Understanding Bash Process Substitution

Process substitution is a powerful feature in Bash and other shells that allows the input or output of a command to be treated as a file. This technique is particularly useful when you need to use the output of one or more commands as the input to another command, or when you want to capture the output of a process and redirect it to the input of another process. The primary syntax involves using <(command) for input redirection and >(command) for output redirection.

Key Concepts of Process Substitution

• The input or output of a command can appear as a file. This is known as process substitution.
• This technique is useful when we want to use the output of multiple commands as the input to a command.
• Process substitution can also be used to capture output and redirect it to the input of a process.
• The template for process substitution is <(command) for reading from a command's output and >(command) for writing to a command's input.

Practical Examples of Process Substitution

Here are some practical examples demonstrating the utility of process substitution:

• Comparing Sorted Files

  You can easily sort and compare the contents of two files by using process substitution with the sort command:

  diff <(sort file1.txt) <(sort file2.txt)

  This command will output the differences between the sorted versions of file1.txt and file2.txt.

• Comparing Directory Contents

  Process substitution can also be used to compare the contents of two directories. By listing the files in each directory and comparing the output, you can identify differences:

  diff <(ls $first_directory) <(ls $second_directory)

  This example compares the output of ls for two different directories, effectively showing which files are present or absent in each.

Further Resources

• Bash Manual on Process Substitution
• Stack Overflow: What is Process Substitution?