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Re: Shell-escape a string

From: Taylan Ulrich Bayırlı/Kammer
Subject: Re: Shell-escape a string
Date: Sat, 07 Mar 2015 12:00:46 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.4 (gnu/linux)

"Mark A. Hershberger" <address@hidden> writes:

> Is there a piece of standard code or a library that I can use to escape
> a string so it is safe to pass to bash?
> Specifically, I have submitted this bit[1] to add deletion of duplicate
> messages to a guile script included with mu (maildir utils) and now
> we're looking at how to escape the file names to keep this maliciously
> inserted maildir files from causing problems.
> Any hints?
> Footnotes: 
> [1]

I don't know if we already have something for this, but it's very simple
for POSIX sh, and bash:

Replace all occurrences of ' (single-quote) with '\'' (single-quote,
backslash, single-quote, single-quote), then prepend and append a pair
of ' (single-quote) to it.  So e.g.




That is guaranteed to be parsed as one token SO LONG as it's separated
by white-space from other things, and no ${} or $() or such will be

Explanation: in POSIX sh syntax, absolutely no characters have special
meaning within single-quotes, except for the terminating single-quote.
I.e. one can not even use \' to represent a literal single-quote inside
a single-quoted string.  What 'foo'\''bar' does is close a single-quoted
string, insert a bare backslash-escaped single-quote, then reopen the
single-quoted string and continue.


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