I have not been sys admin for a UNIX box for a long time. To me it seems dangerous to edit the passwd file with a script like yours. As a minimum I would ask for "use strict;" in the script. Also, there is a number of modules on CPAN to edit the passwd file in a controlled manner, e.g. Unix::PasswdFile among others.

If your aim is to learn how to do things yourself, then I would think that the passwd file is not the best object for that.

UPDATE: Here is a sample script.

my $pw = new Unix::PasswdFile "passwd";  # for real this should be /et
+c/passwd or similar
$pw->shell( "user1", "/bin/nologin" );
$pw->commit(".bak");
[download]

cp890:

Are you sure your script is actually failing? It may be doing everything correctly and also emitting those warning lines. Typically I see that sort of error because a blank line (such as the end) in a file has no information on it.

For example:

$ cat t.pl
use strict; use warnings;

while (<DATA>) {
    my @f = split /\s+/,$_;
    $f[1] =~ s/foo/bar/g;
    print "result: $f[1]\n";
}

__DATA__
Poor foolish kitty

The empty line above this one might fool us.

$ perl t.pl
$ perl t.pl
result: kitty
Use of uninitialized value $f[1] in substitution (s///) at t.pl line 6
+, <DATA> line 2.
Use of uninitialized value $f[1] in concatenation (.) or string at t.p
+l line 7, <DATA> line 2.
result:
result: barlish
Use of uninitialized value $f[1] in substitution (s///) at t.pl line 6
+, <DATA> line 4.
Use of uninitialized value $f[1] in concatenation (.) or string at t.p
+l line 7, <DATA> line 4.
result:
[download]

A simple fix would be to ignore blank lines:

while (<DATA>) {
    next if /^\s*$/;  # skip blank lines
    ...
}
[download]

Update: Minor code edit. (Made first case actually substitute.)

...roboticus

When your only tool is a hammer, all problems look like your thumb.

I agree with hdb that it's safer (and easier) to use modules. Also consider using usermod --shell <new_shell> system command, on Linux, at least.

One way to replace the shell manually:

use strict;
use warnings;

my $new_shell = "/bin/nologin";

# in-place editing with backup
$^I = ".bak";

# diamond operator "<>" reads from command line arguments or STDIN
while ( my $line = <> ) {

    # skip empty lines
    next if $line =~ /^\s*$/;

    # split on ":" and return the 7th field (list slice)
    my $old_shell = ( split ":", $line )[6];

    $line =~ s/$old_shell/$new_shell/;
    print "$line\n";
}
[download]

Update: using explicit variable $line instead of default variable $_ for better readability.

"...I am focus on user1 in the script below but would also like to see a script example if I wanted to in place edit user1 and user3..."

I agree with hdb and others totally here, however, if you still want to practice with your originally Posted data as shown in your post. You can go through the line one by one substituting for user1 and user3 like so:

use warnings;
use strict;

while(<DATA>){
 chomp;
 if(/^user(1|3):/){
    s{(?<=/bin)(.+?)$}{/nologin};
 }
 
  print $_,$/;
}


__DATA__
user1:x:9760822:3241:USER 1:/home/user1:/bin/bash

user2:x:9760822:3242:USER 2:/home/user2:/bin/ksh

user3:x:9760822:3243:USER 3:/home/user3:/bin/sh
[download]

user1:x:9760822:3241:USER 1:/home/user1:/bin/nologin

user2:x:9760822:3242:USER 2:/home/user2:/bin/ksh

user3:x:9760822:3243:USER 3:/home/user3:/bin/nologin
[download]

thanks for all your responses- I have at least one OS if a process is running - the usermod returns in use. I cant always bring down the app or db associated. some of servers i can use vipw which works, but is manual and subject to more errors and some servers and OS do not have vipw- this is why i going with the script and backup