exec tclsh

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# 1  
Old 10-06-2009
Question exec tclsh

As a part of learning shell scripting I was just going through some already created scripts there I found-

exec tclsh "$0" $(1+"$@")

Here I was able to find what exec ,tclsh does & o/p of same but I could not find usage & output of $(1+"$@").

Can anybody pls expalain me usage details of it?
# 2  
Old 10-06-2009
I do not recognize that syntax, what shell is this?
# 3  
Old 10-07-2009
I ma using this on sh shell.
# 4  
Old 10-07-2009
That just doesn't look right. sh doesn't do string concatenation or math like that, and trying to duplicate it in my shell just gives me syntax errors. Are you sure that script is using sh? Does it have #!/bin/sh at the top or something else?
# 5  
Old 10-07-2009
       An even better approach is to start your script files with the  follow-
       ing three lines:

              # the next line restarts using tclsh \
              exec tclsh "$0" "$@"

       This  approach  has  three advantages over the approach in the previous
       paragraph.  First, the location of the tclsh binary doesn't have to  be
       hard-wired  into  the  script:  it can be anywhere in your shell search
       path.  Second, it gets around the 30-character file name limit  in  the
       previous  approach.   Third,  this  approach will work even if tclsh is
       itself a shell script (this is done on some systems in order to  handle
       multiple  architectures or operating systems:  the tclsh script selects
       one of several binaries to run).  The three lines  cause  both  sh  and
       tclsh  to  process the script, but the exec is only executed by sh.  sh
       processes the script first;  it treats the second line as a comment and
       executes  the  third  line.  The exec statement cause the shell to stop
       processing and instead to  start  up  tclsh  to  reprocess  the  entire
       script.   When  tclsh starts up, it treats all three lines as comments,
       since the backslash at the end of the second line causes the third line
       to be treated as part of the comment on the second line.

-- excerpt from man tclsh

For example:
# the next line restarts using tclsh \
exec tclsh "$0" "$@"

# @(#) tcl5     Demonstrate tclsh feature.

set version [ info tclversion ]
set message " Hello, world from tclsh ($version), $auto_path"
puts stdout $message

puts stdout ""
if { $argc == 0 } then {
  puts stdout " No parameters provided."
} else {
  set i 0
  foreach arg $argv {
    set m1 " $i $arg"
    incr i
    set m2 ""
    if { [ file exists $arg ] } then {
      set m2 [ join [ list "(" [ file type $arg ] ")" ] ]
    puts stdout "$m1 $m2"

exit 0

% ./tcl5
 Hello, world from tclsh (8.4),...

 No parameters provided.

% ./tcl5 a x=1 3.14
 Hello, world from tclsh (8.4), ...

 0 a 
 1 x=1 
 2 3.14

The "1+ .." part might be taken from perl:
           #!/bin/sh -- # -*- perl -*- -p
           eval 'exec perl -wS $0 ${1+"$@"}'
               if $running_under_some_shell;
-- excerpt from man perlrun

You might be able to fiddle with single quotes, specific shells, etc., to make the command in the OP work . Best wishes ... cheers, drl
# 6  
Old 10-08-2009
What does ${1+"$@"} mean?


An explanation for the syntax:

is not so easy to find, however, some information is at What does ${1+"$@"} mean?

basically, it's for very old Bourne shells -- not so many around these days, but there are a few as noted in the page at the URL cited ... cheers, drl
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