[SOLVED] Bash subtract negative number using bc

porphyry5 · 05-15-2011, 07:11 AM

How can i get bc to subtract negative numbers?

Code:

g  echo $bal $amt
347.87 -742
g  echo "$bal-$amt" | bc
(standard_in) 1: syntax error
g  amt=742
g  echo "$bal-$amt" | bc
-394.13
g

catkin · 05-15-2011, 07:17 AM

bc needs a space between the subtraction operator and the unary subtraction operator:

Code:

c@CW8:~$ bal=347.87; amt=-742; echo "$bal-$amt" | bc
(standard_in) 1: syntax error
c@CW8:~$ bal=347.87; amt=-742; echo "$bal-i $amt" | bc
-394.13

EDIT: Oops! vi editing at the command line! Should have been:

Code:

c@CW8:~$ bal=347.87; amt=-742; echo "$bal-$amt" | bc
(standard_in) 1: syntax error
c@CW8:~$ bal=347.87; amt=-742; echo "$bal- $amt" | bc
1089.87

druuna · 05-15-2011, 07:20 AM

Hi,

Yopu need spaces around the - in the echo statement:

Code:

$ A="347.87"
$ B="-742"
$ echo "$A-$B" | bc
(standard_in) 1: parse error
$ echo "$A - $B" | bc
1089.87

/Edit: Beaten by catkin

porphyry5 · 05-15-2011, 07:45 AM

Quote:

Originally Posted by catkin

bc needs a space between the subtraction operator and the unary subtraction operator:

Code:

c@CW8:~$ bal=347.87; amt=-742; echo "$bal-$amt" | bc
(standard_in) 1: syntax error
c@CW8:~$ bal=347.87; amt=-742; echo "$bal-i $amt" | bc
-394.13

EDIT: Oops! vi editing at the command line! Should have been:

Code:

c@CW8:~$ bal=347.87; amt=-742; echo "$bal-$amt" | bc
(standard_in) 1: syntax error
c@CW8:~$ bal=347.87; amt=-742; echo "$bal- $amt" | bc
1089.87

Thank you. Two consecutive - signs must upset it, as it doesn't require that space separation when the number is unsigned. But its more prudent to use space separation always.

porphyry5 · 05-15-2011, 07:48 AM

Quote:

Originally Posted by druuna

Hi,

Yopu need spaces around the - in the echo statement:

Code:

$ A="347.87"
$ B="-742"
$ echo "$A-$B" | bc
(standard_in) 1: parse error
$ echo "$A - $B" | bc
1089.87

/Edit: Beaten by catkin

Thank you for your help.

catkin · 05-15-2011, 07:49 AM

Quote:

Originally Posted by porphyry5

But its more prudent to use space separation always.

That would be a good KISS convention.

druuna · 05-15-2011, 07:56 AM

Hi,

Quote:

Originally Posted by porphyry5

Thank you for your help.

You're welcome

porphyry5 · 05-15-2011, 08:16 AM

Quote:

Originally Posted by catkin

That would be a good KISS convention.

Meaning that space separation is generally true for all linux command-line apps? I find it confusing trying to keep track of situations that require it and those that don't. Learning by example from the net is convenient, but examples that don't use space separation carry the implication that in that particular situation one shouldn't.

Telengard · 05-15-2011, 09:33 AM

In bc it is always safe to enclose quantities in parentheses.

Quote:

foo$ minuend=5
foo$ subtrahend=-5
foo$ echo "$minuend-($subtrahend)"
5-(-5)
foo$ echo "$minuend-($subtrahend)" | bc
10
foo$

catkin · 05-15-2011, 09:52 AM

Quote:

Originally Posted by porphyry5

Meaning that space separation is generally true for all linux command-line apps? I find it confusing trying to keep track of situations that require it and those that don't. Learning by example from the net is convenient, but examples that don't use space separation carry the implication that in that particular situation one shouldn't.

There are few situations where it is wrong to include a space; the only ones that come to mind right now (there are probably others) are bash assignments (foo=bar, not foo = bar), awk function calls (myfunc( ) not myfunc ( ) ) and dd's if=<file name> etc. not if = <file name> (but that's derived from IBM mainframe JCL) and sometimes long options need --<option>=<value>. OTOH there are situations where it is wrong not to have a space such as bash' [[ <test expression> ]] not [[<test expression>]] and bc as you have found out.

porphyry5 · 05-17-2011, 11:12 AM

Quote:

Originally Posted by catkin

There are few situations where it is wrong to include a space; the only ones that come to mind right now (there are probably others) are bash assignments (foo=bar, not foo = bar), awk function calls (myfunc( ) not myfunc ( ) ) and dd's if=<file name> etc. not if = <file name> (but that's derived from IBM mainframe JCL) and sometimes long options need --<option>=<value>. OTOH there are situations where it is wrong not to have a space such as bash' [[ <test expression> ]] not [[<test expression>]] and bc as you have found out.

Yeah, immediately after I wrote my previous post I tried foo = bar in bash and saw my hopes dashed. But thanks for this list; even if there are other examples this still brings a lot of order to an area that seemed quite chaotic to me.