bash: value too great for base (error token is ...)

value too great for base (error token is "08") means the arithmetic expression contains a number that is not valid in the base bash chose for it. The usual trigger is a leading zero: bash treats 08 and 09 as octal numbers, and 8 and 9 are not octal digits. Prefix the value with 10# to force decimal, for example $((10#$month + 1)).

Reproduce the error

month=08
echo $((month + 1))

bash: line 2: 08: value too great for base (error token is "08")

The error token is the exact text bash could not read. With x=3abc the token is 3abc: text that starts with a digit is also parsed as a number first. Text that starts with a letter is treated as a variable name instead, and an unset name evaluates to 0, which is why x=abc gives 1 for $((x + 1)) instead of an error.

How bash picks the base

Only the leading zero is a trap. Any value that comes from date, a log file, a CSV column or a zero-padded loop such as seq -w carries it.

Fix it

Force decimal with 10#:

month=08
echo $((10#$month + 1))

9

Or remove the padding before using the value. Parameter expansion with ${var#0} removes one leading zero, which is enough for two-digit values such as months, days and hours (10# is the safer choice for longer padding):

hour=09
hour=${hour#0}
echo $((hour + 1))

10

Confirm the fix by printing the result: bash returns a number and exit status 0. If the value can be empty, give it a default, ${hour:-0}, because an empty value inside 10# is another error (10#: invalid integer constant).

Validate input from users and files

When the number is not under your control, check it before the calculation:

if [[ $count =~ ^[0-9]+$ ]]; then
  echo $((10#$count * 2))
else
  echo "not a number: $count" >&2
fi

This also stops values with signs, spaces and text from reaching the arithmetic parser. Related comparison errors in [ ] look different: integer expression expected comes from test, not from $(( )).

Platform notes

The octal rule is the same in bash on Linux, macOS and Git Bash. date +%m and date +%d return zero-padded values everywhere, so the error is common in scripts that compute dates.

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