In a bash script, loop through a bash array for input to a binary

Question

I'm running bash.

• Suppose I have a binary mybinary that takes interactive user input.
• Suppose that mybinary takes attendee names as input until the user types q and presses Enter (q for "quit").
• I can automate the input process in a bash script by surrounding the input by <<EOF/EOF. For example, suppose I want to enter predefined user input from an array myarray. I can do:

#!/bin/bash

myarray=("Alice" "Bob" "Charlie" "q") # q quits the binary

mybinary -f inputfile.txt -o outputfile.txt<<EOF
${myarray[0]}
${myarray[1]}
${myarray[2]}
${myarray[3]}
EOF

I tested the above, and it works. But it's laborious to type ${myarray[X]} for every index in myarray if myarray has hundreds of (string) elements.

How can I modify the above bash script to loop through myarray?

The following won't work because the for, do, and done parts will be taken as input to mybinary:

#!/bin/bash

myarray=("Alice" "Bob" "Charlie" "q") # q quits the binary

mybinary -f inputfile.txt -o outputfile.txt<<EOF
for element in "${myarray[@]}"
do
${myarray[0]}
${myarray[1]}
${myarray[2]}
${myarray[3]}
done
EOF

Moreover, I can't simply place the for loop around the call to mybinary, because I don't want multiple calls to mybinary. So how can I loop through an array to use the array's elements as input to mybinary?

Answer 1

You can use printf to add newlines after each entry in the array:

myarray=("Alice" "Bob" "Charlie" "q")

mybinary -f inputfile.txt -o outputfile.txt<<EOF
$(printf '%s\n' "${myarray[@]}")
EOF

As doneal24 and steeldriver pointed out, you could also use a pipe rather than a here-document:

printf '%s\n' "${myarray[@]}" | mybinary -f inputfile.txt -o outputfile.txt

Answer 2

If I understand well your needs:

#!/bin/bash

myarray=("Alice" "Bob" "Charlie" "q") # q quits the binary

mybinary -f inputfile.txt -o outputfile.txt < <(printf '%s\n' "${myarray[@]}")

If your binary is well coded, it should be deal with STDIN transparently.

Answer 3

To print the elements of an array one per line, in zsh, you do:

print -rC1 -- "$array[@]"

Or:

print -rNC1 -- "$array[@]"

for NUL-delimited records instead of lines.

(print -rC1 -- $array would only print the non-empty elements).

bash has a print builtin (initially from ksh), but only as an optional loadable builtin and it doesn't support the -C option (nor -N).

printf '%s\n' "${array[@]}" doesn't work correctly for empty arrays as it prints an empty line (as if the array had one empty element) instead of no line at all.

The common way in bash (or POSIX shells¹ in general) is to define some helper functions:

println() { [ "$#" -eq 0 ] || printf '%s\n' "$@"; }
print0() { [ "$#" -eq 0 ] || printf '%s\0' "$@"; }

Then, you can do:

println "${array[@]}" q | your-cmd

---

^{¹ beware though that some POSIX shells (mostly ksh88 and some pdksh-based ones) don't have printf builtin, so won't be able to output large lists as you'll soon reach the limit on the size of the arguments that can be passed to an external command.}