2014-10-14



How does the shell search for some command to be executed?  Where
does it search if I enter:

     foo/bar a b

Would it search for a directory `foo', containing a  file  `bar',
inside of each of the directories in $PATH?

To find answers to such questions, it's worth to have a  look  at
POSIX.  [0]

     Command Search and Execution

     If a simple command results in a command name and an optional
     list of arguments, the following actions shall be performed:

     1. If the command name does not contain any <slash>
        characters, the first successful step in the following
        sequence shall occur:

         a) If the command name matches the name of a special
            built-in utility, that special built-in utility
            shall be invoked.

         b) If the command name matches the name of a function
            known to this shell, the function shall be invoked
            as described in Function Definition Command. [...]

         c) If the command name matches the name of a utility
            listed in the following table, that utility shall
            be invoked. [...] alias, bg, cd, [...]

         d) Otherwise, the command shall be searched for using
            the PATH environment variable as described in XBD
            Environment Variables: [...]

     2. If the command name contains at least one <slash>, the
        shell shall execute the utility in a separate utility
        environment with actions equivalent to calling the execl()
        function defined in the System Interfaces volume of
        POSIX.1-2008 with the path and arg0 arguments set to the
        command name, and the remaining execl() arguments set to
        the command arguments (if any) and the null terminator.

        If the execl() function fails due to an error equivalent
        to the [ENOEXEC] error, the shell shall execute a command
        equivalent to having a shell invoked with the command name
        as its first operand, with any remaining arguments passed
        to the new shell.  If the executable file is not a text
        file, the shell may bypass this command execution. In this
        case, it shall write an error message and shall return an
        exit status of 126.

        [...]
     [1]

Seems as if there someone should start to announce POSIX snippets
of the week. ;-)

[0]  http://pubs.opengroup.org/onlinepubs/9699919799/
[1]  http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_09_01


http://marmaro.de/lue/        markus schnalke <meillo@marmaro.de>