LAUNCH EXTERNAL PROCESS
LAUNCH EXTERNAL PROCESS ( fileName {; inputStream {; outputStream {; errorStream}}}{; pid} )
| Parameter | Type | Description | |
|---|---|---|---|
| fileName | String | 🡒 | File path and arguments of file to launch |
| inputStream | String, BLOB | 🡒 | Input stream (stdin) |
| outputStream | String, BLOB | 🡘 | Output stream (stdout) |
| errorStream | String, BLOB | 🡘 | Error stream (stderr) |
| pid | Longint | 🡘 | Unique identifier for external process |
Compatibility
Starting with 4D v19 R4, we recommend the use of the SystemWorker class to run and control external processes. However, this command is still supported.
Description
The LAUNCH EXTERNAL PROCESS command launches an external process from 4D under macOS and Windows.
Under macOS, this command provides access to any executable application that can be launched from the Terminal.
Pass the fixed file path of the application to execute, as well as any required arguments (if necessary), in the fileName parameter.
Under macOS, you can also pass the application name only; 4D will then use the PATH environment variable to locate the executable.
Warning: This command can only launch executable applications; it cannot execute instructions that are part of the shell (command interpreter). For example, under macOS it is not possible to use this command to execute the echo instruction or indirections.
The inputStream parameter (optional) contains the stdin of the external process. Once the command has been executed, the outputStream and errorStream parameters (if passed) return respectively the stdout and stderr of the external process. You can use BLOB parameters instead of strings if you are working with binary data (such as pictures).
4D provides three specific environment variables that can be set using SET ENVIRONMENT VARIABLE and are available for use in the context of LAUNCH EXTERNAL PROCESS:
- _4D_OPTION_CURRENT_DIRECTORY: Used to set the current directory of the external process to be launched. In varValue, you must pass the pathname of the directory (HFS type syntax on macOS and DOS on Windows).
- _4D_OPTION_HIDE_CONSOLE (Windows only): Used to hide the window of the DOS console. You must pass "true" in varValue to hide the console or "false" to display it.
- _4D_OPTION_BLOCKING_EXTERNAL_PROCESS: Used to execute the external process in asynchronous mode, in other words, non-blocking for other applications. You must pass "false" in varValue to set an asynchronous execution or "true" to set a synchronous execution (default). For this variable, passing "" in varValue does nothing.
If you set the _4D_OPTION_BLOCKING_EXTERNAL_PROCESS environment variable to "false" via the SET ENVIRONMENT VARIABLE command (asynchronous execution), the outputStream and errorStream parameters are not returned.
These variables are valid in the current process for the next call to LAUNCH EXTERNAL PROCESS.
When passed, the pid parameter (longint) returns the system level ID for the process created to launch the command, regardless of the _4D_OPTION_BLOCKING_EXTERNAL_PROCESS option status. With this information, it is easier to interact with a created external process thereafter, e.g. to stop it. If the process launch fails, the pid parameter is not returned.
Examples under macOS
The following examples use the macOS Terminal available in the Application/Utilities folder.
1. To change permissions for a file (chmod is the macOS command used to modify file access):
LAUNCH EXTERNAL PROCESS("chmod +x /folder/myfile.txt")
2. To edit a text file (cat is the macOS command used to edit files). In this example, the full access path of the command is passed:
var input;output : Text
input:=""
LAUNCH EXTERNAL PROCESS("/bin/cat /folder/myfile.txt";input;output)
3. To get the contents of the "Users" folder (ls -l is the macOS equivalent of the dir command in DOS):
var $In;$Out : Text
LAUNCH EXTERNAL PROCESS("/bin/ls -l /Users";$In;$Out)
4. To launch an independent "graphic" application, it is preferable to use the open system command (in this case, the LAUNCH EXTERNAL PROCESS statement has the same effect as double-clicking the application):
LAUNCH EXTERNAL PROCESS("open /Applications/Calculator.app")
Examples under Windows
5. To open NotePad:
LAUNCH EXTERNAL PROCESS("C:\\WINDOWS\\notepad.exe")
6. To open Notepad and open a specific document:
LAUNCH EXTERNAL PROCESS("C:\\WINDOWS\\notepad.exe C:\\Docs\\new folder\\res.txt")
7. To launch the Microsoft® Word® application and open a specific document (note the use of the two ""):
$mydoc:="C:\\Program Files\\Microsoft Office\\Office10\\WINWORD.EXE \"C:\\Documents and
Settings\\Mark\\Desktop\\MyDocs\\New folder\\test.xml\""
LAUNCH EXTERNAL PROCESS($mydoc;$tIn;$tOut)
8. To execute a Perl script (requires ActivePerl):
var $input;$output : Text
SET ENVIRONMENT VARIABLE("myvariable";"value")
LAUNCH EXTERNAL PROCESS("D:\\Perl\\bin\\perl.exe D:\\Perl\\eg\\cgi\\env.pl";$input;$output)
9. To launch a command with the current directory and without displaying the console:
SET ENVIRONMENT VARIABLE("_4D_OPTION_CURRENT_DIRECTORY";"C:\\4D_VCS")
SET ENVIRONMENT VARIABLE("_4D_OPTION_HIDE_CONSOLE";"true")
LAUNCH EXTERNAL PROCESS("mycommand")
10. To allow the user to open an external document on Windows:
$docname:=Select document("";"*.*";"Choose the file to open";0)
If(OK=1)
SET ENVIRONMENT VARIABLE("_4D_OPTION_HIDE_CONSOLE";"true")
LAUNCH EXTERNAL PROCESS("cmd.exe /C start \"\" \""+$docname+"\"")
End if
11. The following examples request the process list on Windows:
var $pid : Integer
var $stdin;$stdout;$stderr : Text
LAUNCH EXTERNAL PROCESS("tasklist";$pid) //gets PID only
LAUNCH EXTERNAL PROCESS("tasklist";$stdin;$stdout;$stderr;$pid) //gets all information
System variables and sets
If the command has been executed correctly, the system variable OK is set to 1. Otherwise (file not found, insufficient memory, etc.), it is set to 0.