Can Bash substring offset be omitted?

Question

str=abcde
echo ${str:0:2} # output: ab
echo ${str::2} # outpub: ab

The code above leads to same result. As the documentation describes:

This is referred to as Substring Expansion. It expands to up to length characters of the value of parameter starting at the character specified by offset. If parameter is @ or *, an indexed array subscripted by @ or *, or an associative array name, the results differ as described below. If length is omitted, it 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 Shell Arithmetic).

If offset evaluates to a number less than zero, the value is used as an offset in characters from the end of the value of parameter. If length evaluates to a number less than zero, it is interpreted as an offset in characters from the end of the value of parameter rather than a number of characters, and the expansion is the characters between offset and that result. Note that a negative offset must be separated from the colon by at least one space to avoid being confused with the :- expansion.

There is no description about omitting offset but in fact it can be omitted.

I wonder if there are any documents I haven't noticed.

Answer 1

In the Arithmetic Evaluation section of the manual it is mentioned that a null value is interpreted as 0, and as we know the argument of that parameter expansion is subject to arithmetic evalation.

A shell variable that is null or unset evaluates to 0 when referenced by name without using the parameter expansion syntax.

A null value evaluates to 0.

This might be the closest reference you can get as to why an empty value evaluates to 0.

However, I still think that allowing empty arguments should still be documented.

Answer 2

When you have a look at man bash you can read:

${parameter:offset}, ${parameter:offset:length} Substring Expansion. Expands to up to length characters of the value of parameter starting at the character specified by offset. If parameter is @, an indexed array subscripted by @ or *, or an associative array name, the results differ as described below. 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).

It must be made clear that it is not possible to omit the offset value. Here, omit implies that also the <colon>-character is missing. From the table below, you see that there is an ambiguity in this case:

	offset	omitted offset
length	${parameter:offset:length}	${parameter:length}
omitted length	${parameter:offset}	${parameter}

From a syntactic point of view, you cannot omit length as you can not distinguish whether length or offset is omitted.

It is possible to leave it empty. man bash clearly states that both length and offset are arithmetic expressions and in that section we find.

A null value evaluates to 0.

This entails both unset variables as well as empty expressions:

$ unset v
$ echo $(( v )) $(( ))
0 0

As offset is a arithmetic expression, an empty value will valuate to the same value as $(( )) which is 0.

So the following are all equivalent:

${parameter:0:length} == ${paramter::length}
${parameter:offset:0} == ${parameter:offset:} == ""
${parameter:0:0} == ${parameter::} == ""