How to Divide in the Linux Console

How can I do division with variables in a Linux shell?

Those variables are shell variables. To expand them as parameters to another program (ie expr), you need to use the $ prefix:

expr $x / $y

The reason it complained is because it thought you were trying to operate on alphabetic characters (ie non-integer)

If you are using the Bash shell, you can achieve the same result using expression syntax:

echo $((x / y))

Or:

z=$((x / y))
echo $z

Divide two variables in bash

shell parsing is useful only for integer division:

var1=8
var2=4
echo $((var1 / var2))

output:
2

instead your example:

var1=3
var2=4
echo $((var1 / var2))

ouput:
0

it's better to use bc:

echo "scale=2 ; $var1 / $var2" | bc

output:
.75

scale is the precision required

How do I divide the output of a command by two, and store the result into a bash variable?

middle=$((`wc -l < file` / 2))

How to divide with awk?

You can use this awk:

awk '$0+0 == $0 { printf "%.3f\n", $0 / .03 }' file
-31137.346
-31068.467
-31007.352

$0+0 == $0 will make sure to execute this division for lines with valid numbers only.
printf "%.3f" will print result with 3 precision points.

Best way to divide in bash using pipes?

Using bc:

$ bc -l <<< "scale=2;$(find . -name '*.mp4' | wc -l)/3"
2.33

In contrast, the bash shell only performs integer arithmetic.

Awk is also very powerful:

$ find . -name '*.mp4' | wc -l | awk '{print $1/3}'
2.33333

You don't even need wc if using awk:

$ find . -name '*.mp4' | awk 'END {print NR/3}'
2.33333

division in shell script

Bash does integer math, not floating point. You will need to use awk or bc to provide floating point output. e.g. with bc

scalingFactor=$(printf "scale=3; %d/%d\n" $normNum $mappedReads | bc)

In bash how do I divide two variables and output the answer rounded upto 5 decimal digits?

The problem here is that you missed the echo (or printf or any other thing) to provide the data to bc:

$ echo "scale=5; 12/7" | bc
1.71428

Also, as noted by cnicutar in comments, you need to use $ to refer to the variables. sum is a string, $sum is the value of the variable sum.

All together, your snippet should be like:

sum=12
n=7
output=$(echo "scale=5;$sum/$n" | bc)
echo "$output"

This returns 1.71428.

Otherwise, with "scale=5;sum/n"|bc you are just piping an assignment and makes bc fail:

$ "scale=5;sum/n"|bc
bash: scale=5;sum/n: No such file or directory

You then say that you want to have the result rounded, which does not happen right now:

$ sum=3345699
$ n=1000000
$ echo "scale=5;($sum/$n)" | bc
3.34569

This needs a different approach, since bc does not round. You can use printf together with %.Xf to round to X decimal numbers, which does:

$ printf "%.5f" "$(echo "scale=10;$sum/$n" | bc)"
3.34570

See I give it a big scale, so that then printf has decimals numbers enough to round properly.

How to divide a column to a number in bash shell if the other column had a condition true?

Try this:

awk '$1 =="G" {$8 = $7 / 255} 1' file > new_file

How to divide columns by a constant in bash

In awk:

$ awk '{for(i=2;i<=NF;i++)$i/=3}1' file
ERR843978.746 5 18 23.6667
ERR843978.1705 2.33333 15 17
ERR843978.5030 1 15.3333 19.3333
ERR843978.8855 5 16 17.3333
ERR843978.4162 5.33333 16 21.3333
ERR843978.421 4.66667 17 18
ERR843978.4599 4 20 15.3333

Pipe it to column for nices output:

$ awk ... | column -t
ERR843978.746   5        18       23.6667
ERR843978.1705  2.33333  15       17
ERR843978.5030  1        15.3333  19.3333