NAME
       open - Open a file-based or command pipeline channel

SYNOPSIS
       open fileName
       open fileName access
       open fileName access permissions


DESCRIPTION
       This  command opens a file or command pipeline and returns
       a channel identifier that may be used  in  future  invoca-
       tions  of  commands  like  read,  puts, and close.  If the
       first character of fileName is  not  |  then  the  command
       opens a file: fileName gives the name of the file to open,
       and it must conform to the conventions  described  in  the
       filename manual entry.

       The  access  argument,  if  present,  indicates the way in
       which the file (or command pipeline) is  to  be  accessed.
       In  the  first  form  access may have any of the following
       values:

       r              Open the file for reading  only;  the  file
                      must  already  exist.  This  is the default
                      value if access is not specified.

       r+             Open the file for both reading and writing;
                      the file must already exist.

       w              Open  the  file for writing only.  Truncate
                      it if it exists.  If it doesn't exist, cre-
                      ate a new file.

       w+             Open  the  file  for  reading  and writing.
                      Truncate it if it exists.   If  it  doesn't
                      exist, create a new file.

       a              Open  the  file for writing only.  The file
                      must already exist, and the file  is  posi-
                      tioned  so that new data is appended to the
                      file.

       a+             Open the file for reading and writing.   If
                      the  file doesn't exist, create a new empty
                      file.  Set the initial access position   to
                      the end of the file.

       In  the  second  form, access consists of a list of any of
       the following flags, all of which have the standard  POSIX
       meanings.   One of the flags must be either RDONLY, WRONLY
       or RDWR.

       RDONLY         Open the file for reading only.
       WRONLY         Open the file for writing only.

       RDWR           Open the file for both reading and writing.

       APPEND         Set the file pointer to the end of the file
                      prior to each write.

       CREAT          Create the file if it doesn't already exist
                      (without  this  flag it is an error for the
                      file not to exist).

       EXCL           If CREAT is also  specified,  an  error  is
                      returned if the file already exists.

       NOCTTY         If the file is a terminal device, this flag
                      prevents the file from  becoming  the  con-
                      trolling terminal of the process.

       NONBLOCK       Prevents  the  process  from blocking while
                      opening the file, and  possibly  in  subse-
                      quent  I/O  operations.  The exact behavior
                      of  this  flag  is  system-   and   device-
                      dependent;   its  use is discouraged (it is
                      better to use the fconfigure command to put
                      a  file  in nonblocking mode).  For details
                      refer to your system documentation  on  the
                      open system call's O_NONBLOCK flag.

       TRUNC          If  the file exists it is truncated to zero
                      length.

       If a new file is created as part of  opening  it,  permis-
       sions  (an integer) is used to set the permissions for the
       new file in conjunction with the process's file mode  cre-
       ation mask.  Permissions defaults to 0666.

COMMAND PIPELINES
       If  the  first  character  of  fileName  is ``|'' then the
       remaining characters of fileName are treated as a list  of
       arguments  that  describe a command pipeline to invoke, in
       the same style as the arguments for exec.  In  this  case,
       the  channel  identifier  returned  by open may be used to
       write to the command's input pipe or read from its  output
       pipe,  depending  on  the  value of access.  If write-only
       access is used (e.g. access is w),  then  standard  output
       for  the pipeline is directed to the current standard out-
       put unless overridden by the command.  If read-only access
       is  used  (e.g.  access  is  r),  standard  input  for the
       pipeline is taken from the current standard  input  unless
       overridden by the command.


SEE ALSO
       close(n), filename(n), gets(n), read(n), puts(n)
KEYWORDS
       access  mode,  append,  create,  file, non-blocking, open,
       permissions, pipeline, process
