Help with my first script ... Can you concatenate variables? {solved]

pseudonomous · 2008-07-04 01:29:26

Hello every body,

What I'd like to now is: is there a way for you to concatenate the expansions of variables to get a string which is the same as the name of a
variable and then expand that string?

like i have variables x = var and y = iable and then i want to make z = variable by using x and y.

---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Here's some context:

I'm trying to right my fisrt bash script that does something meaningful, I've found a number of good resources on the topic, but I'm still
banging my head into a brick wall as far as making the script do what I want to do.

Basically, I would like to use the script to simultanously change the theme for SLiM and my background for fluxbox at the same time. I
figured this could be accomplished by creating a folder for "current" theme (in /usr/share/slim/themes ) and a wallpaper in /usr/share/wallpapers
called "dummy", with my overlay file for fluxbox would set as background.

The script would copy a given background into the "current" and copy it's background to "dummy", then I could leave my slim.conf and overlay files
alone. There would also be a "current_theme_number" file to keep track of which theme I'm using at the moment:

Here's how I though I would implement the script:

Name the themes I want to rotate as variables like theme1 , theme2 , etc and give a variable for the total number of themes.

Delete all the theme files in "current" folder

Figure out the current theme by reading the "current_theme_number" file. (this is where the problem comes in)
I can get a number from the current_theme_number file and then do arithmetic mod no. of themes to get the number
for the next theme, but then I need to use this to get the name for next theme, which I cant do.

once I do that I copy the theme files from the next theme into the current folder

then I I change the number in the theme1 file.

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Here's what I have of the script so far:

#!/bin/bash
#this is a script to rotate a number of backgrounds
#to be shared by the fluxbox desktop and SLiM login manager.
#it is the first meaningful script I've written.  It may be
#freely distributed and used with the understanding that all
#use is at your own risk. 
#
#Fred Drueck 2008

#first check you are running this script as super-user:

if [ $(id -u) != "0" ]; then
    echo "You must be the superuser to run this script" >&2
    exit 1
fi

echo "rotating themes"

#Define here all themes to be rotated and the total number of
#themes to rotate:

rotate_number=9

theme1=/usr/share/slim/themes/default
theme2=/usr/share/slim/themes/lake
theme3=/usr/share/slim/themes/lunar
theme4=/usr/share/slim/themes/flower2
theme5=/usr/share/slim/themes/the-light
theme6=/usr/share/slim/themes/mindlock
theme7=/usr/share/slim/themes/parallel-dimensions
theme8=/usr/share/slim/themes/wave
theme9=/usr/share/slim/themes/fingerprint

#figure out which theme is currently set, then name it as the variable
#"current theme number", otherwise quite w/ error message

if [ -f /usr/share/slim/themes/current/current_theme_number ]
then
   echo "checking current theme"
   cd /usr/share/slim/themes/current/
   last_theme_number=$(cat /usr/share/slim/themes/current/current_theme_number) 
   echo $last_theme_number
else
   echo "no theme is currently set!"
   exit 1
fi

#set the new theme number

new_theme_number=$(($(($last_theme_number % $rotate_number))+1)) 

#select the new theme

declare new_theme=$"theme$new_theme_number"
echo $new_theme

#now clean out the "current" theme where I keep the current
#theme for slim
       
rm /usr/share/slim/themes/current/background*
rm /usr/share/slim/themes/current/panel*
rm /usr/share/slim/themes/current/slim.theme

#the wildcards are there since the themes use jpg and png files

I realize that I might not be able to write the script using my current approach, but I thought
I would ask before I go through and adapt a new approach. If I just keep a fixed number of
themes, then I can use "if ... then ... else" to rotate them, but this would make it alot of work to
add more themes to the script.

I might be able to do something similar to my approach by renaming the themes; folders
theme1 , theme2, etc, but this seems inelegant since everytime I install a new theme I want to
add to the rotation I will need to edit both the script and rename the theme ... plus then I will have
no idea which theme is which when I just look at the folder names.

If somebody has a more elegent approach they would like to suggest, then I would welcome the suggestion,

So anyway, thanks in advance for any help you can give me.

Last edited by pseudonomous (2008-07-04 01:55:43)

Cerebral · 2008-07-04 01:36:40

pseudonomous wrote:

Hello every body,
What I'd like to now is: is there a way for you to concatenate the expansions of variables to get a string which is the same as the name of a
variable and then expand that string?
like i have variables x = var and y = iable and then i want to make z = variable by using x and y.

I haven't read your entire post, but yes, you can do this:

x=var
y=iable
z=$x$y

pseudonomous · 2008-07-04 01:38:38

yes and you get a string of characters:

"variable"

however, if you already defined "variable" to mean something, say:

/usr/share/slim/themes/wave

I don't get

/usr/share/slim/themes/wave

I just get "variable".

What I'd like to get is

/usr/share/slim/themes/wave

-thanks though

Last edited by pseudonomous (2008-07-04 01:41:04)

Cerebral · 2008-07-04 01:40:03

Oh bullocks, that's not quite what you want to do it seems. You want something like

x=var
y=1
var1=pie
eval theme=\$$x$y
echo $theme

-edit- perhaps a bit of explanation why this works. eval is the key here - eval takes all its parameters and executes them as if it were a bash statement, for example.

So, if you did "eval theme=hi" then $theme would be set to "hi". The example above is a bit more complex though - as arguments to eval I pass "theme=\$$x$y". I'll try to explain what bash does behind the scenes here.

First, bash looks through the line for variables it can substitute. It reaches the \$, and since it's escaped, it leaves that dollar sign alone. Then bash parses $x and replaces it with 'var', and $y which it replaces with '1'. Then it passes the entire argument to the 'eval' command.

So, when I write eval theme=\$$x$y, bash evaluates that line and does the above. Then eval gets theme=$var1 as its argument, and sets the variable appropriately.

I hope that was clear, it's a bit confusing.

pseudonomous · 2008-07-04 01:48:49

Maybe I'm missing something, but I still get back (transfering to your langauge)

"echo $var1"

instead of

"pie"

edit:oops!

Last edited by pseudonomous (2008-07-04 01:49:35)

pseudonomous · 2008-07-04 01:52:59

I missed that "eval" in your post.

Thanks a whole bunch!

Cerebral · 2008-07-04 01:57:08

Yeah, you can split it up if it makes it any clearer I suppose, too.

# Set up your variables
front=var
var1=pie
var2=cake
var3=pudding

# Iterate over all variables
for back in 1 2 3; do
    varname=\$$front$back   # Sets varname="$var1", varname="$var2", etc...
    eval dessert=$varname   # Bash substitutes $varname for its value, then passes the whole thing to eval
    echo $dessert
done

pseudonomous · 2008-07-07 21:57:35

By the way, a working version of this script can be found at:

http://bbs.archlinux.org/viewtopic.php?id=51398

It's still not elegant, but it does work.

Arch Linux

#1 2008-07-04 01:29:26

Help with my first script ... Can you concatenate variables? {solved]

#2 2008-07-04 01:36:40

Re: Help with my first script ... Can you concatenate variables? {solved]

#3 2008-07-04 01:38:38

Re: Help with my first script ... Can you concatenate variables? {solved]

#4 2008-07-04 01:40:03

Re: Help with my first script ... Can you concatenate variables? {solved]

#5 2008-07-04 01:48:49

Re: Help with my first script ... Can you concatenate variables? {solved]

#6 2008-07-04 01:52:59

Re: Help with my first script ... Can you concatenate variables? {solved]

#7 2008-07-04 01:57:08

Re: Help with my first script ... Can you concatenate variables? {solved]

#8 2008-07-07 21:57:35

Re: Help with my first script ... Can you concatenate variables? {solved]

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