How can I join elements of an array in Bash?

Question

If I have an array like this in Bash:

FOO=( a b c )

How do I join the elements with commas? For example, producing a,b,c.

Answer 1

If you build the array in a loop, here is a simple way:

arr=()
for x in $(some_cmd); do
   arr+=($x,)
done
arr[-1]=${arr[-1]%,}
echo ${arr[*]}

Answer 2

Here's a 100% pure Bash function that does the job:

join() {
    # $1 is return variable name
    # $2 is sep
    # $3... are the elements to join
    local retname=$1 sep=$2 ret=$3
    shift 3 || shift $(($#))
    printf -v "$retname" "%s" "$ret${@/#/$sep}"
}

Look:

$ a=( one two "three three" four five )
$ join joineda " and " "${a[@]}"
$ echo "$joineda"
one and two and three three and four and five
$ join joinedb randomsep "only one element"
$ echo "$joinedb"
only one element
$ join joinedc randomsep
$ echo "$joinedc"

$ a=( $' stuff with\nnewlines\n' $'and trailing newlines\n\n' )
$ join joineda $'a sep with\nnewlines\n' "${a[@]}"
$ echo "$joineda"
 stuff with
newlines
a sep with
newlines
and trailing newlines


$

This preserves even the trailing newlines, and doesn't need a subshell to get the result of the function. If you don't like the printf -v (why wouldn't you like it?) and passing a variable name, you can of course use a global variable for the returned string:

join() {
    # $1 is sep
    # $2... are the elements to join
    # return is in global variable join_ret
    local sep=$1 IFS=
    join_ret=$2
    shift 2 || shift $(($#))
    join_ret+="${*/#/$sep}"
}

Answer 3

Shorter version of top answer:

joinStrings() { local a=("${@:3}"); printf "%s" "$2${a[@]/#/$1}"; }

Usage:

joinStrings "$myDelimiter" "${myArray[@]}"

Answer 4

Right now I'm using:

TO_IGNORE=(
    E201 # Whitespace after '('
    E301 # Expected N blank lines, found M
    E303 # Too many blank lines (pep8 gets confused by comments)
)
ARGS="--ignore `echo ${TO_IGNORE[@]} | tr ' ' ','`"

Which works, but (in the general case) will break horribly if array elements have a space in them.

(For those interested, this is a wrapper script around pep8.py)

Answer 5

$ foo=(a "b c" d)
$ bar=$(IFS=, ; echo "${foo[*]}")
$ echo "$bar"
a,b c,d

Answer 6

x=${"${arr[*]}"// /,}

This is the shortest way to do it.

Example,

arr=(1 2 3 4 5)
x=${"${arr[*]}"// /,}
echo $x  # output: 1,2,3,4,5