Bash error: "let: division by 0"

Error String

bash: let: i++: division by 0 (error token is "0")

Tldr

Bash's let and $(( )) arithmetic evaluate expressions using integer math, so any division or modulo by zero — including accidental ones from typos like a stray / where you meant a different operator — aborts with this error. Guard the divisor or fix the expression, and remember `let x++` returns a false exit status when x was 0 before incrementing, which trips set -e.

Cause

Arithmetic contexts (let, $(( )), (( ))) in Bash perform C-style integer arithmetic and raise "division by 0" for any / or % with a zero right-hand side. A related and very common gotcha is unrelated: `let "i++"` evaluates to the pre-increment value as its exit status, so `let "i++"` when i is 0 returns exit status 1 (failure), which under set -e kills the script even though no division happened — easy to misdiagnose as this error.

Repro

#!/usr/bin/env bash
set -e
total=0
count=0
average=$((total / count))   # bash: division by 0

Fix

#!/usr/bin/env bash
set -euo pipefail

total=0
count=0

# Guard against zero before dividing
if (( count == 0 )); then
  average=0
else
  average=$((total / count))
fi

# For the let ++ / set -e gotcha, use (( )) with || true, or ((i++)) ||:
i=0
((i++)) || true    # does not abort the script even though i started at 0

Explanation

Always validate a divisor is non-zero before using it in $(( )) or let, especially when it comes from user input, a loop counter, or a count of matched items that could legitimately be zero. Separately, remember that (( expr )) and let return the arithmetic truthiness of the *result* as their exit status, which combined with set -e can end scripts unexpectedly on expressions like `((count--))` when count starts at 1.

Related Shellcheck

SC2004

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