Tcl中利用open函数创建pipe

Running other programs from Tcl - exec, open

[from: http://www.tcl.tk/man/tcl/tutorial/Tcl26.html]

So far the lessons have dealt with programming within the Tcl interpreter. However, Tcl is also useful as a scripting language to tie other packages or programs together. To accomplish this function, Tcl has two ways to start another program:

• open...... run a new program with I/O connected to a file descriptor
• exec...... run a new program as a subprocess

Theopencall is the same call that is used to open a file. If the first character in the file name argument is a "pipe" symbol (|), thenopenwill treat the rest of the argument as a program name, and will run that program with thestandard inputoroutputconnected to a file descriptor. This "pipe" connection can be used to read the output from that other program or to write fresh input data to it or both.

If the "pipe" is opened for both reading and writing you must be aware that the pipes are buffered. The output from aputscommand will be saved in an I/O buffer until the buffer is full, or until you execute aflushcommand to force it to be transmitted to the other program. The output of this other program will not be available to areadorgetsuntil its output buffer is filled up or flushed explicitly.

(Note:as this isinternalto this other program, there is no way that your Tcl script can influence that. The other program simply must cooperate. Well, that is not entirely true: theexpectextension actually works around this limitation by exploiting deep system features.)

Theexeccall is similar to invoking a program (or a set of programs piped together) from the prompt in an interactive shell or a DOS-box or in a UNIX/Linux shell script. It supports several styles of output redirection, or it can return the output of the other program(s) as the return value of theexeccall.

open | progName ?access?

Returns a file descriptor for the pipe. The progNameargument must start with the pipe symbol. If progNameis enclosed in quotes or braces, it can include arguments to the subprocess.

exec ?switches? arg1 ?arg2?... ?argN?

exectreats its arguments as the names and arguments for a set of programs to run. If the first argsstart with a "-", then they are treated as switchesto the execcommand, instead of being invoked as subprocesses or subprocess options.

switchesare:

-keepnewline

Retains a trailing newline in the pipeline's output. Normally a trailing newline will be deleted.

--

Marks the end of the switches. The next string will be treated as arg1, even if it starts with a " -"

arg1... argNcan be one of:

• the name of a program to execute
• a command line argument for the subprocess
• an I/O redirection instruction.
• an instruction to put the new program in the background:

  exec myprog &
  

  will start the programmyprogin the background, and return immediately. There is no connection between that program and the Tcl script, both can run on independently.
  The & must be the last argument - you can use all other types of arguments in front of it.

  [NOTE:add information on how to wait for the program to finish?]

There are many I/O redirection commands. The main subset of these commands is:

|

Pipes the standard outputof the command preceding the pipe symbol into the standard inputof the command following the pipe symbol.

< fileName

The first program in the pipe will read input from fileName.

<@ fileID

The first program in the pipe will read input from the Tcl descriptor fileID. fileIDis the value returned from an open ... "r"command.

<< value

The first program in the pipe will read valueas its input.

> fileName

The output of the last program in the pipe will be sent to fileName. Any previous contents of fileNamewill be lost.

>> fileName

The output of the last program in the pipe will be appended to fileName.

2> fileName

The standard errorfrom all the programs in the pipe will be sent to fileName. Any previous contents of fileNamewill be lost.

2>> fileName

The standard errorfrom all the programs in the pipe will be appended to fileName.

>@ fileID

The output from the last program in the pipe will be written to fileID. fileIDis the value returned from an open ... "w"command.

If you are familiar with shell programming, there are a few differences to be aware of when you are writing Tcl scripts that use theexecandopencalls.

• You don't need the quotes that you would put around arguments to escape them from the shell expanding them. In the example, the argument to thesedcommand is not put in quotes. If it were put in quotes, the quotes would be passed tosed, instead of being stripped off (as the shell does), andsedwould report an error.
• If you use theopen|cmd"r+"construct, you must follow each puts with a flush to force Tcl to send the command from its buffer to the program. The output from the program itself may be buffered in its output buffer.

  You can sometimes force the output from the external program to flush by sending anexitcommand to the process.

  You can also use thefconfigurecommand to make a connection (channel) unbuffered.

  As already mentioned,expectextension to Tcl provides a much better interface to other programs, which in particular handles the buffering problem.
  [NOTE:add good reference to expect]

• If one of the commands in anopen|cmdfails theopendoes not return an error. However, attempting to read input from the file descriptor withgets$filewill return an empty string. Using thegets$fileinputconstruct will return a character count of -1.
• Tcldoes not expand file nameslike the UNIX/Linux shells do. So:

  exec ls *.tcl
  

  will fail - there is most probably no file with the literal name "*.tcl".

  If you need such an expansion, you should use theglobcommand:

  eval exec ls [glob *.tcl]
  

  or, from Tcl 8.5 onwards:

  exec ls {expand}[glob *.tcl]
  

  where the{expand}prefix is used to force the list to become individual arguments.
• If one of the commands in anexeccall fails to execute, theexecwill return an error, and the error output will include the last line describing the error.

Theexectreats any output tostandard errorto be an indication that the external program failed. This is simply a conservative assumption: many programs behave that way and they are sloppy in setting return codes.

Some programs however write tostandard errorwithout intending this as an indication of an error. You can guard against this from upsetting your script by using thecatchcommand:

if { [catch { exec ls *.tcl } msg] } {
   puts "Something seems to have gone wrong but we will ignore it"
}

To inspect the return code from a program and the possible reason for failure, you can use the globalerrorInfovariable:

if { [catch { exec ls *.tcl } msg] } {
   puts "Something seems to have gone wrong:"
   puts "Information about it: $::errorInfo"
}

---

Example

#
# Write a Tcl script to get a platform-independent program:
#
# Create a unique (mostly) file name for a Tcl program
set TMPDIR "/tmp"
if { [info exists ::env(TMP)] } {
    set TMPDIR $::env(TMP)
}
set tempFileName "$TMPDIR/invert_[pid].tcl"

# Open the output file, and
# write the program to it

set outfl [open $tempFileName w]

puts $outfl {
    set len [gets stdin line]
    if {$len < 5} {exit -1}

    for {set i [expr {$len-1}]} {$i >= 0} {incr i -1} {
        append l2 [string range $line $i $i]
    }
    puts $l2
    exit 0
}

# Flush and close the file
flush $outfl
close $outfl

#
# Run the new Tcl script:
#
# Open a pipe to the program (for both reading and writing: r+)
#
set io [open "|[info nameofexecutable] $tempFileName" r+]

#
# send a string to the new program
#     *MUST FLUSH*
puts $io "This will come back backwards."
flush $io

# Get the reply, and display it.
set len [gets $io line]

puts  "To reverse: 'This will come back backwards.'"
puts "Reversed is: $line"
puts "The line is $len characters long"

# Run the program with input defined in an exec call

set invert [exec [info nameofexecutable] $tempFileName << /
       "ABLE WAS I ERE I SAW ELBA"]

# display the results
puts "The inversion of 'ABLE WAS I ERE I SAW ELBA' is /n $invert"

# Clean up
file delete $tempFileName

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