Simple math statements in bash in a for loop

bash math syntax for-loop

30,394

Solution 1

Here's a decent solution:

for i in {1..20}
do
   for j in {1..20}
   do
       echo "scale = 3; sqrt($i*$i + $j*$j)" | bc
   done
done

Output will be:

1.414
2.236
3.162
2.236
[...etc...]

Solution 2

Arithmetic expansion needs $((...)) notation, so something like:

echo $((i*i + j*j))

However, bash only uses integers so you may need to use an external tool such as dc.

E.g.

dc -e "18k $i $i * $j $j * + v p"

Solution 3

Shell math can be done in several ways.

echo $(( i*i + j*j ))
echo $[ i*i + j*j ]
expr "$i" '*' "$i" '+' "$j" '*' "$j"

However, this can only handle integer arithmetic. Instead, you can use bc:

echo "scale = 5; sqrt( $i*$i + $j*$j)" | bc

Change scale to the number of decimal places desired.

Solution 4

#!/bin/bash
for i in {1..20}; do
    for j in {1..20}; do
        echo 5k$i $i\* $j $j\*+vp | dc
    done
done

Solution 5

Use double paren to evaluate a variable.

variableA=$((variableB*variableC))

Only for ints though.

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30,394

Author by

physicsmichael

Particle physics in my rear view mirror; data science in my headlights. My mother tongue is python, but I used C++ often while I was stuck/blessed with ROOT in physics. I dabble in Tex, Arduino, Raspi, bash, cluster computing, and more!

Updated on July 13, 2020

Comments

physicsmichael almost 4 years
I'm quite new to bash scripting and usually avoid it all costs but I need to write a bash script to execute some simple things on a remote cluster. I'm having problems with a for loop that does the following:
```
for i in {1..20}
do
    for j in {1..20}
    do
        echo (i*i + j*j ) **.5  <--- Pseudo code!
    done
done
```
Can you help me with this simple math? I've thrown $'s everywhere and can't write it properly. If you could help me understand how variables are named/assigned in bash for loops and the limitations of bash math interpretation (how do you do the square root?) I'd be very grateful. Thanks!
dustmachine over 14 years

You're probably missing the .. between your 1 and 20 in your {1..20} range
ephemient over 14 years

Weird, it should work in any POSIX bc. What if you use dc? i.e. dc -e "5 k $i $i * $j $j * + v p"
ephemient over 14 years

"almost certainly"? Certainly not standard on any Linux distribution I've seen. Perhaps you mean *BSD?
physicsmichael over 14 years

Bah. I shorted the j loop so it wouldn't flood the screen and introduced a typo.
wallyk over 14 years

ksh is on Fedora and CentOS.
ephemient over 14 years

It's available as a package, but not part of the default install. Most Linux systems use Bash as the default shell; the BSDs tend to gravitate around various non-GNU shells.
Chris Johnsen over 14 years

My variant: printf "%s %s 10kd*rd*+vp" "$i" "$j" | dc