Properly restore IFS

Following some discussion with vagrantc and stgraber on IRC, I'm going
to commit a non-obvious change to IFS saving/restoring, so I'm sending a
script to clarify it and for future reference.

#!/bin/sh
# This script demonstrates a better way to save/restore $IFS.
# An unset IFS is not the same as a null IFS, for example:
#   $ IFS='' && var='      ' && echo x${var}x
#   x      x
#   $ unset IFS && var='      ' && echo x${var}x
#   x x
# So I propose to use this line to save it:
#   oldifs="${IFS-not set}"
# and this line to restore it:
#   [ "$oldifs" = "not set" ] && unset IFS || IFS="$oldifs"

do_something_with_IFS()
{
    # Save $IFS, or "not set" if it's unset.
    # Note the '-' below, it isn't the regular ':-' syntax, it means
    # "use alternate value ONLY if IFS is unset, not if it's null".
    oldifs="${IFS-not set}"

    # Modify IFS and do something with it
    IFS=':'
    echo "...changing, using, and restoring IFS..."

    # Now restore IFS
    # This is an "if ... then ... else" construct but in one line.
    [ "$oldifs" = "not set" ] && unset IFS || IFS="$oldifs"
}

report_IFS()
{
    if [ "${IFS-not set}" = "not set" ]; then
        echo "IFS is unset"
    elif [ -z "$IFS" ]; then
        echo "IFS is null"
    else
        echo -n "IFS="
        echo -n "$IFS" | hexdump -e '8/1 "%02X ""\t"" "' -e '8/1 "%c""\n"'
    fi
}

# Testing with various IFS values.

# First, try with the system default:
report_IFS; do_something_with_IFS; report_IFS; echo

unset IFS
report_IFS; do_something_with_IFS; report_IFS; echo

IFS=""
report_IFS; do_something_with_IFS; report_IFS; echo

IFS=" \t\n"
report_IFS; do_something_with_IFS; report_IFS; echo

# And, on this crazy case, the new code has problems.
# But setting IFS to "not set" is a bug on its own,
# as it contains the letter "t" twice. :-)
IFS="not set"
report_IFS; do_something_with_IFS; report_IFS; echo



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