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Full Discussion: Can I do this without eval? (zsh)
Top Forums Shell Programming and Scripting Can I do this without eval? (zsh) Post 302990286 by rovf on Tuesday 24th of January 2017 11:26:57 AM
Old 01-24-2017
Can I do this without eval? (zsh)
I have in one shell variable, op, a string which represents a "test operator" in a [[ ... ]] construct, for instance -d or -n or -s, an in another shell variable, arg, some arbitrary string. What I want to achieve, is basically this:

Code:
#This is INCORRECT code. I just want to get you the idea, what I'm looking for.
if  [[ ! $op $arg ]]
then
   do_something
fi

Of course, this does not work, because the operator - being a syntax construct - can not be substituted.

The closest solution I found is this:

Code:
if eval "[[ ! $op '$arg' ]]"
then
   do_something
fi

I'm not happy with this for two reasons:

1. It breaks if $arg contains sequences of backslashes or single quotes
2. While I have good control of what is stored into $op, I don't have so good control of what is stored on $arg, so it is a security hole.

I could scan arg before and reject it completely, if arg gontains either backslash or single quote - I reckon that any exploit of the security hole would require either this or the other -, but I wonder whether there is a simpler way to code this.

A proposed solution can be zsh-specific, but should be upward compatible from zsh 5.0. Any ideas?
 

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LEARN ABOUT DEBIAN

ns_sockasyncconnect2

Ns_Sock(3aolserver)					   AOLserver Library Procedures 				       Ns_Sock(3aolserver)

__________________________________________________________________________________________________________________________________________________

NAME

       Ns_BindSock,  Ns_GetSockAddr,  Ns_SockAccept,  Ns_SockAsyncConnect,  Ns_SockAsyncConnect2,  Ns_SockBind, Ns_SockCloseLater, Ns_SockConnect,
       Ns_SockConnect2, Ns_SockListen, Ns_SockListenEx, Ns_SockPipe, Ns_SockSetBlocking, Ns_SockSetNonBlocking, Ns_SockStrError,  Ns_SockTimedCon-
       nect, Ns_SockTimedConnect2, ns_pipe, ns_sockpair - library procedures

SYNOPSIS

       #include "ns.h"

       Ns_BindSock(arg, arg)

       Ns_GetSockAddr(arg, arg)

       Ns_SockAccept(arg, arg)

       Ns_SockAsyncConnect(arg, arg)

       Ns_SockAsyncConnect2(arg, arg)

       Ns_SockBind(arg, arg)

       Ns_SockCloseLater(arg, arg)

       Ns_SockConnect(arg, arg)

       Ns_SockConnect2(arg, arg)

       Ns_SockListen(arg, arg)

       Ns_SockListenEx(arg, arg)

       Ns_SockPipe(arg, arg)

       Ns_SockSetBlocking(arg, arg)

       Ns_SockSetNonBlocking(arg, arg)

       Ns_SockStrError(arg, arg)

       Ns_SockTimedConnect(arg, arg)

       Ns_SockTimedConnect2(arg, arg)

       ns_pipe(arg, arg)

       ns_sockpair(arg, arg)
_________________________________________________________________

DESCRIPTION

       These functions ...

SEE ALSO

       nsd(1), info(n)

KEYWORDS

AOLserver								4.0						       Ns_Sock(3aolserver)
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