SCRIPT(7)					       BSD Miscellaneous Information Manual						 SCRIPT(7)

NAME

     script -- interpreter script execution

DESCRIPTION

     The system is capable of treating a text file containing commands intended for an interpreter, such as sh(1) or awk(1), as an executable pro-
     gram.

     An ``interpreter script'' is a file which has been set executable (see chmod(2)) and which has a first line of the form:

	   #! pathname [argument]

     The ``#!'' must appear as the first two characters of the file.  A space between the ``#!'' and pathname is optional.  At most one argument
     may follow pathname, and the length of the entire line is limited (see below).

     If such a file is executed (such as via the execve(2) system call), the interpreter specified by the pathname is executed by the system.
     (The pathname is executed without regard to the PATH variable, so in general pathname should be an absolute path.)

     The arguments passed to the interpreter will be as follows.  argv[0] will be the path to the interpreter itself, as specified on the first
     line of the script.  If there is an argument following pathname on the first line of the script, it will be passed as argv[1].  The subse-
     quent elements of argv will be the path to the interpreter script file itself (i.e. the original argv[0]) followed by any further arguments
     passed when execve(2) was invoked to execute the script file.

     By convention, it is expected that an interpreter will open the script file passed as an argument and process the commands within it.  Typi-
     cal interpreters treat '#' as a comment character, and thus will ignore the initial line of the script because it begins ``#!'', but there is
     no requirement for this per se.

     On NetBSD, the length of the ``#!'' line, excluding the ``#!'' itself, is limited to PATH_MAX (as defined in <limits.h>).	Other operating
     systems impose much smaller limits on the length of the ``#!'' line (see below).

     Note that the interpreter may not itself be an interpreter script.  If pathname does not point to an executable binary, execution of the
     interpreter script will fail.

   Trampolines and Portable Scripts
     Different operating systems often have interpreters located in different locations, and the kernel executes the passed interpreter without
     regard to the setting of environment variables such as PATH.  This makes it somewhat challenging to set the ``#!'' line of a script so that
     it will run identically on different systems.

     Since the env(1) utility executes a command passed to it on its command line, it is often used as a ``trampoline'' to render scripts porta-
     ble.  If the leading line of a script reads
	   #! /usr/bin/env interp
     then the env(1) command will execute the ``interp'' command it finds in its PATH, passing on to it all subsequent arguments with which it
     itself was called.  Since /usr/bin/env is found on almost all POSIX style systems, this trick is frequently exploited by authors who need a
     script to execute without change on multiple systems.

   Historical Note: Scripts without ``#!''
     Shell scripts predate the invention of the ``#!'' convention, which is implemented in the kernel.	In the days of Version 7 AT&T UNIX, there
     was only one interpreter used on the system, /bin/sh, and the shell treated any file that failed to execute with an ENOEXEC error (see
     intro(2)) as a shell script.

     Most shells (such as sh(1)) and certain other facilities (including execlp(3) and execvp(3) but not other types of exec(3) calls) still pass
     interpreter scripts that do not include the ``#!'' (and thus fail to execute with ENOEXEC) to /bin/sh.

     As this behavior is implemented outside the kernel, there is no mechanism that forces it to be respected by all programs that execute other
     programs.	It is thus not completely reliable.  It is therefore important to always include
	   #!/bin/sh
     in front of Bourne shell scripts, and to treat the traditional behavior as obsolete.

EXAMPLES

     Suppose that an executable binary exists in /bin/interp and that the file /tmp/script contains:

	   #!/bin/interp -arg

	   [...]

     and that /tmp/script is set mode 755.

     Executing

	   $ /tmp/script one two three

     at the shell will result in /bin/interp being executed, receiving the following arguments in argv (numbered from 0):

	   "/bin/interp", "-arg", "/tmp/script", "one", "two", "three"

   Portability Note: Multiple arguments
     The behavior of multiple arguments on the ``#!'' line is highly non-portable between different systems.  In general, only one argument can be
     assumed to work consistently.

     Consider the following variation on the previous example.	Suppose that an executable binary exists in /bin/interp and that the file
     /tmp/script contains:

	   #!/bin/interp -x -y

	   [...]

     and that /tmp/script is set mode 755.

     Executing

	   $ /tmp/script one two three

     at the shell will result in /bin/interp being executed, receiving the following arguments in argv (numbered from 0):

	   "/bin/interp", "-x -y", "/tmp/script", "one", "two", "three"

     Note that "-x -y" will be passed on NetBSD as a single argument.

     Although most POSIX style operating systems will pass only one argument, the behavior when multiple arguments are included is not consistent
     between platforms.  Some, such as current releases of NetBSD, will concatenate multiple arguments into a single argument (as above), some
     will truncate them, and at least one will pass them as multiple arguments.

     The NetBSD behavior is common but not universal.  Sun's Solaris would present the above argument as "-x", dropping the " -y" entirely.  Per-
     haps uniquely, recent versions of Apple's OS X will actually pass multiple arguments properly, i.e.:

	   "/bin/interp", "-x", "-y", "/tmp/script", "one", "two", "three"

     The behavior of the system in the face of multiple arguments is thus not currently standardized, should not be relied on, and may be changed
     in future releases.  In general, pass at most one argument, and do not rely on multiple arguments being concatenated.

SEE ALSO

     awk(1), csh(1), ksh(1), sh(1), chmod(2), execve(2), intro(2), execlp(3), execvp(3), fd(4), options(4), setuid(7)

STANDARDS

     The behavior of interpreter scripts is obliquely referred to, but never actually described in, IEEE Std 1003.1-2004 (``POSIX.1'').

     The behavior is partially (but not completely) described in the System V Interface Definition, Fourth Edition (``SVID4'').

     Although it has never been formally standardized, the behavior described is largely portable across POSIX style systems, with two significant
     exceptions: the maximum length of the ``#!'' line, and the behavior if multiple arguments are passed.  Please be aware that some operating
     systems limit the line to 32 or 64 characters, and that (as described above) the behavior in the face of multiple arguments is not consistent
     across systems.

HISTORY

     The behavior of the kernel when encountering scripts that start in ``#!'' was not present in Version 7 AT&T UNIX.	A Usenet posting to
     net.unix by Guy Harris on October 16, 1984 claims that the idea for the ``#!'' behavior was first proposed by Dennis Ritchie but that the
     first implementation was on BSD.

     Historical manuals (specifically the exec man page) indicate that the behavior was present in 4BSD at least as early as April, 1981.  Infor-
     mation on precisely when it was first implemented, and in which version of UNIX, is solicited.

SECURITY CONSIDERATIONS

     Numerous security problems are associated with setuid interpreter scripts.

     In addition to the fact that many interpreters (and scripts) are simply not designed to be robust in a setuid context, a race condition
     exists between the moment that the kernel examines the interpreter script file and the moment that the newly invoked interpreter opens the
     file itself.

     Because of these security issues, NetBSD does not allow setuid interpreter scripts by default.  In order to turn on setuid interpreter
     scripts,
	   options SETUIDSCRIPTS
     must be set in the configuration of the running kernel.  Setting this option implies the FDSCRIPTS option, which causes the kernel to open
     the script file on behalf of the interpreter and pass it in argv as /dev/fd/[fdnum].  (See fd(4) for an explanation of the /dev/fd/[fdnum]
     devices.)	This design avoids the race condition, at the cost of denying the interpreter the actual name of the script file.  See options(4)
     for more information.

     However, the FDSCRIPTS mechanism is not a cure-all for security issues in setuid interpreters and scripts.  Subtle techniques can be used to
     subvert even seemingly well written scripts.  Scripts executed by Bourne type shells can be subverted in numerous ways, such as by setting
     the IFS variable before executing the script.  Other interpreters possess their own vulnerabilities.  Turning on SETUIDSCRIPTS is therefore
     very dangerous, and should not be done lightly if at all.

BSD
								    May 6, 2005 							       BSD