#! /bin/bash# protect_literal.sh# set -vx:<<-'_Protect_Literal_String_Doc'    Copyright (c) Michael S. Zick, 2003; All Rights Reserved    License: Unrestricted reuse in any form, for any purpose.    Warranty: None    Revision: $ID$    Documentation redirected to the Bash no-operation.    Bash will '/dev/null' this block when the script is first read.    (Uncomment the above set command to see this action.)    Remove the first (Sha-Bang) line when sourcing this as a library    procedure.  Also comment out the example use code in the two    places where shown.    Usage:        _protect_literal_str 'Whatever string meets your ${fancy}'        Just echos the argument to standard out, hard quotes        restored.        $(_protect_literal_str 'Whatever string meets your ${fancy}')        as the right-hand-side of an assignment statement.    Does:        As the right-hand-side of an assignment, preserves the        hard quotes protecting the contents of the literal during        assignment.    Notes:        The strange names (_*) are used to avoid trampling on        the user's chosen names when this is sourced as a        library._Protect_Literal_String_Doc# The 'for illustration' function form_protect_literal_str() {# Pick an un-used, non-printing character as local IFS.# Not required, but shows that we are ignoring it.    local IFS=$'\x1B'               # \ESC character# Enclose the All-Elements-Of in hard quotes during assignment.    local tmp=$'\x27'$@$'\x27'#    local tmp=$'\''$@$'\''         # Even uglier.    local len=${#tmp}               # Info only.    echo $tmp is $len long.         # Output AND information.}# This is the short-named version._pls() {    local IFS=$'x1B'                # \ESC character (not required)    echo $'\x27'$@$'\x27'           # Hard quoted parameter glob}# :<<-'_Protect_Literal_String_Test'# # # Remove the above "# " to disable this code. # # ## See how that looks when printed.echoecho "- - Test One - -"_protect_literal_str 'Hello $user'_protect_literal_str 'Hello "${username}"'echo# Which yields:# - - Test One - -# 'Hello $user' is 13 long.# 'Hello "${username}"' is 21 long.#  Looks as expected, but why all of the trouble?#  The difference is hidden inside the Bash internal order#+ of operations.#  Which shows when you use it on the RHS of an assignment.# Declare an array for test values.declare -a arrayZ# Assign elements with various types of quotes and escapes.arrayZ=( zero "$(_pls 'Hello ${Me}')" 'Hello ${You}' "\'Pass: ${pw}\'" )# Now list that array and see what is there.echo "- - Test Two - -"for (( i=0 ; i<${#arrayZ[*]} ; i++ ))do    echo  Element $i: ${arrayZ[$i]} is: ${#arrayZ[$i]} long.doneecho# Which yields:# - - Test Two - -# Element 0: zero is: 4 long.           # Our marker element# Element 1: 'Hello ${Me}' is: 13 long. # Our "$(_pls '...' )"# Element 2: Hello ${You} is: 12 long.  # Quotes are missing# Element 3: \'Pass: \' is: 10 long.    # ${pw} expanded to nothing# Now make an assignment with that result.declare -a array2=( ${arrayZ[@]} )# And print what happened.echo "- - Test Three - -"for (( i=0 ; i<${#array2[*]} ; i++ ))do    echo  Element $i: ${array2[$i]} is: ${#array2[$i]} long.doneecho# Which yields:# - - Test Three - -# Element 0: zero is: 4 long.           # Our marker element.# Element 1: Hello ${Me} is: 11 long.   # Intended result.# Element 2: Hello is: 5 long.          # ${You} expanded to nothing.# Element 3: 'Pass: is: 6 long.         # Split on the whitespace.# Element 4: ' is: 1 long.              # The end quote is here now.#  Our Element 1 has had its leading and trailing hard quotes stripped.#  Although not shown, leading and trailing whitespace is also stripped.#  Now that the string contents are set, Bash will always, internally,#+ hard quote the contents as required during its operations.#  Why?#  Considering our "$(_pls 'Hello ${Me}')" construction:#  " ... " -> Expansion required, strip the quotes.#  $( ... ) -> Replace with the result of..., strip this.#  _pls ' ... ' -> called with literal arguments, strip the quotes.#  The result returned includes hard quotes; BUT the above processing#+ has already been done, so they become part of the value assigned.##  Similarly, during further usage of the string variable, the ${Me}#+ is part of the contents (result) and survives any operations#  (Until explicitly told to evaluate the string).#  Hint: See what happens when the hard quotes ($'\x27') are replaced#+ with soft quotes ($'\x22') in the above procedures.#  Interesting also is to remove the addition of any quoting.# _Protect_Literal_String_Test# # # Remove the above "# " to disable this code. # # #exit 0