Porting GNU sed command to BSD sed

Question

I’m porting a shell script written for Linux to FreeBSD, and have come across this line:

local from_conf="$(sed -rn "s/^$1=('|\"|)(.*)\1/\2/ip" "$CONF" | tail -n 1)"

It’s used as part of a function to get properties out of a configuration file ($CONF), so adding in actual variable values it may look something like:

local from_conf="$(sed -rn "s/^USERNAME=('|\"|)(.*)\1/\2/ip" "/etc/msm.conf" | tail -n 1)"

Under GNU sed (v4.2.2 in Ubuntu) this works as required, however under BSD sed (as shipped in FreeBSD 10.1) it complains:

sed: 1: "s/^USERNAME=('|"|)(.*)\ ...": RE error: empty (sub)expression

It seems to be the second pipe in ('|"|) part of the pattern, removing it ( ('|") ) clears the error, and I get the desired result on FreeBSD. It also appears to give the correct results under Ubuntu, but since I’m trying to make code that is portable between the two systems, I want to better understand this part of the expression and why it doesn't work under BSD sed.

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My understanding of the original pattern ( ('|"|) ) is:

Match:

• a single quote (') — e.g. USERNAME='foo'
• a double quote (") — e.g. LOCALE="bar"
• nothing () — e.g. DELAY=10

How can I port this pattern to BSD without breaking it for GNU sed?

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There’s another element that doesn’t work:

sed -rn "s/^$1=('|\"|)(.*)\1/\2/ip" "$CONF"

The \1 is part of the pattern, and under GNU sed will insert whatever the outcome of ('|\"|) is. This doesn't work under BSD sed, is there a different, portable, notation? Or would I have to repeat the first pattern verbatim?

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EDIT: The contents of /etc/msm.conf might look something like this:

USERNAME='foo'
LOCALE="bar"
DELAY=10
SERVER_NAME="Ben's Server"

The expected output of sed -rn "s/^USERNAME=('|\"|)(.*)\1/\2/ip" "/etc/msm.conf" should be “foo”.

Answer 1

This should work with the first capture group removed

sed "s/^USERNAME=\(['\"]\)\{0,1\}\([^'\"]*\)['\"]\{0,1\}/\2/p;d" "/etc/msm.conf"

Answer 2

Your enclosing script has bad quoting. Eliminate the outer double quotes like this:

    local from_conf=$(sed -rn "s/^$1=('|\"|)(.*)\1/\2/ip" "$CONF" | tail -n 1)

and you should be good to go.

As for the (...) issue, use \( and \). That's the portable notation.