Loop through variable 2 elements at a time in bash - bash

Question

I have a bash variable which is a white-space separated list of strings and would like a loop that iterates 2 elements at a time from that list. I've made sure the length of the list is divisible by 2. So I want something like:
x="a bb cccc d"
while read first second; do
echo "($first,$second)"
done <<< $x
output should be:
(a,bb)
(cccc,d)
currently the above yields:
(a,bb cccc d)
Note: I need the assignment of $first and $second in my loop. echo was put in as a placeholder.
I'm looking for an efficient answer (preferably without a counter).

Answer 1

Depending on what you want to do with the values, you could do something like this:
$ x="one two buckle myShoe"
$ xargs -n 2 printf "(%s,%s)\n" <<<$x
(one,two)
(buckle,myShoe)
I assume that you really want to assign the values to shell variables, though, and that the body of your anticipated loop is more complicated than a simple formated print. In fact, since printf automatically repeats the format until its arguments are exhausted, xargs in the above example was unnecessary; it could have just been:
printf "(%s,%s)\n" $x
Unfortunately, xargs is not a built-in, so you can't xargs an arbitrary bash pipeline. Nor can you just define a shell function and give xargs its name. You can, however, xargs a bash subshell, which provides a very general solution:
# Shell function which takes two arguments
doit() {
echo "Number of arguments: $#"
echo "First argument: $1"
echo "Second argument: $2"
}
# Make the function visible to subshells
export -f doit
x="one two buckle myShoe"
xargs -n2 bash -c 'doit "$#"' _ <<<$x
Finally, as per a discussion in comments, you can use either printf or xargs (but probably printf is more efficient, since it is a builtin) to reorganize the list into pairs, and then feed that into a while read loop:
printf "%s %s\n" $x |
while read -r first second; do
echo "($first,$second)"
done

Answer 2

Using shift and $1, $2, etc.
You can use set -- $x to assign each part of x to the bash variables $1, $2, $3, etc.
You can use shift <n> to shift the numbered variables down by n. (I.e. shift 2 moves the value of $3 to $1, and $4 to $2, etc.)
Example
x="a bb cccc d"
set -- $x
while [ ! -z "$1" ] # while $1 is not empty
do
# do whatever you want here with $1 and $2. you can take more
# than two at a time by calling shift with a higher argument
# (and use $1, $2, and $3).
echo "($1,$2)"
shift 2
done
This prints:
(a,bb)
(cccc,d)
If you need the command line arguments
Save them to a temporary variable:
ARGS=( "$#" ) # save the command line args
set -- $x
...
set -- "${ARGS[#]}" # restore them back

Answer 3

You can tell read to read specific # of characters:
while read -n4 first second; do echo "($first,$second)"; done <<< "$x"
(a,b)
(c,d)
EDIT: For generic solution use awk:
x="aa bb cccc de"
awk '{for (i=1; i<=NF; i+=2) printf "(%s,%s)\n", $i, $(i+1) }' <<< "$x"
(aa,bb)
(cccc,de)

Answer 4

I would declare your variable like this :
x="a b c d"
A=($x)
And then
$ for ((i=0; i<${#A[*]}; i=i+2)); do echo "("${A[$i]},${A[$i+1]}")"; done
(a,b)
(c,d)