Shell Scripting Question: planets[9]=Pluto ??

koodoo · 08-11-2006, 08:49 AM

Hi,

I was revising shell scripting and came across the following script:

Code:

#!/bin/sh
set -a planets mecury venus 
planets[9]=Pluto 
printf ${planets[*]} 
exit 0

I could not understand what the satement planets[9]=Pluto did?
Could anyone please just give me a hint as to how the above script works?

The output of the script is:
Pluto

Thanks for any help.

unSpawn · 08-11-2006, 10:19 AM

In Bash:
# make array named "planets" and add lowercase named members "mercury" and "venus"
# (else use declare -a planets; planets[0]=mercury etc, etc):
planets=( mecury venus )
# the 10th array member has contents "Pluto". Note this doesn't make Pluto a planet. Pluto still is a rock ;-p:
planets[9]=Pluto
# is missing a newline. Prints the contents of the array as string:
printf ${planets[*]}
# always exit a script in OK state:
exit 0

koodoo · 08-11-2006, 02:53 PM

Thanks a lot for the reply. It kinda feels special when the moderator replies

I tried the following script (Sorry, should've tried this before starting this thread) :

Code:

#!/bin/sh
planets[9]=Pluto 
printf ${planets[*]} 
exit 0

and the output was same: Pluto
So, this is what I understand:
The line planets[9]=Pluto declares/defines an array that can store 9 members and it's ninth member is Pluto. Then the statement printf ${planets[*]} Prints the contents of the array as a string. Also since the other members of the array planets are not defined the shell substitues "" whenever they are requested.
Is my interpretation correct?

I also want to ask that what does the following statement do:

Code:

set -a planets mecury venus

I looked up the man page of the set command, but could not find any refernece to the -a flag. What I know about the set command is that a simple set command like:

Code:

set planets mecury venus

would set $1 to planets, $2 to mercury, $3 to venus etc. There were also other uses of set like storing a random number in a variable, but I could not find what the -a flag did.

Thanks for all the help.

unSpawn · 08-12-2006, 06:02 AM

The line planets[9]=Pluto declares/defines an array that can store 9 members and it's ninth member is Pluto.
The line planets[9]=Pluto declares/defines (or adds to) an array named "planets" with one element at position nine whose content is the string "Pluto". The name of the element is planets[9] and it does not define the amount of elements an array can contain.

Then the statement printf ${planets[*]} Prints the contents of the array as a string. Also since the other members of the array planets are not defined the shell substitues "" whenever they are requested.
Since no other elements are defned the shell doesn't print a thing. Use "set -x" to see when running.

set -a planets mecury venus
I explained that to make an array named "planets" you can simply use "planets=( mecury venus )" or else use declare -a planets; planets[0]=mercury etc, etc. set doesn't work for this.

I looked up the man page of the set command
It's a Bash shell builtin: run "help set". BTW, Bash arrays start count at element number zero.

koodoo · 08-14-2006, 09:28 AM

Thanks a lot for the reply. That answer clarified a lot of things