Bash tilde *substring* expansion - undocumented feature?

Question

I was surprised that this expansion:

$ echo "${foo:~abc}"

yielded the empty string when foo was unset. I expected that it would parse like this:

$ echo "${foo:(~abc)}"

and yield the string "~abc". But instead, I found that if I did define

$ foo='abcdefg'
$ echo "${foo:~abc}"
g

In fact, it's taking "abc" in arithmetic context and doing. "${foo:~0}". Likewise

$ foo='abcdefg'
$ echo "${foo:~3}"
defg

It gets you the last n+1 characters of the expansion. I looked in the "Parameter Expansion" section of the manpage. I see no mention of tildes there. Bash Hackers Wiki only mentions tildes as (also undocumented) case modifiers.

This behavior goes back to at least 3.2.57.

Am I just missing where this form of substring expansion is documented, or is it not documented at all?

Answer 1

It's not undocumented (you may have been confusing ${foo:~abc} with ${foo-~abc}).

${parameter:offset}
${parameter:offset:length}
     Substring Expansion.  Expands to up to length characters of  the
     value  of  parameter starting at the character specified by off-
     set.  [...]  If length is omitted, expands to the  substring  of  the
     value of parameter starting at the character specified by offset
     and extending to the end of the value.  length  and  offset  are
     arithmetic expressions (see ARITHMETIC EVALUATION below).

Here, ~abc is the offset field of the expansion, and ~ is the bitwise negation operator in the arithmetic expression. An undefined parameter evaluates to 0 in an arithmetic expression, and ~0 == -1.