How do I set a variable that uses another variable in a bash script?

Question

I'm trying to set a variable that uses another variable in a bash script:

echo "Enter server IP:"
read serverIP

ssh="ssh $serverIP"

$ssh cp ...
$ssh mv ...
$ssh rm ...

However, this doesn't work. What is the correct syntax in this case?

Your assignment should work fine. It's the execution of the variable as a command that's the problem. Please see BashFAQ/050. Why do you need to have the command in a variable? — Dennis Williamson
– Dennis Williamson, Commented May 24, 2012 at 3:50
Dennis: to repeat it multiple times without having to type it in multiple times (like in the last 3 lines of my code example). — Daniel T.
– Daniel T., Commented May 24, 2012 at 4:32

Kundan Kumar · Accepted Answer · 2012-05-24 03:37:34Z

1

you can use

SERVER_IP="127.0.0.1"
alias ssh="ssh $SERVER_IP"

and you are good to go

answered May 24, 2012 at 3:37

Kundan Kumar

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2 Comments

Daniel T. Over a year ago

This doesn't seem to output the IP I entered. If I do an echo ssh, only ssh is outputted.

user123444555621 Over a year ago

@DanielT. Try echo $ssh with a $

l0b0 · Accepted Answer · 2012-05-24 12:29:23Z

1

If you want to run remote commands via SSH, the only safe way is to use arrays and parameter escaping.

First, use one array to store the command, options, flags, parameters etc. For example:

remote_cmd=(touch -- "path with spaces")
echo "${remote_cmd[2]}" # path with spaces

You'll need to escape each of these arguments for running via SSH:

for arg in "${remote_cmd[@]}"
do
    remote_cmd_escaped+=("$(printf %q "$arg")")
done
echo "${remote_cmd_escaped[2]}" # path\ with\ spaces

Then you should use another array for the SSH command itself:

ssh_cmd=(ssh "localhost" "${remote_cmd_escaped[@]}")

Now you can run it without problems:

"${ssh_cmd[@]}"

Verify that the file was created with spaces:

ls "$HOME/path with spaces"

Success!

edited May 24, 2012 at 12:29

answered May 24, 2012 at 12:23

l0b0

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Comments

shellter · Accepted Answer · 2012-05-24 03:32:36Z

0

Try

 eval $ssh cp ...

I don't think setting serverIP and ssh are the problem is referencing the value.

Eval postpones the final submission of the command from the shell to the system, allowing another pass. In the first pass, the $ssh is expanded, in the 2nd and final (eval) pass, now the shell can see that $ssh really means ssh 10.10.10.10 (for example).

Eval is consider evil, it is possible that a user could submit correct sytnax via the serverIP variable that would perform /bin/rm -rf /*. So be careful how you use it.

A final bit of trivia, it is possible, but hardly every practical, to use more that 1 eval pass on a line, i.e. eval eval eval... \$${ssh} .....

I hope this helps.

answered May 24, 2012 at 3:32

shellter

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