mapfile - Linux

Overview

mapfile, also known as readarray, is a bash built-in command used to read lines from the standard input into an array variable. This command is useful for scripts that need to process lists or outputs from other commands line-by-line efficiently. It is particularly valuable in scenarios where handling large amounts of data or complex string manipulation tasks are necessary.

Syntax

The basic syntax of the mapfile command is as follows:

mapfile [-d delim] [-n count] [-O origin] [-s count] [-t] [-u fd] [-C callback] [-c quantum] [array]
  • array: The name of the array to create or modify. If omitted, the default array name is MAPFILE.

Options/Flags

  • -d delim: Specify a delimiter character. The default delimiter is a newline.
  • -n count: Limit the number of lines read to count. By default, mapfile reads all lines until EOF.
  • -O origin: Begin storing lines in the array at the index origin. The default index is 0.
  • -s count: Discard the first count lines read.
  • -t: Remove a trailing newline from each line read.
  • -u fd: Read input from file descriptor fd.
  • -C callback: Execute callback every quantum lines. The callback is evaluated after the line is read and before the array element is assigned.
  • -c quantum: The number of lines read between each invocation of the callback.

Examples

Basic Usage:

mapfile my_array < myfile.txt

This reads lines from myfile.txt into the array named my_array.

Using a Custom Delimiter:

mapfile -d ':' my_array < /etc/passwd

Here, mapfile reads the /etc/passwd file using ‘:’ as the delimiter, useful for parsing colon-separated files.

Reading a Specific Number of Lines and Skipping Lines:

mapfile -n 10 -s 2 my_array < myfile.txt

This command skips the first 2 lines and reads the next 10 lines from myfile.txt into my_array.

Common Issues

  • Memory Limitations: Large files can cause performance issues. In such cases, consider reading the file in chunks.
  • Handling Special Characters: Ensure the -d option’s delimiter does not occur in the file unexpectedly, as it can distort the array’s formation.

Integration

mapfile can be integrated with other commands like find or grep in scripts to handle complex data processing tasks:

find /var/log -name "*.log" | mapfile -t log_files
cat "${log_files[@]}" | grep "ERROR" > error_logs.txt

This script finds all log files, reads them into an array, then concatenates them and filters lines containing “ERROR”.

  • read: Reads a single line of input.
  • declare: Used to declare array variables which can be used with mapfile.

Further information can be found in the Bash official documentation or man pages, accessible via man bash on your Linux terminal.