protect_literal.sh

BASH Shell 编程 经典教程 《高级SHELL脚本编程》中文版

#! /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.
echo
echo "- - 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.
done
echo

# 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.
done
echo

# 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