bash: help with script

[mglenney]

Running the following script:

#!/bin/bash

a="one two three"
b="a"

echo $a
echo $b

Results in:

one two three
a

Is it possible to return the value of $a by calling $b? For example:

echo ${$b} would result in "one two three" (it doesn't )

I hope that is clear.

Thanks,

MG

[pmcoleman]

~$ a="one two three"
~$ b=$a
~$ echo $b
one two three

Works here....

[mglenney]

You're setting b to the variable $a. I have b set to the string "a". Consider the following script:

a="one two three"
b="four five six"

echo -n "Please select a or b"
read choice

Depending on the input, choice could be set to "a" or "b". Now if I wanted to reference the variable $a or $b based on that choice how would I do it?

I can do this:

output="$""$choice"

but if I do that then $output is seen as variable containing a string, not a variable containing a variable (eg, echo $output results in "$a" instead of "one two three")

MG

[pmcoleman]

You're setting b to the variable $a. I have b set to the string "a". Consider the following script:

a="one two three"
b="four five six"

echo -n "Please select a or b"
read choice

Depending on the input, choice could be set to "a" or "b". Now if I wanted to reference the variable $a or $b based on that choice how would I do it?

I can do this:

output="$""$choice"

but if I do that then $output is seen as variable containing a string, not a variable containing a variable (eg, echo $output results in "$a" instead of "one two three")

MG

OK. I understand better...
I went back through my own scripts looking for an example and found one that I have adapted to your original post. My approach to such scenarios is to use arrays in the script.

Thus the following. Script is "self documenting" with output at the end.

## Initiate variables
a=0
b=1
## Initiate array and populate elements 0 and 1
arrayX[0]="one two three"
arrayX[1]="four five six"
## Display current array elements
echo "The current array element contents:"
echo "Variable 'a'  ${arrayX[0]}" # Var from original example (variable a)
echo "Variable 'b'  ${arrayX[1]}" # Var from original example (variable b)
echo -n "Please select a or b :" # Per original post
read choice
echo "The value of choice is   $choice" # Display current value

# convert for use with array
if [ $choice = "a" ]
	then choice=$a
fi
if [ $choice = "b" ]
	then choice=$b
fi
echo "The value of choice is now   $choice" # Display converted value
echo "Your selection is:  "
# Display the "value" of the original post vars "a" or "b"
echo ${arrayX[$choice]}
exit 0

### OUTPUT ###
# ~/Coding/WorkingScripts/Dev$ ./makeAchoice.bash 
# The current array element contents:
# Array 'a'  one two three
# Array 'b'  four five six
# Please select a or b :b
# The value of choice is   b
# The value of choice is now   1
# Your selection is:  
# four five six
# ~/Coding/WorkingScripts/Dev$ ./makeAchoice.bash 
# The current array element contents:
# Array 'a'  one two three
# Array 'b'  four five six
# Please select a or b :a
# The value of choice is   a
# The value of choice is now   0
# Your selection is:  
# one two three
# ~/Coding/WorkingScripts/Dev$

As you can see the output changes based on the user input of selecting a or b.
Instead of assigning values to a and b directly, the strings are held in the arrayX, elements 0 and 1. The user selection determines what element will be displayed.

Good luck.. I have to get back to my other work, may not be able to check this forum until tomorrow.

[drl]

Hi.

The eval feature is present in many languages. It allows one to do indirect references (among other things). There is also a special syntax to do this in most recent versions of bash:

#!/bin/bash

a="one two three"
b="a"

echo $a
echo $b
echo $$b
eval echo $$b
eval echo \$$b

echo ${!b}

producing:

% ./user3
one two three
a
31635b
31635b
one two three
one two three

See Indirect References for more ... cheers, drl

[ghostdog74]

Running the following script:

#!/bin/bash

a="one two three"
b="a"

echo $a
echo $b

Results in:

one two three
a

Is it possible to return the value of $a by calling $b? For example:

echo ${$b} would result in "one two three" (it doesn't )

I hope that is clear.

Thanks,

MG

why would you want to declare your "b" variable like that in the first place?

a="one two three"
b="$a"

[pmcoleman]

Hi.

The eval feature is present in many languages. It allows one to do indirect references (among other things). There is also a special syntax to do this in most recent versions of bash:

#!/bin/bash

a="one two three"
b="a"

echo $a
echo $b
echo $$b
eval echo $$b
eval echo \$$b

echo ${!b}

producing:

% ./user3
one two three
a
31635b
31635b
one two three
one two three

See Indirect References for more ... cheers, drl

Very nice. I thought someone would jump in with a more effecient method. That is what I like about this forum, everyone can learn something new. ...adding your link to new things to check out. I was not aware of indirect references.

Edit: Your link gave me back the name of the work I was looking for. I had lost the pdf version of the book some time back and my hard copy scripting volumes... well lets just say all is well now.. Thanks again.

[mglenney]

Thanks drl!! This is exactly what I was looking for. Works like a charm.

pmcoleman - Thanks for taking the time to try and work through this issue with me.

MG

[drl]

Hi.

Thanks drl!! This is exactly what I was looking for. Works like a charm.
MG

You're welcome. The tlpd is a very long document, too long for most people to take in all at once. However, it is one of the single best references around for bash scripting. I suggest you skim it every now and then if you are going to be doing scripting ... cheers, drl

[mglenney]

ghostdog -

The reason I had to do this is because where I work they have these bash scripts that are nothing but a bunch of variables containing the hostnames of our servers and they source these in other scripts so they can perform actions against a group of servers at one time.

So they might have the following hosts.sh:

appHosts="host1 host2 host3"
dbHosts="dbhost1 dbhost2 dbhost3"

servers1="host1 dbhost1"
servers2="host2 dbhost2"
servers3="host3 dbhost3"

allServers="$appHosts $dbhosts"

And then the following getHostname.sh script:

#!/bin/bash

. /nfs1/hosts.sh

for name in $servers1
do
  result=`ssh $name "hostname"`
  echo -ne "$name\t$result"
done

exit

So I wanted to create a script to simplify running commands on remote hosts and was looking for a simpler way to call one of the host groups from the hosts.sh file without having to edit the script each time. What I did was add an additional variable to the end of hosts.sh called menuList:

menuList="appHosts dbHosts servers1 servers2 servers3 allServers"

I use $menuList to generate a menu which the script user can use to select a group

function menu
{
i=1
for name in `echo $menuList`
do
  menu[$i]=$name
  echo -ne "$i)\t$name\n"
  let "i += 1"
done

echo ""
echo -n "Please enter number: "
read selection

menuchoice=${menu[$selection]}
}

I just needed a way to tack the "$" back on to the front of the selected group to turn it into a variable.

Hope that explains it. I wanted to post all of this in case you or someone else has a better way to accomplish it.

Thanks,

MG