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Running external processes in Tcl and Tcl/Tk

If you want to run external processes from a Tcl based program (Tcl, Tcl/Tk, expect), there are various ways of doing it.

Firstly, the whole purpose of the Expect extension is to allow you to control other processes via its three major commands of spawn which starts another process, send which sends characters to that other process, and expect which causes your program to wait until it gets a particular response, or one of a series of possible responses.

If you don't need to go to these lengths, though, there are three other options at least.

You can use an exec command to run an external process and capture the result returned in a variable:

set ff [exec ls]
puts $ff


You can use the open command and open a process by preceeding it with a "pipe" character in place of its default behaviour which would be to open a file. This lets you handle a process that generates a lot of output line by line, and by using r+, w+ modes and flush you can also set it up for both read and write. Here's a simple example of opening a piped process for read:

set fh [open |df r]
while {[gets $fh line] >= 0} {
puts $line
}


Finally, you can talk to a remote process via the network, using socket to open a remote connection. Like the open command, the socket command opens a channel so once the channel is set up you can handle the data stream exactly as if it was a file or pipe. Again, don't forget flush. Example (getting a file off a web server):

# Through a socket - a remote process

set fh [socket 192.168.200.66 80]
puts $fh "GET /index.html HTTP/1.0\n"
flush $fh
while {[gets $fh line] >= 0} {
puts $line
}

(written 2006-06-29, updated 2006-06-30)

 
Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
T209 - Tcl/Tk - File and Directory Handling
  [4524] Tcl - a new example for data reformatting - (2015-10-10)
  [4523] Catching failed commands and not crashing the program in Tcl - (2015-10-10)
  [4461] Reading from a URL, and reading Json, from your Tcl script - (2015-03-12)
  [3617] The fileutil package and a list of file system commands in Tcl - (2012-02-18)
  [3429] Searching through all the files in or below a directory - Ruby, Tcl, Perl - (2011-09-09)
  [3320] Reading the nth line from a file (Perl and Tcl examples) - (2011-06-09)
  [3192] Tcl - Some example of HOW TO in handling data files and formats - (2011-03-04)
  [2467] Tcl - catching an error before your program crashes - (2009-10-22)
  [1467] stdout v stderr (Tcl, Perl, Shell) - (2007-12-10)
  [1426] Buffering up in Tcl - the empty coke can comparison - (2007-11-10)
  [1407] Reading from another process in Tcl (pipes and sockets) - (2007-10-26)
  [779] The fragility of pancakes - and better structures - (2006-06-26)

T244 - Tcl/Tk - Socket Programming
  [4462] Server program written in Tcl using sockets - (2015-03-13)


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