What's wrong with this sh script ?

[Paul Falstad]

luj at gus17.ecn.purdue.edu (Jun Lu) wrote:
>Your "at" job "7148" produced the following output:
>
>gus ...  10:56pm  up 2 days,  5:05,  1 user,  load average: 5.82, 6.24, 6.34
>OK
>___
>
>But where is the output for 2nd machine ?

...

>echo -n "$HOST ..."
>if rsh -l $USER $HOST uptime
>then
>    echo OK
>else
>    echo died
>fi

The problem is that rsh eats up its standard input, which also happens
to be the shell script you're executing, since at gives your script to
the shell as standard input.  So when you rsh to the first
host, while you're waiting for uptime to execute, rsh reads its input
(your shell script), and sends it over the socket.  Then uptime
exits without reading anything.  Now when the shell wakes up and tries
to read the rest of your script, it gets an EOF, since rsh has eaten up
all its input.  The reason that the shell managed to print the OK is because
the rsh was part of a control structure; the shell read in the whole if ... fi
structure before executing rsh.  Nothing after the fi was executed.  You can
try for yourself that if you put the whole script in braces { }, forcing the
shell to parse all of it before executing, it will work fine.

The easiest way to fix this is just to use the -n flag of rsh.

rsh -n -l $USER $HOST uptime

Read the FM for more information.

--
Paul Falstad, pfalstad at phoenix.princeton.edu | 10 PRINT "PRINCETON CS"
[Your blood pressure just went up.]          | 20 GOTO 10
Princeton University would like to apologize to everyone for this article.