topic shell substitution in Operating System - HP-UX

shell substitution

Igor Sovin — Sun, 24 Apr 2005 23:57:40 GMT

Hi!

Working on the following problem

I have script for example:
export rx1=hostname1
export rx2=hostname2
export rx3=hostname3
for i in 1 2 3
do
ssh $(rx$i) <- need to get here hostname1, hostname2 etc
.....
.....
done

How to do that?
i only get rx1 or rx2 or rx3 instead of hostname1, hostname2 or hostname3

thank you

Re: shell substitution

T G Manikandan — Mon, 25 Apr 2005 00:11:40 GMT

for i in hostname1 hostname2 hostname3
do
ssh $i

..
done

OR

make entries in file /tmp/a

hostname1
hostname2
hostname3

Then in the script

for i in `cat /tmp/a`
do
ssh $i
..
..
done

Re: shell substitution

Igor Sovin — Mon, 25 Apr 2005 00:13:20 GMT

Shame on me! its so easy :)

thank you!

Re: shell substitution

Paul_481 — Mon, 25 Apr 2005 00:19:59 GMT

Hi Igor,

Use array.

insert this line

rx=3
export rx[1]=hostname1
export rx[2]=hostname2
export rx[3]=hostname3

for i in 1 2 3
do
ssh ${rx[$i]}

.....
.....
done

Hope it helps.

Regards,
Paul

Re: shell substitution

Igor Sovin — Mon, 25 Apr 2005 00:22:23 GMT

thank you guys!
Both methods are work!

Re: shell substitution

Muthukumar_5 — Mon, 25 Apr 2005 04:51:09 GMT

You got the answer. You can use array's effectivly as,

#!/bin/ksh

export rx[1]=hostname1
export rx[2]=hostname2
export rx[3]=hostname3
let i=0

while [[ $i -lt ${#rx[*]} ]]
do
ssh ${rx[$i]}
let i=i+1
done

exit 0
##

HTH.

Re: shell substitution

Bill Hassell — Mon, 25 Apr 2005 07:08:44 GMT

The construct $() has the results of a command execution. When you place just the variable inside the parens (), the shell tries to run the string rx1$hostname1 which is not what you want. Just remove the command substitution and it works fine:

export rx1=hostname1
export rx2=hostname2
export rx3=hostname3
for i in 1 2 3
do
ssh rx$i ...
done

Although the ${} curly braces are not needed in this context, it is always good script writing practice to explicitly separate variables from plaintext as in:

ssh rx${i} ...

Re: shell substitution

Nguyen Anh Tien — Tue, 26 Apr 2005 01:58:15 GMT

Hi logor
1. Make one file contains all hosts
#vi allhosts
hostname1
hostname2
hostname3
#vi yourscript
for i in `cat hosts`
do
ssh $i
done
Try it now
HTH
tienna

Re: shell substitution

Bob_Vance — Tue, 26 Apr 2005 08:22:49 GMT

A much simpler way to use the array method (which I like) is:

rx[1]=hostname1
rx[2]=hostname2
rx[3]=hostname3
export rx

for i in ${rx[@]}
do
echo ssh ${i}
done

This [@] is much more general and expands to the entire array list.
You don't have to do any counting or know how big the array is!!

HTH
bv

Re: shell substitution

Stuart Browne — Tue, 26 Apr 2005 23:04:17 GMT

So basically you're wanting to do multi-level evaluation.

You first want to evaluate $i to be a digit, which is working.. You then want to re-evaluate the new string to get the contents of the next variable.. i.e.

rx$i => rx1

You then want to get:

$rx1 =>

but that second part wasn't working.

You can use one of the other solutions here, of which I'd highly reccomend.. But for the sake of the argument, there's usually a few ways to do the multi-level evaluation.

One is to use the 'eval' command:

ssh $(eval "echo \$rx$i")

The 'eval' command allows for the second leevl of evaluation. So the shell would work like this:

eval "echo \$rx$i" => echo $rx1 => hostname1

thus the answer you wanted.

Anyway, have fun :)