED="/path/to/delete"
sed -i "\,$PATH_TO_BE_DELETED,d" /path/to/file

I used commas as I know that my paths won’t ever include them, but you can use whatever you want or need.

Note that you also have to use double quotes instead of single ones, or your variable is interpreted as literal!

A nice list of useful one-liners for sed can be found here.