Expansion and Data

Arithmetic

Updating Counters

Bash arithmetic is integer arithmetic. It is useful for counters, totals, and small control decisions.

Program

Play the script to watch count change, then feed a second calculation.

count
arithmetic.sh
Replay: real traced execution (multi-file project)
#!/usr/bin/env bash

count=7
count=$((count + 5))
half=$((count / 2))
echo "$count:$half"

#!/usr/bin/env bash

count=3
count=$((count + 5))
half=$((count / 2))
echo "$count:$half"

#!/usr/bin/env bash

count=11
count=$((count + 5))
half=$((count / 2))
echo "$count:$half"

  1. count ← 7

    3count=74count=$((count + 5))
    values this step7count

  2. count ← 12

    3count=74count=$((count + 5))5half=$((count / 2))
    values this step7 → 12count

  3. half ← 6

    4count=$((count + 5))5half=$((count / 2))6echo "$count:$half"
    values this step6half12count

  4. echo "$count:$half"

    5half=$((count / 2))6echo "$count:$half"
    output12:6
    values this step12count6half

  1. count ← 3

    3count=34count=$((count + 5))
    values this step3count

  2. count ← 8

    3count=34count=$((count + 5))5half=$((count / 2))
    values this step3 → 8count

  3. half ← 4

    4count=$((count + 5))5half=$((count / 2))6echo "$count:$half"
    values this step4half8count

  4. echo "$count:$half"

    5half=$((count / 2))6echo "$count:$half"
    output8:4
    values this step8count4half

  1. count ← 11

    3count=114count=$((count + 5))
    values this step11count

  2. count ← 16

    3count=114count=$((count + 5))5half=$((count / 2))
    values this step11 → 16count

  3. half ← 8

    4count=$((count + 5))5half=$((count / 2))6echo "$count:$half"
    values this step8half16count

  4. echo "$count:$half"

    5half=$((count / 2))6echo "$count:$half"
    output16:8
    values this step16count8half

Follow the Counter

  1. count starts at 7.
  2. The arithmetic update adds 5, so count becomes 12.
  3. half=$((count / 2)) uses integer division.
  4. 12 / 2 gives 6.
  5. The script prints 12:6. | starting count | after + 5 | half | output | | --- | --- | --- | --- | | 3 | 8 | 4 | 8:4 | | 7 | 12 | 6 | 12:6 | | 11 | 16 | 8 | 16:8 |
integer arithmetic Bash arithmetic drops fractional parts because it works with integers.
counter update A variable can be read and assigned again in the same arithmetic expression.

Exercise: arithmetic.sh

Reproduce the default output 12:6, then try the listed count values 3 and 11 and predict each output before running it.