重击比较

Question

I have a variable which stores the output of a command. How do I compare it with a float?

To be more specific I am doing

x=$(tail -n 1 foo| cut -d ' ' -f2)

if (($x < 0)); then ...

where foo is a filename. On doing the above I get the following error

-0.08 < 0 : syntax error: invalid arithmetic operator (error token is "0.08 < 0")

The value I need to compare is -0.08 , but the error token is different

What should I do for such comparisons?

Answer 1

bash doesn't support floating point arithmetics.
You can however use bc which is an external program to do arithmetics.

if (( $(bc <<< "$x < 0") )); then 
    printf "%f is less than 0\n" "$x"; 
fi

from the man page:

The relational operators are

  expr1 < expr2 The result is 1 if expr1 is strictly less than expr2. expr1 <= expr2 The result is 1 if expr1 is less than or equal to expr2. expr1 > expr2 The result is 1 if expr1 is strictly greater than expr2. expr1 >= expr2 The result is 1 if expr1 is greater than or equal to expr2. expr1 == expr2 The result is 1 if expr1 is equal to expr2. expr1 != expr2 The result is 1 if expr1 is not equal to expr2. 

---

one can also use awk that also supports floating point arithmetics.

Answer 2

If ksh is available to you, you can use it to write your script instead of Bash since it supports floats. Zsh also supports floats.

#!/usr/bin/ksh
x=$(tail -n 1 foo| cut -d ' ' -f2)

if ((x < 0))
then
    echo "less than"
fi