[ILUG] Shell quoting mystery

[Brian Foster]

  | Date: Fri, 23 May 2008 14:40:32 +0100 (IST)
  | From: Lars Hecking <lhecking at users.sourceforge.net>
  | 
  |  bash 2.05b.1(1) on RHEL3.
  | 
  |  I have a script that builds a search expression for find
  |  and then runs find.  For some reason, an expression which
  |  includes "*" gets quoted with single quotes and therefore
  |  literally looks for 'foo.*' instead of all foo.*.
  | 
  | FINDRX_C="-name core.\* -o -name core"
  | FINDRX="${FINDRX_C}${FINDRX_N}${FINDRX_X}"
  | ...
  | find ${d} -type d -name .snapshot -prune -o -type f \( $FINDRX \)
  | ...
  | 
  |  becomes
  | sh -x ...
  | ...
  | + FINDRX=-name core.\* -o -name core
  | ++ find /home -type d -name .snapshot -prune -o -type f '(' -name 'core.\*' -o -name core ')'

 that seems correct, yes.  it is what you asked for.

  |  I tried all sorts of quoting tricks, but can't get rid of the single
  |  quotes surrounding core.\*. Any ideas?

 broadly, the problem is yer trying to convert
 a string into a list of arguments.  that can
 be tricky, esp. if those strings contain (as
 in this case) shell metachars.  quoting them
 when they need to be quoted, and not quoting
 them when they don't, and getting the string
 split up into the desired argument strings,
 can be a nightmare.

 the solution is to stop doing so.  since yer
 using ‘bash’, I suggest using an array (i.e.,
 an ordered list of strings — which is exactly
 what you want in the first place):

    args=( -name core.\* -o -name core )
    find "$d" -type d -name .snapshot -prune -o -type f \( "${args[@]}" \)

 other solutions can be devised, but the above
 is a no-brainer.

cheers!
	-blf-

p.s. unfortunately, array assignment syntax
    (at least) differs in various Bourne-ish
    shells — I don't know if arrays are in
    POSIX or not? — and so, e.g., the above
    would need a minor change to work with
    the Korn shell (ksh(1)).  and the older
    Bourne-ish shells don't have arrays at
    all (except for $*).

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