In scripts, first line should specify the path to interpreter.
But on different servers Linux, Unix, or BSD this path could be different.
What is more preferable?
#!/usr/bin/env bash
or
#!/bin/bash
If you want to use the system-installed version of a given interpreter that's installed in a standard location, use the direct path. If you want to use whatever version of the interpreter appears first in the user's $PATH, use #!/usr/bin/env ....
The env command invokes a specified command, letting you set or unset environment variables:
env FOO=BAR do-something
env DISPLAY=:0.0 xterm -ls &
If you don't specify any environment variables or other options, it will just invoke the named command. (Using it this way is arguably a bit of a hack.)
The purpose of writing the shebang as
#!/usr/bin/env interp
is to invoke whatever interp appears first in $PATH.
This means you don't have to know, when writing the script, exactly where interp is (say, if it could be in either /bin, /usr/bin, or /usr/local/bin). Of course you do have to know that env is /usr/bin/env, but that seems to be reasonably universal.
The advantage is that it invokes whichever version of the interpreter appears first in the user's $PATH. The disadvantage is that it invokes whichever version of the interpreter appears first in the user's $PATH.
For example, suppose I've installed a personal build of perl under my home directory, as $HOME/bin/perl, and I have $HOME/bin at the front of my $PATH. If I run a script whose shebang is
#!/usr/bin/env perl
then it's going to be run with my own installed perl executable -- which might not be a good thing. The author of the script probably hasn't tested it with the bleading-edge Perl that I built from source a month ago.
For something like Perl or Bash that's likely to be installed in a consistent location on most systems (/usr/bin/perl and /bin/bash, respectively), I'd use the direct path to the command. For something more obscure that could be installed differently on different systems, I'd either use the /usr/bin/env trick, or I'd write an installer that adjusts the shebang line as the script is being installed. (I used to have to do that for my Perl scripts.)
UPDATE : I've gone into a bit more detail in this answer to this question on the Unix & Linux site.
