What does ! do in ${!VARNAME}

rwcooper · 06-04-2010, 09:29 AM

In BLFS there are a few functions that contain lines like:

for DIR in ${!PATHVARIABLE} ; do

and

export $PATHVARIABLE="$1${!PATHVARIABLE:+:${!PATHVARIABLE}}"

I can't seem to find anything that tells me what the ! does in these statements.

Thanks,
Randy

pixellany · 06-04-2010, 09:36 AM

In BASH, "!" is typically used for negation....but I'm not quite sure what it means in your context. Off to run some experiments.....

forrestt · 06-04-2010, 09:38 AM

It is a way of saying the value of the variable that is named the value of this variable.

Here is an example. Say you set the value of FOO to asdf, and you set the value of BAR to foo. You can get the value of FOO with ${!BAR}. Here it is in code w/ an example of why you would want to do it:

Code:

FOO1='This is fun'
FOO2='So is this'
FOO3='No fun here'

for BAR in FOO1 FOO2 FOO3 ; do
        echo ${!BAR}
done

HTH

Forrest

pixellany · 06-04-2010, 09:55 AM

Quote:

the value of the variable that is named the value of this variable

Arrgh!!!---instant headache....

Obviously not negation in this context...

From ABS**:

Quote:

Example 34−2. Indirect variable references − the new way
#!/bin/bash
# Indirect variable referencing.
# This has a few of the attributes of references in C++.
a=letter_of_alphabet
letter_of_alphabet=z
echo "a = $a" # Direct reference.
echo "Now a = ${!a}" # Indirect reference.
# The ${!variable} notation is greatly superior to the old "eval var1=\$$var2"
echo
t=table_cell_3
table_cell_3=24
echo "t = ${!t}" # t = 24
table_cell_3=387
echo "Value of t changed to ${!t}" # 387

**The Advanced Bash Scripting Guide (AKA the "boat anchor")---get it at http://tldp.org

rwcooper · 06-04-2010, 09:55 AM

Forrest,

Thanks, that's exactly what I wanted to know.

Randy

catkin · 06-04-2010, 10:04 AM

Interesting! Could be used to approximate content-addressable-arrays

And surely can be used to write obfuscated bash

forrestt · 06-04-2010, 10:06 AM

Quote:

Originally Posted by pixellany

Arrgh!!!---instant headache....

Yeah, but I couldn't think of a better way to say it with only one cup of coffee under my belt.

Forrest

rwcooper · 06-04-2010, 10:08 AM

Hi,

I just downloaded the Advanced Bash Scripting Guide from the Linux Documentation Project and the example is now in chapter 36 as 36-2.

Thanks for the pointer.

Randy