Store Output of the Command Into Array in Bash

Both mapfile and read -a can be used to store the output of a command or a list of values into an array. However, the mapfile command is generally preferred when reading lines from a file, while read -a is well-suited for reading space-separated values from a string.

Let's assume that we want to store all directories (top-level) that are located in projects forlder. In other words, keeping all projects (dir names) as array elements.

projects=("$HOME"/projects/*)

# Using 'find' command with '-print0' to handle directory names with special characters
while IFS= read -r -d $'\0' line; do
    projects+=("$line")
done < <(find "${projects[@]}" -maxdepth 0 -type d -print0)

In the provided code, the read command is used together with some parameters. Here is a brief explanation:

• -a : This option is used when we want to read from input and store it in an array. In the given code snippet, the input is obtained from a subshell command that lists directories (ls -d ${projects[@]}).

• -r : This option prevents backslash escapes from being interpreted. It helps you to read the strings "as is".

• -d $'\0' : This tells read to continue until it encounters a null byte (\0), which is the delimiter used by find . -print0.

So read -r -d $'\0' line reads input separated by null characters into the variable line. This is done inside a while loop, which continues to perform this reading operation for each directory returned by find, assigned to the projects array one by one.

The while loop structure while IFS= read -r -d $'\0' line; do is commonly used in shell scripting to read lines from a file (or in this case, results from a command substitution) in a safe manner that preserves whitespace and special characters.

IFS= is used to temporarily clear the Internal Field Separator variable, which is used by read to split the input line into separate fields. By clearing it, we ensure that read treats each line as a whole, even if it includes spaces.

In this script, the find command is used, ill-equipped with the -print0 option to output names using a null character as a delimiter, which helps in dealing with directory names that include spaces or other special characters. The -maxdepth 0 option ensures that only the directories (not their subdirectories) are listed. The -type d filter ensures that only directories are returned.

The while loop with IFS= read -r -d $'\0' handles the null delimited output from find. Within the loop, each line is appended to the projects array. Lastly, the elements of the projects array are added to the 'list' array.