Re: bin/44246: enhance mv to work better with xargs

[Mouse <mouse%Rodents-Montreal.ORG@localhost>]

The following reply was made to PR bin/44246; it has been noted by GNATS.

From: Mouse <mouse%Rodents-Montreal.ORG@localhost>
To: tech-userlevel%netbsd.org@localhost
Cc: gnats-bugs%netbsd.org@localhost
Subject: Re: bin/44246: enhance mv to work better with xargs
Date: Sat, 12 Jan 2013 23:46:24 -0500 (EST)

 > I've taken a look at bin/44246 and attempted to implement an
 > enhancement to resolve the PR.
 
 IMO this is not a defect in mv, but a defect in xargs.
 
 mv is (even "mv and cp and ln are") hardly the only case(s) where more
 than just appending arguments to a command would be useful.
 
 With my xargs, this would be, eg, "ls -1 | xargs -kX mv X newdir".
 Here's the comment header describing the functionality.  I don't expect
 NetBSD would want it directly, if only because I doubt NetBSD agrees
 with me that the standard-specified xargs behaviour with respect to
 whitespace and quoting is a bug or at best a misfeature.  But perhaps
 something along the lines of -k and -r could be adopted to fix this
 once, at the source of the problem, rather than working around it in a
 few of the commonest cases by kludging up other tools.  (For example,
 mv -x doesn't permit this, which is relatively common for me:
        ls .. | grep something | xargs -kX mv ../X .
 In this case it's not all that hard to adapt by running a similar
 command in .., but that's a workaround for this case, not a general
 fix.)
 
 Of course, NetBSD is also welcome to the whole thing.  I just don't
 expect that to be desired; I can easily make it available if it is.
 
 Here's that comment header.
 
 /*
  * xargs - accumulate arguments for a program
  *
  * xargs reads its standard input and accumulates the strings it gets.
  *  When it has enough, either total length or number of strings, it
  *  executes the program named as its argument.  This is useful for
  *  using programs like rm in conjunction with find:
  *
  *     find / -name \*.CKP -print | xargs rm -f
  *
  * Any options to xargs must of course occur before the name of the
  *  command xargs is to execute.  The first argument not beginning with
  *  a - sign (or being an argument to a flag) indicates the beginning
  *  of the command.
  *
  * The options are:
  *
  *     -l      causes xargs to log the command it is about to execute
  *               to stderr just before executing it.
  *
  *     -n      don't execute the commands.  This is pretty useless
  *               except in conjunction with -l.
  *
  *     -N<n>   tells xargs to fork and exec as soon as it has <n>
  *               arguments accumulated (not counting any on the xargs
  *               command line).  It may still fork/exec earlier than
  *               this.  (In the presence of -n, it doesn't actually
  *               fork and exec.)
  *
  *     -0      causes xargs to break its input into arguments on NULs
  *               rather than newlines.
  *
  *     -f      forces xargs to execute the command at least once, with
  *               no arguments if necessary.
  *
  *     -p<n>   causes xargs to execute <n> commands in parallel.
  *               Normally, once a command is started no further
  *               commands are run until it exits.  With this option,
  *               xargs does not block until <n> commands are running,
  *               and, as soon as any one of them exits, xargs unblocks.
  *
  *     -k<str> indicates that instead of taking the given command and
  *               arguments and appending the arguments derived from
  *               the input, xargs should modify and replicate certain
  *               arguments.  Any argument containing <str> will be
  *               replicated, once per string, as in
  *
  *                     .... | xargs -kX mv dir1/X dir2
  *
  *               which will run commands like
  *
  *                     mv dir1/foo dir1/bar dir1/blee dir2
  *
  *               If -k is not used, the effect is as if it were used
  *               with a string that does not otherwise occur, with one
  *               extra trailing argument, equal to that string, on the
  *               command.
  *
  *     -r<c><s1><c><s2>
  *             indicates that certain argument(s) (or portions
  *               thereof) to the command are to be replicated.  If
  *               <s1> appears as an argument to the command, all
  *               arguments between that and the next argument equal to
  *               <s2> (or the last argument, if no argument equals
  *               <s2>) are replicated, appearing once per string.
  *               (Normally, -k will be used as well, and at least one
  *               of the replicated arguments will include the -k
  *               string, but this does not have to be so.)  If an
  *               argument to the command contains but is not equal to
  *               <s1>, then part of that argument is replicated.
  *               Specifically, everything from immediately after the
  *               <s1>, up to but not including the next <s2> (or the
  *               end of the argument if there is no following <s2>),
  *               is replicated.  If -k is used and the replicated
  *               portion contains the -k string, each copy will have
  *               the -k string replaced as appropriate.  For example,
  *
  *                     xargs -kXX -r/A/B cmd mAjB blivet A -q XX -f mXX B -t
  *
  *               will run commands like
  *
  *                     cmd mjjj blivet -q foo -f mfoo -q bar -f mbar -q blee 
-f mblee -t
  *
  *               while
  *
  *                     xargs -kXX -r/A/B cmd -t mAjXXkB
  *
  *               will run commands like
  *
  *                     cmd -t mjfookjbarkjbleek
  *
  *               Neither <s1> nor <s2> may be zero-length; it is also
  *               an error for the second <c> to be missing.
  *
  * If -k is used, any argument containing the -k string but not under
  *  the influence of -r (either because -r is not given or because the
  *  argument is not bracketed appropriately) will nevertheless be
  *  replicated.
  *
  * If -k is used with a string that can overlap itself (ie, it has a
  *  proper suffix that is also a proper prefix), only nonoverlapping
  *  occurrences found in a simple left-to-right scan are used.  Thus,
  *  for example, with -kXX, an argument 1XXXX2 will generate arguments
  *  such as 1foofoo2 and 1barbar2, even though in a sense there are
  *  *three* occurrences of XX in 1XXXX2.
  *
  * In arguments that are being repeated due to -r, no -r strings are
  *  recognized except for the argument that is equal to the second
  *  string; in particular, arguments that contain but are not equal to
  *  the -r string(s) are not noticed.
  *
  * When the -k and -r strings are such that they can overlap, and an
  *  argument contains overlapping instances, which one is recongized is
  *  unspecified.
  *
  * It is an error for an argument to both contain but be different from
  *  the first -r string and also contain the -k string in a position
  *  where it is not being repeated due to -r.
  *
  * Unlike the "standard" System V xargs, this one doesn't consider a line
  *  containing whitespace as more than one argument.  (I consider this a bug
  *  in the SV xargs; the absence of a flag to fix it is definitely a
  *  misfeature.)
  */
 
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