Background

To find out a way to loop through all elements in an array consisting of more than one element, it’s easy.

$ for f in {"hello","world"}; do echo $f; done
hello
world

Problem

When I try to take away the second element in the above array so that it becomes an array consisting in one single element, then I get {hello} instead of hello.

$ for f in {"hello"}; do echo $f; done
{hello}

How can one get back an output consistent with the case of arrays consisting of multiple elements?

Motivation

This problem seems nonsense—loops are supposed to do repetitive tasks. If the loop has only one iteration, then we can directly type in the command, instead of adapting it into a loop and setting up the scope of the loop.

Nevertheless, if one converts multiple lines of short texts into one long single line with xargs, then the items will be separated by white spaces. This won’t work with the above for loop. To change the delimiter from white space to comma, one may use paste -d, -s.

Surely, there are other tools to do this, such as sed. However, I think that paste -d, -s should be the simplest way to do this. Note that the white space character between the two flags are important. Otherwise, this command won’t work.

Without prior knowledge of the text to be processed, it’s possible that the output of paste -d, -s consists of only one item. This single case is easy to deal with, even though the handling may be a bit different from the case of multiple elements. However, if the syntax for the command that handles the output of paste -d, -s isn’t exactly the same in the case of one single element and the case of multiple elements, then we need to look at the content and make manual judgement–this is tedious and error-prone.

In the problem posed in the previous section, if the list in the for loop has only one item, then we need to do something different from a list having two or more items. If we observe this difference with our naked eyes, the whole process will lack efficiency.

To solve this problem, we seek an unified approach. In other words, I try to change the syntax of the above for loop so that the for loop will work for any one of these two cases.

Solution

1. Surround each string in the array by a pair of double quotes.
2. Add a comma either before the first element or after the last element.

The first step is needed if in the array, there exists an element having two or more words separated by a white space; the second step is needed for an array with one single element.

$ for f in {"hello world",}; do echo $f; done
hello world

Remarks

As the number of steps increases, the difficulty of constructing a one-line command to do the thing rises tremendously. It’s possible that a shell script is easier to write. By searching “bash array tutorial” on Google, I got a tutorial on the Geek Stuff in the first search result.

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#!/bin/bash

arr=("hello")
for f in ${arr[@]}; do
    echo $f
done