SCRIPT(7) | Miscellaneous Information Manual | SCRIPT(7) |
script
—
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 subsequent 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. Typical 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.
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 portable. If the leading line of a script reads
#! /usr/bin/env interp
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.
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
#!/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):
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):
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. Perhaps uniquely, recent versions of Apple's OS X will actually pass multiple arguments properly, i.e.:
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.
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.
Historical manuals (specifically the exec man page) indicate that the behavior was present in 4BSD at least as early as April, 1981. Information on precisely when it was first implemented, and in which version of UNIX, is solicited.
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
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.
May 6, 2005 | NetBSD 9.4 |