[Linux] Bash readarray Usage: Read lines into an array

Overview

The readarray command in Bash is used to read lines from standard input into an indexed array. This is particularly useful when you want to store multiple lines of text or data into a single variable for further processing.

Usage

The basic syntax of the readarray command is as follows:

readarray [options] [array_name]

Common Options

• -n N: Read only the first N lines.
• -s N: Skip the first N lines before reading.
• -t: Remove the trailing newlines from each line read.
• -O N: Specify the index to start storing the lines in the array.

Common Examples

Example 1: Basic Usage

To read lines from a file into an array:

readarray my_array < my_file.txt

Example 2: Reading from a Command Output

You can also read lines from the output of a command:

readarray my_array < <(ls -1)

Example 3: Skipping Lines

To skip the first line of a file and read the rest into an array:

readarray -s 1 my_array < my_file.txt

Example 4: Removing Trailing Newlines

To read lines from a file and remove trailing newlines:

readarray -t my_array < my_file.txt

Example 5: Specifying Starting Index

To start storing lines in an array from a specific index:

readarray -O 5 my_array < my_file.txt

Tips

• Use the -t option to avoid issues with trailing newlines when processing the array.
• Always check the contents of your array after reading to ensure it has been populated as expected, using echo "${my_array[@]}".
• Combine readarray with other commands like grep or awk for more powerful data manipulation.