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arraysbashsyntax
Katie Byers
Katie Byers

Reputation: 1010

Explain ${#arrayname[@]} syntax for array length in bash?

I know that one can get the length of an array in bash by doing ${#arrayname[@]}.

My question is: is this just something that I have to memorize, or can this syntax be broken down into understandable parts? For instance, what does the @ symbol mean where one would expect to find the index? Why the #?

Upvotes: 10

Views: 8312

Answers (4)

clt60
clt60

Reputation: 63892

You should memorize. :) The # usually means number. e.g. the

  • $# - is the number of arguments
  • ${#str} - length of the string $str
  • ${#arr[@]}" - length (number of elements) of the array arr
  • ${#arr} - the length of the 1st element of the array (like the str above)

Unfortunately the ${parameter#word} or ${parameter##word} has nothing with numbers. (it removes the shortest/longest word from the beginning of the parameter.

And also, the # .... is comment ;)

Upvotes: 9

Barmar
Barmar

Reputation: 780724

# at the beginning of a variable reference means to get the length of the variable's value. For a normal variable this means its length in characters. # is the "number" sign, so you can remember this as meaning "the number of things in the variable".

@ or * in an array index means to use the whole array, not a specific element, and instead of returning the number of characters, it returns the number of array elements. * is used as a wildcard in many contexts, so this should be easy to remember. Also, $* and $@ are used to mean all the arguments to a shell script, so the parallel with all the array elements should be obvious.

You can't just write ${#arrayname} because when you use an array variable without a subscript, it's equivalent to element 0 of the array. So ${#arrayname} is the same as ${#arrayname[0]}, which is the number of characters in the first element of the array.

Upvotes: 20

Inian
Inian

Reputation: 85550

In general usage of form ${#PARAMETER} returns the length in number of characters and NOT bytes of the parameter's value.

myString="Hello StackOverflow!"
printf "%s\n" "${#myString}"
20

But for arrays, this expansion type has two meanings:

  1. For individual elements, it reports the string length of the element (as for every "normal" parameter)
  2. For the mass subscripts @ and * it reports the number of set elements in the array

Consider an example over arrays,

myArray=(1 2 3 4 15)
printf "%s\n" "${myArray[@]}"    # <-- Gives me list of elements 
1
2
3
4
15
printf "%s\n" "${#myArray[@]}"   # <-- Gives me number of elements
5

It gets interesting now, the length of the last element 2 can be obtained by doing

printf "%s\n" "${#myArray[4]}"
2

Upvotes: 4

Brian H
Brian H

Reputation: 1041

The '@' acts the same way as '*'. Instead of providing a specific index this references the full thing.

The '#' is telling bash you want the length

https://www.cyberciti.biz/faq/finding-bash-shell-array-length-elements/

Upvotes: 1

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