Re: [Help-bash] something about bc

[Eric Blake]

On 12/28/2011 08:44 AM, lina wrote:
> Hi,
> 
> I tried but still don't know how to achieve it with *bc*
> 
> some wrong try:
> 
> $ a=1.5 ; a+=1 ; echo $a
> 1.51

This says make a shell variable 'a' with contents "1.5", then take those
contents and concatenate "1", for an end result of "1.51".

> 
> $ a=1.5 ; a+=1 | bc; echo $a
> 1.5

This declares a shell variable a with contents "1.5", then executes a
pipeline (which spawns a subshell); in the pipeline, you pass the output
of appending to a (there is none) to bc; and since it was a subshell,
the parent shell saw no change to $a.

> 
> $ a=1.5 ; let "a+=1" | bc; echo $a
> bash: let: 1.5: syntax error: invalid arithmetic operator (error token
> is ".5")
> 1.5

Like the last time, except by using 'let' instead of variable
concatenation, you are now forcing bash to attempt a numeric operation
on the contents of $a; but bash doesn't do floating point.

If you want bc to math, then pass the _entire_ bc script in to bc:

echo 'a=1.5; a+=1; print a' | bc

-- 
Eric Blake   address@hidden    +1-919-301-3266
Libvirt virtualization library http://libvirt.org

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