Doriano Blengino schrieb: > Rolf-Werner Eilert ha scritto: >> Could someone explain me why there are two different ways of executing >> shell commands and how they differ in practice? I mean, when do I want >> EXEC and when will I want SHELL? What's the idea behind them? >> >> Thanks for all hints :-) >> > SHELL invokes /bin/sh and passes it a single command line. /bin/sh > parses this command line exactly the same way you do on a normal shell. So, > > SHELL "ls -l *.o >/tmp/list" > > will do exactly the same as you typed "ls -l *.o >/tmp/list" in a > terminal emulator under a shell. This is a lot of things, because the > shell has a tremendous power. This command does: > > 1. split the command line in several parts: executable to run, > parameters to pass it, other constructs... > 2. search for an executable named ls in the PATH environment. > 3. substitute "*.o" with all the ".o" files in the current directory > 4. prepare a redirection (the normal output of /bin/ls is redirected > in /tmp/list) > > To make it short, the shell can do a lot of things, and the gambas SHELL > command brings that power to you. > After /bin/sh has done with all this parsing/computing work, it invokes > an exec() system call, which loads and executes an executable, passing > it a number of parameters. > > The gambas EXEC instruction calls the exec() system call, bypassing the > shell (/bin/sh). This is faster and less memory hungry, because you > invoke an external command without invoking /bin/sh, but you loose all > the power the shell has. In fact, if you want to list all the ".o" files > in the current directory and put the result in /tmp/list without using > the powerful shell, you have to: > > 1. search by yourself the files > 2. create an array of the names of those files > 3. invoke /bin/ls and pass it an array which contains the "-l" and > all the files > 4. redirect its standard output in a file > > To conclude. If you run an EXEC in gambas, you must simply supply the > program name to execute and all its parameter. If you issue: > > EXEC ["/bin/ls", "-l", "*.o", ">/tmp/list"] > > you will invoke /bin/ls passing it the above parameters. /bin/ls will > (correctly) recognize the "-l" as a switch; but "*.o" and ">/tmp/list" > will be recognized as files to look for, and no files named "*.o" will > exist. The ">/tmp/list" is a shell syntax, not a /bin/ls one, and > /bin/ls will look again to for file named ">/tmp/list". > > You can type "man sh" at the shell prompt; all of what you will read > there are shell capabilities, and none of them are available in EXEC. > The three most important things which are available in the shell, and > not available in EXEC are: > > 1. Pattern substitution. *.o and the like are shell construct. > 2. Redirections and pipes. ">/...", "</...", "2>&1 |some_command" > and so on. > 3. Variables like $HOME, $PATH and so on > > But exec has a good advantage over SHELL. If you have to invoke an > external command which has (or can have) unusual characters in the > command line, like "firefox > http://someserver.com/doit.cgi?name=foo&reply=bar";, SHELL (or, better, > /bin/sh) will interpret characters like "?" and "&", whilst EXEC will not. > > The reply to your answer is: if you need some shell capability, use > SHELL; otherwise use EXEC. Using SHELL saves typing, on the other hand, > if you are sure that no strange characters ("?", "&", "$", spaces, and > others) can appear in the command you are constructing. > > Hope this is a good start - regards, >
Oh yeah, thank you very much for that comprehensive explanation! In fact, I find your answer so well written that I want to propose to take it up into the Gambas wiki, linked between SHELL and EXEC or just as part of both texts. What do you think? Regards Rolf ------------------------------------------------------------------------------ Crystal Reports - New Free Runtime and 30 Day Trial Check out the new simplified licensing option that enables unlimited royalty-free distribution of the report engine for externally facing server and web deployment. http://p.sf.net/sfu/businessobjects _______________________________________________ Gambas-user mailing list Gambas-user@lists.sourceforge.net https://lists.sourceforge.net/lists/listinfo/gambas-user
