Using array to dynamically pass files to a command in UNIX

Question

I have four files (file_one to file_four), the contents of these files isn't really important. I want to pass three of these files to a command (i.e. paste or awk) in a particular order as defined by an array. My array is order=(two one four). I was hoping to use the array to pass the desired files to the command as input similarly as how you could use * (i.e. paste file_* > combined_file);

paste file_${order[@]} > combined_file

paste file_"${order[@]}" > combined_file

paste file_"${order[*]}" > combined_file

paste file_"{${order[@]}}" > combined_file

paste file_{"${order[@]}"} > combined_file

I have looked at different pages (1, 2, 3 or 4) but I can't get it to to work. A loop through the files doesn't work in the case of paste or awk. I want to pass the files all at once to a command. Seeing as my UNIX knowledge is limited I may have misinterpreted some solutions/answers. From what I understand from this answer there might be some issues with how arrays were originally developed in bash.

Desired result: paste file_two file_one file_four > combined_file

Answer 1

You could use printf:

paste $(printf "file_%s " ${order[@]}) > combined_file

This avoids having to loop through all elements of the order array.

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Alternatively, using bashism, you could use this:

paste ${order[@]/#/file_} > combined_file

Note the # that matches the start of the pattern as mentioned in the bash man page:

${parameter/pattern/string}

(...) If pattern begins with #, it must match at the beginning of the expanded value of parameter.

Answer 2

You can have the for loop within $():

paste $(for i in ${order[@]}; do echo file_$i; done) > output_file