Re: [Help-bash] something about bc

[Greg Wooledge]

On Wed, Dec 28, 2011 at 09:56:00AM -0600, address@hidden wrote:
> On Wed, 28 Dec 2011, lina wrote:
> >some wrong try:
> >
> >$ a=1.5 ; a+=1 ; echo $a
> >1.51

You are doing string concatenation here.  The two strings "1.5" and "1"
are concatenated to make "1.51".

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

Here, you ran three separate commands:

a=1.5        # variable assignment
a+=1 | bc    # variable assignment piped to bc
echo $a      # expand a variable, perform word splitting and globbing, write

Since the second variable assignment does not write anything to standard
output, the pipe to bc has no input, and therefore bc does nothing.

Since the second variable assignment was performed inside a pipeline, it
takes place in a subshell (child process), and no change to the variable
takes place in the main shell.

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

Bash cannot perform floating-point arithmetic.  It only has integer
arithmetic.

imadev:~$ let a=1.5+1
bash: let: a=1.5+1: syntax error: invalid arithmetic operator (error token is 
".5+1")

If you want to perform floating-point math, you have to use bc *instead of*
let, not *in addition to* let.

> echo "a=1.5; a+=1; print a;" | bc

This must be GNU bc.  It doesn't work with standard bc:

imadev:~$ echo "a=1.5; a+=1; print a;" | bc
syntax error on line 1, 

Here is how it works with standard bc:

imadev:~$ echo "a=1.5; a=a+1; a" | bc
2.5

Or more simply:

imadev:~$ echo "1.5+1" | bc
2.5

And, taking a wild-ass guess, here is what I think lina wanted to do:

imadev:~$ a=1.5
imadev:~$ a=$(echo "$a + 1" | bc)
imadev:~$ echo "$a"
2.5