exec command help

Shell Programming and Scripting
1

Hi,

I have the following lines in a script :

.
.
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

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...

1 Like
3

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 :

#!/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:

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

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:

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

But in ksh:

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

and in dash:

$ 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:

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:

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

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

See: Bash POSIX Mode, item #18. Bash Reference Manual

1 Like
5

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

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:

And also:

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..