OPEN(5) File Formats Manual OPEN(5)
NAME
open, create - prepare a fid for I/O on an existing or new file
SYNOPSIS
Topen tag[2] fid[2] mode[1]
Ropen tag[2] fid[2] qid[8]
Tcreate tag[2] fid[2] name[28] perm[4] mode[1]
Rcreate tag[2] fid[2] qid[8]
DESCRIPTION
The open request asks the file server to check permissions and prepare a fid for I/O with subsequent read and write messages. The mode
field determines the type of I/O: 0, 1, 2, and 3 mean read access, write access, read and write access, and execute access, to be checked
against the permissions for the file. In addition, if mode has the OTRUNC (0x10) bit set, the file is to be truncated, which requires
write permission (if the file is append-only, and permission is granted, the open succeeds but the file will not be truncated); if the mode
has the ORCLOSE (0x40) bit set, the file is to be removed when the fid is clunked, which requires permission to remove the file from its
directory. If other bits are set in mode they will be ignored. It is illegal to write a directory, truncate it, or attempt to remove it
on close. If the file is marked for exclusive use (see stat(5)), only one client can have the file open at any time. That is, after such
a file has been opened, no other open will succeed until fid has been clunked. All these permissions are checked at the time of the open
request; subsequent changes to the permissions of files do not affect the ability to read, write, or remove an open file.
The create request asks the file server to create a new file with the name supplied, in the directory (dir) represented by fid, and
requires write permission in the directory. The owner of the file is the implied user id of the request, the group of the file is the same
as dir, and the permissions are the value of
(perm&(~0777|0111)) | (dir.perm&perm&0666)
if a regular file is being created and
(perm&~0777) | (dir.perm&perm&0777)
if a directory is being created. This means, for example, that if the create allows read permission to others, but the containing direc-
tory does not, then the created file will not allow others to read the file.
Finally, the newly created file is opened according to mode, and fid will represent the newly opened file. Mode is not checked against the
permissions in perm. The qid for the new file is returned with the create reply message.
Directories are created by setting the CHDIR bit (0x80000000) in the perm.
The names . and .. are special; it is illegal to create files with these names.
It is an error for either of these messages if the fid is already the product of a successful open or create message.
An attempt to create a file in a directory where the given name already exists will be rejected; in this case, the create system call (see
open(2)) uses open with truncation. The algorithm used by create is: first walk to the directory to contain the file. If that fails,
return an error. Next walk to the specified file. If the walk succeeds, send a request to open and truncate the file and return the
result, successful or not. If the walk fails, send a create message. If that fails, it may be because the file was created by another
process after the previous walk failed, so (once) try the walk and open again. For the behavior of create on a union directory, see
bind(2).
ENTRY POINTS
Open and create both generate open messages; only create generates a create message.
OPEN(5)
Check Out this Related Man Page
CHMOD(1) BSD General Commands Manual CHMOD(1)
NAME
chmod -- change file modes
SYNOPSIS
chmod [-fhv] [-R [-H | -L | -P]] mode file ...
DESCRIPTION
The chmod utility modifies the file mode bits of the listed files as specified by the mode operand.
The options are as follows:
-H If the -R option is specified, symbolic links on the command line are followed. (Symbolic links encountered in the tree traversal
are not followed by default.)
-L If the -R option is specified, all symbolic links are followed.
-P If the -R option is specified, no symbolic links are followed. This is the default.
-R Change the modes of the file hierarchies rooted in the files instead of just the files themselves.
-f Do not display a diagnostic message if chmod could not modify the mode for file.
-h If the file is a symbolic link, change the mode of the link itself rather than the file that the link points to.
-v Cause chmod to be verbose, showing filenames as the mode is modified. If the -v flag is specified more than once, the old and new
modes of the file will also be printed, in both octal and symbolic notation.
The -H, -L and -P options are ignored unless the -R option is specified. In addition, these options override each other and the command's
actions are determined by the last one specified.
Only the owner of a file or the super-user is permitted to change the mode of a file.
DIAGNOSTICS
The chmod utility exits 0 on success, and >0 if an error occurs.
MODES
Modes may be absolute or symbolic. An absolute mode is an octal number constructed from the sum of one or more of the following values:
4000 (the set-user-ID-on-execution bit) Executable files with this bit set will run with effective uid set to the uid of the file
owner. Directories with the set-user-id bit set will force all files and sub-directories created in them to be owned by the
directory owner and not by the uid of the creating process, if the underlying file system supports this feature: see chmod(2)
and the suiddir option to mount(8).
2000 (the set-group-ID-on-execution bit) Executable files with this bit set will run with effective gid set to the gid of the file
owner.
1000 (the sticky bit) See chmod(2) and sticky(8).
0400 Allow read by owner.
0200 Allow write by owner.
0100 For files, allow execution by owner. For directories, allow the owner to search in the directory.
0040 Allow read by group members.
0020 Allow write by group members.
0010 For files, allow execution by group members. For directories, allow group members to search in the directory.
0004 Allow read by others.
0002 Allow write by others.
0001 For files, allow execution by others. For directories allow others to search in the directory.
For example, the absolute mode that permits read, write and execute by the owner, read and execute by group members, read and execute by oth-
ers, and no set-uid or set-gid behaviour is 755 (400+200+100+040+010+004+001).
The symbolic mode is described by the following grammar:
mode ::= clause [, clause ...]
clause ::= [who ...] [action ...] action
action ::= op [perm ...]
who ::= a | u | g | o
op ::= + | - | =
perm ::= r | s | t | w | x | X | u | g | o
The who symbols ``u'', ``g'', and ``o'' specify the user, group, and other parts of the mode bits, respectively. The who symbol ``a'' is
equivalent to ``ugo''.
The perm symbols represent the portions of the mode bits as follows:
r The read bits.
s The set-user-ID-on-execution and set-group-ID-on-execution bits.
t The sticky bit.
w The write bits.
x The execute/search bits.
X The execute/search bits if the file is a directory or any of the execute/search bits are set in the original (unmodified) mode.
Operations with the perm symbol ``X'' are only meaningful in conjunction with the op symbol ``+'', and are ignored in all other
cases.
u The user permission bits in the original mode of the file.
g The group permission bits in the original mode of the file.
o The other permission bits in the original mode of the file.
The op symbols represent the operation performed, as follows:
+ If no value is supplied for perm, the ``+'' operation has no effect. If no value is supplied for who, each permission bit specified in
perm, for which the corresponding bit in the file mode creation mask is clear, is set. Otherwise, the mode bits represented by the
specified who and perm values are set.
- If no value is supplied for perm, the ``-'' operation has no effect. If no value is supplied for who, each permission bit specified in
perm, for which the corresponding bit in the file mode creation mask is clear, is cleared. Otherwise, the mode bits represented by the
specified who and perm values are cleared.
= The mode bits specified by the who value are cleared, or, if no who value is specified, the owner, group and other mode bits are
cleared. Then, if no value is supplied for who, each permission bit specified in perm, for which the corresponding bit in the file
mode creation mask is clear, is set. Otherwise, the mode bits represented by the specified who and perm values are set.
Each clause specifies one or more operations to be performed on the mode bits, and each operation is applied to the mode bits in the order
specified.
Operations upon the other permissions only (specified by the symbol ``o'' by itself), in combination with the perm symbols ``s'' or ``t'',
are ignored.
EXAMPLES
644 make a file readable by anyone and writable by the owner only.
go-w deny write permission to group and others.
=rw,+X set the read and write permissions to the usual defaults, but retain any execute permissions that are currently set.
+X make a directory or file searchable/executable by everyone if it is already searchable/executable by anyone.
755
u=rwx,go=rx
u=rwx,go=u-w make a file readable/executable by everyone and writable by the owner only.
go= clear all mode bits for group and others.
g=u-w set the group bits equal to the user bits, but clear the group write bit.
BUGS
There's no perm option for the naughty bits.
COMPATIBILITY
The -v option is non-standard and its use in scripts is not recommended.
SEE ALSO
chflags(1), install(1), chmod(2), stat(2), umask(2), fts(3), setmode(3), symlink(7), chown(8), mount(8), sticky(8)
STANDARDS
The chmod utility is expected to be IEEE Std 1003.2 (``POSIX.2'') compatible with the exception of the perm symbol ``t'' which is not
included in that standard.
HISTORY
A chmod command appeared in Version 1 AT&T UNIX.
BSD
March 31, 1994 BSD