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Suppose a bash script expects an argument to be a file. But we have a bash array, and would like to provide the array in some way as an argument to the script, as if there were a file whose lines are the elements of the array. I wonder how to do that?

For a simple example, I have a script myscrpt containing a single command, which expects a file argument

cat $1

and I have an array

arr=( 1 3 5 )

and how can I provide arr as an argument to the script, as if the argument is actually a file whose content is

1
3
5

does myscript <(echo "${arr[@]}") work?

Thanks.

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  • Could you describe with words what the combination of your two scripts is doing? It looks like it could be done in a much more efficient way... Commented Nov 5, 2018 at 16:56
  • Thanks. See my update. I guess comm may solve my ultimate problem (but needs sort on the file arguments, which change the line order, undesirably). But right now I just wonder if a script is expecting a file argument, how can I provide an array to it? Commented Nov 5, 2018 at 17:04

2 Answers 2

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If you're using bash or some other array-capable shell you can use a "${var[@]}" type construct.

a=(one two 'three four' five)
printf "%s\n" "${a[@]}"

Pipe the printf into whatever expects elements one per line, for example

printf "%s\n" "${a[@]}" | myscript

It's really important to double-quote the array expansion; otherwise the @ character is treated as * and the array expansion gets (wrongly) treated to word splitting on whitespace.

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So, you have your script (only a bit modified to add #!-line and quotes around the expansion of $1 as to support reading from files with unusual filenames),

#!/bin/sh
cat "$1"

Now you want to call this script with an array

arr=( 1 2 3 4 5 )

so that instead of reading a file (which your script is doing), it reads newline-delimited value from you array, producing

1
2
3
4
5

as output?

This is how this may be done:

printf '%s\n' "${arr[@]}" | ./script -

or

printf '%s\n' "${arr[@]}" | ./script /dev/stdin

This would pass - or /dev/stdin to your script as the file to read from. The cat will interpret - as standard input, and if given /dev/stdin will likewise read from standard input.

The data is provided on standard input by printf which provides it as newline-delimited elements from the array.

The same effect would be had with 

( IFS=$'\n'; sh ./script.sh - <<<"${arr[*]}" )

where the data is provided via a here-string. By using "${arr[*]}", we join the elements of the array into a single string separated by the first character of $IFS (which we set to a newline). I'm using a subshell here to avoid modifying IFS in the surrounding environment.

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