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Re: style of assignment to variable changes command selection

From: Bob Proulx
Subject: Re: style of assignment to variable changes command selection
Date: Fri, 7 Mar 2008 23:01:28 -0700
User-agent: Mutt/1.5.13 (2006-08-11)

Eric Blake wrote:
> Doug McIlroy wrote:
> | So there's a bug in the manual, which does not breathe a word about
> | time being executed by the shell.  And the shell covers its tracks, too:
> Like I said, there's a difference between a builtin (for example,
> 'builtin' or 'exec') and a reserved word (for example, 'time' or 'if').

In the bash manual I am reading I see:

       Reserved words are words that have a special meaning to the shell.  The
       following words are recognized as reserved when unquoted and either the
       first word of a simple command (see SHELL GRAMMAR below) or  the  third
       word of a case or for command:

       !  case  do done elif else esac fi for function if in select then until
       while { } time [[ ]]

       If  the  time reserved word precedes a pipeline, the elapsed as well as
       user and system time consumed by its execution are  reported  when  the
       pipeline  terminates.   The -p option changes the output format to that
       specified by POSIX.  The TIMEFORMAT variable may be  set  to  a  format
       string  that  specifies how the timing information should be displayed;
       see the description of TIMEFORMAT under Shell Variables below.

       A pipeline is a sequence of one or more commands separated by the char‐
       acter |.  The format for a pipeline is:

              [time [-p]] [ ! ] command [ | command2 ... ]

And a command is defined as:

   Simple Commands
       A simple command is a sequence of optional  variable  assignments  fol‐
       lowed  by  blank-separated  words and redirections, and terminated by a
       control operator.  The first word specifies the command to be executed,
       and  is  passed  as  argument  zero.  The remaining words are passed as
       arguments to the invoked command.

> >   % export X=x; time --version; unset X
> >   -bash: --version: command not found

Here time is the shell's version because it occurs at the start of the
pipeline, which in bash does not implement a --version option to it.

> >   % unset X; X=x time --version
> >   GNU time 1.7

Here the environment variable assignment matches a 'command' and
therefore 'time' is an external command.

Further, because for an individual command do not affect the current
environment this implies to me that the command must have its own
environment.  The manual says:

       If no command name results, the variable assignments affect the current
       shell  environment.  Otherwise, the variables are added to the environ‐
       ment of the executed command and do not affect the current shell  envi‐
       ronment.   If  any  of  the assignments attempts to assign a value to a
       readonly variable, an error occurs, and the command exits with  a  non-
       zero status.

This implies to me that a variable assignment forces subsequent
commands to be external commands because the current environment is
not affected.  In your case above the external /usr/bin/time program
is forced by the variable assignment requiring the environment
variable set for it.

> | Were I still the keeper of Unix manuals, my fix to the manual
> | would be to list this wart under BUGS.
> 'man bash' correctly lists time under the section RESERVED WORDS.  Also,
> 'help time' gives evidence that it is special to bash (as 'help' is the
> bash builtin that reports about both reserved words and builtins).

I will leave that decision up to the maintainer.  I could go either
way.  However there are so many edge cases that might trip up someone
that if every one of them were documented in the bugs section then
that section would be far longer than the rest of the manual.  In the
end many of these cases simply become judgement calls.


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