Meaning of word=$word"`expr substr '${ -b :board}' 1 3`"?

Question

I am working on some bash scripting conversion to C++ and came across this line...

word="yahoo"

word=$word"`expr substr '${ -b :board}' 1 3`"

I understand what expr substr does, but the argument that I provide "${ -b :board}" does not make any sense to me.

When I tried to run that on terminal:

 echo $word

Output:

 yahoo${

I would appreciate any input, thanks.

Answer 1

This question is not about Bash or sh, but about expr, a standalone command that is part of, e.g., the GNU Coreutils. If we consult the manual, we find

expr evaluates an expression and writes the result on standard output. Each token of the expression must be a separate argument.

and

substr string position length
Returns the substring of string beginning at position with length at most length. If either position or length is negative, zero, or non-numeric, returns the null string.

So the command

expr substr '${ -b :board}' 1 3

takes the string ${ -b :board} and extracts a substring of length 3, starting at position 1, which is ${ .

The command

word=$word"`expr substr '${ -b :board}' 1 3`"

puts the expr command into a command substitution (the backticks) and appends the result to the expansion of $word, which at this point contains yahoo, so that's how you end up with yahoo${ .

This all being said, I don't see the reason to do it like that. The output of the expr command is a constant string, so you could really just replace everything with

word='yahoo${ '

---

As a side note, in modern Bash, you could get the same functionality with parameter expansion:

word='yahoo'
var='${ -b :board}'
word+=${var:0:3}

But the result is the same, and without more context seems to not make any sense in the first place.