exec - Invoke subprocess(es)
exec ?switches? arg ?arg ...?
This command treats its arguments as the specification
of one or more subprocesses to execute.
The arguments take the form of a standard shell pipeline
where each arg becomes one word of a command, and
each distinct command becomes a subprocess.
If the initial arguments to exec start with - then
they are treated as command-line switches and are not part
of the pipeline specification. The following switches are
currently supported:
- -keepnewline
-
Retains a trailing newline in the pipeline's output.
Normally a trailing newline will be deleted.
- --
-
Marks the end of switches. The argument following this one will
be treated as the first arg even if it starts with a -.
If an arg (or pair of arg's) has one of the forms
described below then it is used by exec to control the
flow of input and output among the subprocess(es).
Such arguments will not be passed to the subprocess(es). In forms
such as ``< fileName'' fileName may either be in a
separate argument from ``<'' or in the same argument with no
intervening space (i.e. ``<fileName'').
- |
-
Separates distinct commands in the pipeline. The standard output
of the preceding command will be piped into the standard input
of the next command.
- |&
-
Separates distinct commands in the pipeline. Both standard output
and standard error of the preceding command will be piped into
the standard input of the next command.
This form of redirection overrides forms such as 2> and >&.
- < fileName
-
The file named by fileName is opened and used as the standard
input for the first command in the pipeline.
- <@ fileId
-
FileId must be the identifier for an open file, such as the return
value from a previous call to open.
It is used as the standard input for the first command in the pipeline.
FileId must have been opened for reading.
- << value
-
Value is passed to the first command as its standard input.
- > fileName
-
Standard output from the last command is redirected to the file named
fileName, overwriting its previous contents.
- 2> fileName
-
Standard error from all commands in the pipeline is redirected to the
file named fileName, overwriting its previous contents.
- >& fileName
-
Both standard output from the last command and standard error from all
commands are redirected to the file named fileName, overwriting
its previous contents.
- >> fileName
-
Standard output from the last command is
redirected to the file named fileName, appending to it rather
than overwriting it.
- 2>> fileName
-
Standard error from all commands in the pipeline is
redirected to the file named fileName, appending to it rather
than overwriting it.
- >>& fileName
-
Both standard output from the last command and standard error from
all commands are redirected to the file named fileName,
appending to it rather than overwriting it.
- >@ fileId
-
FileId must be the identifier for an open file, such as the return
value from a previous call to open.
Standard output from the last command is redirected to fileId's
file, which must have been opened for writing.
- 2>@ fileId
-
FileId must be the identifier for an open file, such as the return
value from a previous call to open.
Standard error from all commands in the pipeline is
redirected to fileId's file.
The file must have been opened for writing.
- >&@ fileId
-
FileId must be the identifier for an open file, such as the return
value from a previous call to open.
Both standard output from the last command and standard error from
all commands are redirected to fileId's file.
The file must have been opened for writing.
If standard output has not been redirected then the exec
command returns the standard output from the last command
in the pipeline.
If any of the commands in the pipeline exit abnormally or
are killed or suspended, then exec will return an error
and the error message will include the pipeline's output followed by
error messages describing the abnormal terminations; the
errorCode variable will contain additional information
about the last abnormal termination encountered.
If any of the commands writes to its standard error file and that
standard error isn't redirected,
then exec will return an error; the error message
will include the pipeline's standard output, followed by messages
about abnormal terminations (if any), followed by the standard error
output.
If the last character of the result or error message
is a newline then that character is normally deleted
from the result or error message.
This is consistent with other Tcl return values, which don't
normally end with newlines.
However, if -keepnewline is specified then the trailing
newline is retained.
If standard input isn't redirected with ``<'' or ``<<''
or ``<@'' then the standard input for the first command in the
pipeline is taken from the application's current standard input.
If the last arg is ``&'' then the pipeline will be
executed in background.
In this case the exec command will return a list whose
elements are the process identifiers for all of the subprocesses
in the pipeline.
The standard output from the last command in the pipeline will
go to the application's standard output if it hasn't been
redirected, and error output from all of
the commands in the pipeline will go to the application's
standard error file unless redirected.
The first word in each command is taken as the command name;
tilde-substitution is performed on it, and if the result contains
no slashes then the directories
in the PATH environment variable are searched for
an executable by the given name.
If the name contains a slash then it must refer to an executable
reachable from the current directory.
No ``glob'' expansion or other shell-like substitutions
are performed on the arguments to commands.
execute, pipeline, redirection, subprocess
Copyright © 1993 The Regents of the University of California.
Copyright © 1994-1996 Sun Microsystems, Inc.
Copyright © 1995, 1996 Roger E. Critchlow Jr.