# open - Open a file

## SYNOPSIS

open fileName ?access? ?permissions?

## DESCRIPTION

This command opens a file and returns an identifier that may be used in future invocations of commands like read, puts, and close. FileName gives the name of the file to open; if it starts with a tilde then tilde substitution is performed as described for Tcl_TildeSubst. If the first character of fileName is ``|'' then the remaining characters of fileName are treated as a command pipeline to invoke, in the same style as for exec. In this case, the identifier returned by open may be used to write to the command's input pipe or read from its output pipe.

The access argument indicates the way in which the file (or command pipeline) is to be accessed. It may take two forms, either a string in the form that would be passed to the fopen library procedure or a list of POSIX access flags. It defaults to ``r''. In the first form access may have any of the following values:

r
  Open the file for reading only; the file must already exist.

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, create 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 positioned 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 controlling terminal of the process.

NONBLOCK
  Prevents the process from blocking while opening the file. 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, permissions (an integer) is used to set the permissions for the new file in conjunction with the process's file mode creation mask. Permissions defaults to 0666.

If a file is opened for both reading and writing then seek must be invoked between a read and a write, or vice versa (this restriction does not apply to command pipelines opened with open). When fileName specifies a command pipeline and a write-only access is used, then standard output from the pipeline is directed to the current standard output unless overridden by the command. When fileName specifies a command pipeline and a read-only access is used, then standard input from the pipeline is taken from the current standard input unless overridden by the command.

## KEYWORDS

access mode, append, controlling terminal, create, file, non-blocking, open, permissions, pipeline, process