Bash strings evaluation

matiasquestions · 08-20-2008, 12:27 PM

Hi,

I've the following variable:

Code:

# echo $BINMAKE
`if [ -x /usr/obj/usr/src/make.i386/make ]; then echo /usr/obj/usr/src/make.i386/make; else echo make; fi` -m /usr/src/share/mk
#

It's a string wich includes a commmand.

How can I get the subcommand replaced by its output?

I'd like to have a second variable like:

Code:

# echo $newvariable
make -m /usr/src/share/mk 
#

colucix · 08-20-2008, 12:48 PM

It should work the way you have written. Better to use the $(command) syntax instead of back ticks, for various reasons. Anyway it seems correct to me. Can you explain a bit more what the problem is?

matiasquestions · 08-20-2008, 01:00 PM

How would you assign the value to $myvariable so that the output on my main post is consistent?

That's the problem..........

colucix · 08-20-2008, 01:28 PM

Have you any control on how the BINMAKE variable is created? It looks like single quotes have been used, whereas double quotes should do the trick:

Code:

# BINMAKE='`if [ -x /usr/obj/usr/src/make.i386/make ]; then echo /usr/obj/usr/src/make.i386/make; else echo make; fi` -m /usr/src/share/mk'

# echo $BINMAKE
`if [ -x /usr/obj/usr/src/make.i386/make ]; then echo /usr/obj/usr/src/make.i386/make; else echo make; fi` -m /usr/src/share/mk

# BINMAKE="`if [ -x /usr/obj/usr/src/make.i386/make ]; then echo /usr/obj/usr/src/make.i386/make; else echo make; fi` -m /usr/src/share/mk"

# echo $BINMAKE
make -m /usr/src/share/mk

matiasquestions · 08-20-2008, 01:32 PM

Nope, I've no control on how BINMAKE is created.... :-(

colucix · 08-20-2008, 01:48 PM

Ok. This is a complex variable substitution and evaluation problem (please, don't ask me to explain in details). You have to put the whole assignment statement in a dummy variable, then evaluate that variable. Better to explain with a real example:

Code:

# BINMAKE='`if [ -x /usr/obj/usr/src/make.i386/make ]; then echo /usr/obj/usr/src/make.i386/make; else echo make; fi` -m /usr/src/share/mk'

# echo $BINMAKE
`if [ -x /usr/obj/usr/src/make.i386/make ]; then echo /usr/obj/usr/src/make.i386/make; else echo make; fi` -m /usr/src/share/mk

# PLUTO="PIPPO=\"$BINMAKE\""

# eval $PLUTO

# echo $PIPPO
make -m /usr/src/share/mk

in the example above you put the assignment statement in the variable PLUTO, then eval $PLUTO to actually assign the variable PIPPO. After that you can unset PLUTO and use PIPPO the way you want.

PS - PIPPO and PLUTO are italian translations of Walt Disney characters Goofy and... Pluto. I often use them in my examples and as temporary file names.

matiasquestions · 08-20-2008, 05:18 PM

WoW!!!! That works perfect!!! thanks a lot for your help!! :-D