b2 command finds ICU when called directly but not from indirect bash script variable

Question

I have this strange issue with my bash script. I compile boost as part of it. The call from the script looks like this:

./b2 --reconfigure ${PARALLEL} link=static cxxflags=-fPIC install boost.locale.iconv=off boost.locale.posix=off -sICU_PATH="${ICU_PREFIX}" -sICU_LINK="${BOOST_ICU_LIBS}" >> "${BOOST_LOG}" 2>&1

That command works perfectly well. The log file shows that it finds ICU without a problem. However, if I change it to run from a variable, it no longer finds ICU (but it still compiles everything else):

bcmd="./b2 --reconfigure ${PARALLEL} link=static cxxflags=-fPIC install boost.locale.iconv=off boost.locale.posix=off -sICU_PATH=\"${ICU_PREFIX}\" -sICU_LINK=\"${BOOST_ICU_LIBS}\""
$bcmd >> "${BOOST_LOG}" 2>&1

What's the difference? I would like to be able to use the second approach so that I can pass the command into another function before running it.

Answer 1

Don't use a variable to store complex commands involving quotes that are nested. The problem is when you call the variable with just $cmd, the quotes are stripped incorrectly. Putting commands (or parts of commands) into variables and then getting them back out intact is complicated.

Quote removal is part of the one of the word expansions done by the shell. From the excerpt seen in POSIX specification of shell

2.6.7 Quote Removal

The quote characters ( backslash, single-quote, and double-quote) that were present in the original word shall be removed unless they have themselves been quoted.

Your example can be simply reproduced by a simple example. Assuming you have a few command flags (not actual ones)

cmdFlags='--archive --exclude="foo bar.txt"'

If you carefully look through the above, it contains 2 args, one --archive and another for --exclude="foo bar.txt", notice the double-quotes which needs to be preserved when you are passing it.

Notice how the quotes are incorrectly split when I don't quote cmdFlags, in the printf() call below

printf "'%s' " $cmdFlags; printf '\n'
'--archive' '--exclude="foo' 'bar.txt"'

and compare the result with one with proper quoting done below.

printf "'%s' " "$cmdFlags"; printf '\n'
'--archive --exclude="foo bar.txt"'

So along with the suggestion of properly quoting the variable, the general suggestion would be to use an array to store the flags and pass the quoted array expansion

cmdArray=()
cmdArray=(./b2 --reconfigure ${PARALLEL} link=static cxxflags=-fPIC install boost.locale.iconv=off boost.locale.posix=off -sICU_PATH="${ICU_PREFIX}" -sICU_LINK="${BOOST_ICU_LIBS}")

and pass the array as

"${cmdArrray[@]}" >> "${BOOST_LOG}" 2>&1