perl - How can I escape an arbitrary string for use as a command line argument in Bash?

Question

I have a list of strings and I want to pass those strings as arguments in a single Bash command line call. For simple alphanumeric strings it suffices to just pass them verbatim:

> script.pl foo bar baz yes no
foo
bar
baz
yes
no

I understand that if an argument contains spaces or backslashes or double-quotes, I need to backslash-escape the double-quotes and backslashes, and then double-quote the argument.

> script.pl foo bar baz ""yes""no""
foo
bar
baz
"yes""no"

But when an argument contains an exclamation mark, this happens:

> script.pl !foo
-bash: !foo: event not found

Double quoting doesn't work:

> script.pl "!foo"
-bash: !foo: event not found

Nor does backslash-escaping (notice how the literal backslash is present in the output):

> script.pl "!foo"
!foo

I don't know much about Bash yet but I know that there are other special characters which do similar things. What is the general procedure for safely escaping an arbitrary string for use as a command line argument in Bash? Let's assume the string can be of arbitrary length and contain arbitrary combinations of special characters. I would like an escape() subroutine that I can use as below (Perl example):

$cmd = join " ", map { escape($_); } @args;

Here are some more example strings which should be safely escaped by this function (I know some of these look Windows-like, that's deliberate):

yes
no
Hello, world      [string with a comma and space in it]
C:Program Files [path with backslashes and a space in it]
"                 [i.e. a double-quote]
                 [backslash]
\                [two backslashes]
\               [three backslashes]
\\              [four backslashes]
\\             [five backslashes]
"                [double-quote, backslash]
"T               [double-quote, backslash, T]
"\T              [double-quote, backslash, backslash, T]
!1                
!A                
"!/'"            [double-quote, exclamation, backslash, forward slash, apostrophe, double quote]
"Jeff's!"         [double-quote, J, e, f, f, apostrophe, s, exclamation, double quote]
$PATH             
%PATH%            
&                 
<>|&^             
*@$$A$@#?-_       

EDIT:

Would this do the trick? Escape every unusual character with a backslash, and omit single or double quotes. (Example is in Perl but any language can do this)

sub escape {
    $_[0] =~ s/([^a-zA-Z0-9_])/\$1/g;
    return $_[0];
}

Answer 1

If you want to securely quote anything for Bash, you can use its built-in printf %q formatting:

cat strings.txt:

yes
no
Hello, world
C:Program Files
"

\
\
\\
\\
"
"T
"\T
!1
!A
"!/'"
"Jeff's!"
$PATH
%PATH%
&
<>|&^
*@$$A$@#?-_

cat quote.sh:

#!/bin/bash
while IFS= read -r string
do
    printf '%q
' "$string"
done < strings.txt

./quote.sh:

yes
no
Hello, world
C:\Program Files\
"
\
\\
\\\
\\\\
\\\\\
"\
"\T
"\\T
!1
!A
"!\/'"
"Jeff's!"
$PATH
%PATH%
&
<>|&^
*@$$A$@#?-_

These strings can be copied verbatim to for example echo to output the original strings in strings.txt.