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# 1  
Old 03-25-2012
exec command help
Hi,

I have the following lines in a script :

Code:
.
.
exec < some_file
.
.
.

I have very little idea about exec command. I would like to know what this does and what will happen if the file some_file does not exist. Specifically, I would like to know whether the lines following this line will be interpreted if some_file does not exist.
# 2  
Old 03-25-2012
There are broadly speaking two distinct uses of exec:
  1. with a command. This exits the script and executes the command. The lines following the exec statement will not be executed.
  2. with a redirection and without a command. In this case stdin takes it input from "some_file". The lines following exec will be executed while stdin remains redirected.
This is irrespective of whether "some_file" exists or not.. *not entirely correct see post below*
So the answer is: use nr. 2. is the case here and the statements below exec will get executed whether or not the file exists...
Last edited by Scrutinizer; 03-25-2012 at 10:06 AM..
This User Gave Thanks to Scrutinizer For This Post:
jim mcnamara
# 3  
Old 03-25-2012
Quote:
Originally Posted by Scrutinizer
There are broadly speaking two distinct uses of exec:
  1. with a command. This exits the script and executes the command. The lines following the exec statement will not be executed.
  2. with a redirection and without a command. In this case stdin takes it input from "some_file". The lines following will be executed while stdin remains redirected.
This is irrespective of whether "some_file" exists or not..
So the answer is option 2. is used and the statements below exec will get executed whether or not the file exists...
Thanks Scrutinizer for the info on exec...

But, I still have a doubt...

To test it, I made a script file named "test2" having the following lines :

Code:
#!/usr/bin/ksh
exec < test123
echo Hi

As per your explanation, "Hi" should be echoed irrespective of whether test123 exists or not. But, when I run "test2" ( with no file named "test123" in the current directory), I get the following output:

Code:
A file or directory in the path name does not exist.
test2[2]: test123: 0403-016 Cannot find or open the file.

"Hi" is not at all echoed........
# 4  
Old 03-25-2012
You are right, If the file does not exist in use nr. 2, then POSIX compliant shells should exit the script, so that the lines following exec will be executed.

I had only quickly tested it with bash:

Code:
bash ./test2
./test2: line 2: ./test123: No such file or directory
Hi

But in ksh:
Code:
ksh ./test2
./test2[2]: ./test123: cannot open [No such file or directory]

and in dash:
Code:
$ dash ./test2
./test2: 2: ./test1231c: cannot open ./test123: No such file

According to POSIX "exec" is a so-called "special built-in utility", and there is a redirection error, therefore the script should exit ( Shell Command Language: Consequences of Shell Errors )
Special Built-in utilities are:
Code:
break
: (colon)
continue
. (dot)
eval
exec
exit
export
readonly
return
set
shift
times
trap
unset

The reason that bash worked, is that is not fully POSIX compliant by default:
Code:
bash ./test2
./test2: line 2: ./test123: No such file or directory
Hi

Code:
$ bash --posix ./test2
./test2: line 2: ./test123: No such file or directory

See: Bash POSIX Mode, item #18. Bash Reference Manual
Last edited by Scrutinizer; 03-25-2012 at 10:35 AM..
This User Gave Thanks to Scrutinizer For This Post:
elixir_sinari
# 5  
Old 03-25-2012
Even with --posix, Bash is not fully POSIX complaint and makes not claim to be. All that flag does is cause Bash "to conform more closely to the POSIX standard."
# 6  
Old 03-25-2012
A good point, but IMO it goes a bit further than that, and the non-compliance is limited and specific: the same page also says:
Quote:
There is other posix behavior that Bash does not implement by default even when in posix mode. Specifically:

The fc builtin checks $EDITOR as a program to edit history entries if FCEDIT is unset, rather than defaulting directly to ed. fc uses ed if EDITOR is unset.
As noted above, Bash requires the xpg_echo option to be enabled for the echo builtin to be fully conformant.
And also:
Code:
Bash can be configured to be posix-conformant by default, by specifying the --enable-strict-posix-default to configure when building (see Optional Features).

And from my manage..
Quote:
Bash is intended to be a conformant implementation of the Shell and Utilities portion of the IEEE POSIX specification (IEEE Standard 1003.1). Bash can be configured to be POSIX-conformant by default.
Last edited by Scrutinizer; 03-25-2012 at 11:17 AM..
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