Question

For some reason I need to escape forward slash inside some string

# echo "http://google.com" | sed -e 's/[/]/\\\//g'
http:\/\/google.com

But when I try to execute the same inside “ it shows me:

# somevar=`echo "http://google.com" | sed -e 's/[/]/\\\//g'`
sed: -e expression #1, char 10: unknown option to `s'

My sed’s version:

# rpm -q sed
sed-4.1.5-8.el5

Answer 1

This is because the inside the backticks (aka backquotes, grave accents), \\ is replaced with \ before the command is parsed.

From the bash manpage:

When the old-style backquote form of substitution is used, backslash retains its literal meaning except when followed by $, `, or
\. The first backquote not preceded by a backslash terminates the
command sub‐
stitution. When using the $(command) form, all characters between the parentheses make up the command; none are treated
specially.

Just use $(...) instead of backticks, unless you really need pre-posix sh compatibility

somevar=$(echo "http://google.com" | sed -e 's/[/]/\\\//g')

I’d also recommend using a regex delimiter other than / to make this easier, e.g.

somevar=$(echo "http://google.com" | sed -e 's|/|\\/|g')