bash script variables

twantrd · 11-16-2004, 01:12 PM

Hi there,

I have a very very simple bash script:

#!/bin/bash

mail=`/usr/bin/mail`

echo "Hello there" | $mail -s "testing" twantrd
exit 0

However, run trying to run it..it will give me "-s: command not found". If I replace $mail with just the command 'mail' it will work. Any ideas why?

Also with ssh:

#!/bin/bash
ssh=`/usr/local/ssh`
$ssh localhost

It will give me this when trying to run: "./ssh_script: localhost: command not found"

However, if I replace $ssh with just the command 'ssh' then it works.
Why doesn't declaring these certain commands as variables work while commands such as 'date', 'hostname', etc.. work as variables? Hmmm...

-twantrd

ahh · 11-16-2004, 01:17 PM

Have you tried using normal quotes around /usr/bin/mail?

And execute the program.

#!/bin/bash

mail="/usr/bin/mail"

echo "Hello there" | $($mail -s "testing" twantrd)
exit 0

twantrd · 11-16-2004, 05:55 PM

Ahhh thank you. DUH, that makes sense! Thanks!!

-twantrd

twantrd · 11-16-2004, 07:44 PM

Actually, I'm back again

. Ok so:

ssh="/usr/bin/ssh" works
ssh=`/usr/bin/ssh` does not work

But how come:
hostname=`/usr/bin/hostname`works

How come hostname works when you declare it as a variable and ssh doesn't?? Thanks!

-twantrd

ahh · 11-17-2004, 01:12 AM

I think this is whats happening:-

When you use

Code:

hostname=`/usr/bin/hostname`

bash executes /usr/bin/hostname and stores the output in $hostname.
Then when you call $hostname in a script it will be replaced with the hosts name, in my case desktop, which is the same as executing the program hostname.

When you use

Code:

ssh=`/usr/bin/ssh`

bash executes /usr/bin/ssh (without any parameters) and stores the output in $ssh.
Then when you call $ssh in a script it will be replaced with the output from ssh called with no parameters,

Code:

usage: ssh [-1246AaCfghkNnqsTtVvXxY] [-b bind_address] [-c cipher_spec]
           [-D port] [-e escape_char] [-F configfile] [-i identity_file]
           [-L port:host:hostport] [-l login_name] [-m mac_spec] [-o option]
           [-p port] [-R port:host:hostport] [user@]hostname [command]

which is obviously not the same as executing the program ssh with any parameters you may have added.

You could test this by placing an

Code:

echo $ssh

in your script.

jlliagre · 11-17-2004, 01:16 AM

Because hostname is a non interactive command that returns a string that is affected to the variable value, while ssh is an interactive command that returns nothing when launched without options or arguments. It should return its help message on stderr though, but this is not going to the variable. mail is also returning nothing on stdout, thus the initial script problem.
Single and double quotes are used to group strings, while back-quotes are executing the embedded string.
Back quotes are becoming obsolete and should be replaced by the equivalent syntax:
hostname=$(hostname)

twantrd · 11-17-2004, 01:46 AM

Ok, now I fully understand. Thanks to you both!!!

-twantrd

theYinYeti · 11-17-2004, 02:38 AM

It has nothing to do with the program being interactive or not.

var=`/usr/bin/program`
or
var=$(/usr/bin/program)
means execute /usr/bin/program and store the output
It's more or less like a pointer on char in C (char*), the value of which is computed by calling a function.

var='/usr/bin/program'
or
var="/usr/bin/program"
means store the string (value) "/usr/bin/program" into var.
It's more or less like a pointer on a function in C (char*), though in this case it can perfectly be handled as a char*, as in: echo $var.

Yves.