When the shell did the arithmetic anyway

Shell arithmetic has a quiet rule: by the time an expression is evaluated, every $ in it has already been substituted. So a $ you still see in the expression is one you deliberately protected — with a backslash, or with quotes — and bash refuses to expand it a second time.

huck expanded it anyway. The worst shape was a test:

bash

# before
$ x=5
$ [[ '$x' -eq 5 ]]; echo "rc=$?"
rc=0

bash

# after
$ x=5
$ [[ '$x' -eq 5 ]]; echo "rc=$?"
huck: line 1: [[: $x: syntax error: operand expected (error token is "$x")
rc=1

The quotes were there to compare the literal text $x. huck read the variable instead, so the test silently passed. The same happened inside an expression:

bash

# before
$ x=5; echo $(( 1 + "\$x" ))
6

bash

# after
$ x=5; echo $(( 1 + "\$x" ))
huck: line 1: 1 + $x : syntax error: operand expected (error token is "$x ")

Integer variables that swallowed their errors

An integer-flagged variable evaluates whatever you assign to it. When that failed, huck said nothing and stored zero:

bash

# before
$ declare -i total=99
$ total=$(echo 1.5)
$ echo "total=$total"
total=0

bash

# after
$ declare -i total=99
$ total=$(echo 1.5)
huck: line 1: 1.5: syntax error: invalid arithmetic operator (error token is ".5")
$ echo "total=$total"
total=99

A script reading a number from somewhere could not tell a real zero from a value it failed to parse. The += form had the same hole, in a different place: declare -i n=5; n+=@ quietly left n at 5.

Saying which part failed

When arithmetic did fail inside a test or a for loop header, huck named the problem but not the expression:

bash

# before
$ [[ @ -eq 5 ]]; echo "rc=$?"
huck: line 1: [[: syntax error: operand expected
rc=2

bash

# after
$ [[ @ -eq 5 ]]; echo "rc=$?"
huck: line 1: [[: @: syntax error: operand expected (error token is "@")
rc=1

The exit status was wrong too — bash treats a broken comparison as a false-with-error (1), not a usage error (2).

Still open

This pass filed eight neighbouring issues. The one most worth knowing about: a failed operand still abandons the whole test, where bash reports it and keeps going, so [[ @ -eq 5 || 1 -eq 1 ]] is true in bash and false in huck (#718).

Also open: a quoted array subscript is still evaluated (#699), an unusable regex prints the underlying library's error where bash is silent (#716), an unterminated bracket in a pattern errors where bash treats it as a literal (#717), and a number with trailing junk gets the wrong message (#720).