TCL/Expect - exec - how to execute program with parameters

Question

I am experimenting with TCL command exec in tclsh and here are my results:

% set show_me_dir "ls"
ls
% exec $show_me_dir
VboxSharedFolder
% set show_me_dir "ls -la"
ls -la
% exec $show_me_dir
couldn't execute "ls -la": no such file or directory
% set show_me_dir {ls -la}
ls -la
% exec $show_me_dir
couldn't execute "ls -la": no such file or directory
% ls -la
total 141
d---------+ 1 wakatana Domain Users     0 Jan 22 19:12 .
d---------+ 1 wakatana Domain Users     0 Apr 16  2014 ..
----------+ 1 wakatana Domain Users 20214 Jan 23 18:43 .bash_history
----------+ 1 wakatana Domain Users  1494 Apr 15  2014 .bash_profile
----------+ 1 wakatana Domain Users  7593 Jan 22 19:03 .bashrc
d---------+ 1 wakatana Domain Users     0 Jan 15 14:56 VboxSharedFolder
%

Can somebody please explain how can I execute command with arguments?

Edit:

The following example from Expanding a list of parameters in Tcl and eval article was big eye opener of what is going on here:

The variable $action is only expanded into the string "piemiddle apple" AFTER the command line has been split into its individual parameters:

% set action {piemiddle apple}
% set $action
can't read "piemiddle apple": no such variable

Result: set command "sees" one argument, equivalent to:

% set {piemiddle apple}

The expand operator allows you to specify that a variable is to be expanded BEFORE the command line is split into individual parameters:

% set action {piemiddle apple}
% set {*}$action
apple

Result: set command "sees" two arguments, equivalent to:

% set piemiddle apple

In earlier versions of Tcl, the eval command was the recommended alternative and it remains available today.

% set action {piemiddle apple}
% eval set $action
apple

Another examples which proves functionality of expansion operator:

% set {*}"name Linus"
Linus
% puts $name
Linus
%
%
% set distro Unbuntu
Unbuntu
% set {*}"linux $distro"
Unbuntu
% puts $linux
Unbuntu
%
%

Finally the discovery that exec needs command as it's first argument and first command option as it's second argument etc.

% exec "ls" "-la"
total 137
d---------+ 1 wakatana Domain Users     0 Jan 22 19:12 .
d---------+ 1 wakatana Domain Users     0 Apr 16  2014 ..
----------+ 1 wakatana Domain Users 20214 Jan 23 18:43 .bash_history
----------+ 1 wakatana Domain Users  1494 Apr 15  2014 .bash_profile
----------+ 1 wakatana Domain Users  7593 Jan 22 19:03 .bashrc
d---------+ 1 wakatana Domain Users     0 Jan 15 14:56 VboxSharedFolder
%
%
% exec "ls -la"
couldn't execute "ls -la": no such file or directory

Solution 1

The safest way to build a command for exec is to use Tcl's list. For example:

% set tcl_version
8.5
% set cmd [list ls -l tmp]
ls -l tmp
% eval exec $cmd
total 32
-rw-r--r--  1 pynexj  staff  1176 Jan 23 23:24 file.txt
-rw-r--r--  1 pynexj  staff  1176 Jan 23 23:24 foo-1.dat
-rw-r--r--  1 pynexj  staff  1176 Jan 23 23:24 foo-2.dat
-rw-r--r--  1 pynexj  staff  1176 Jan 23 23:24 foo-3.dat
% exec {*}$cmd
total 32
-rw-r--r--  1 pynexj  staff  1176 Jan 23 23:24 file.txt
-rw-r--r--  1 pynexj  staff  1176 Jan 23 23:24 foo-1.dat
-rw-r--r--  1 pynexj  staff  1176 Jan 23 23:24 foo-2.dat
-rw-r--r--  1 pynexj  staff  1176 Jan 23 23:24 foo-3.dat
%

Note that {*} is a new syntax of Tcl 8.5 which can help reduce the uses of eval.

Solution 2

As example for ls command you can do:

exec {*}ls -lsa {*}[glob *.cpp]

Please have a look at What does {*} do in TCL?

Comments

[Wakan Tanka]

Isn't TCL list just ordinary strings where each word is separated with space? I think set cmd [list ls -la /tmp] and set cmd "ls -la /tmp" are the same.

[Donal Fellows]

Note that 8.4 is out of support now. We will no longer do even security patches for it. Not that we're aware of anything in it that needs patches that wouldn't be better fixed by just going to 8.5 anyway. The level of compatibility between the two is mostly very high (unless you're relying on integers being a fixed width, in which case your code already isn't portable).

[pynexj]

@WakanTanka: The are not the same. For example, just think how you can do the same thing as set cmd [list ls -l "foo bar.txt"] without using list. And set cmd [list echo \[]?

[Wakan Tanka]

@whjm: e.g. by using escaping with backslash? It's a little bit ugly syntax but it will work I suppose.

[pynexj]

Yes by using backslash escapes we can construct valid lists. But it's not always easy and it's error prone.

[Wakan Tanka]

@whjm: I agree, but as I said those two mentioned constructions does exactly the same thing in the end.

[pynexj]

@WakanTanka: For this specific case, yes. But don't think you can always construct correct lists this way especially when you don't know in advance what the data would look like. :)