Why Process substitution (<() and >()) in Bash Scripting? - Purpose & Use Cases

The Big Idea

Discover how to skip messy temporary files and make your scripts smarter with process substitution!

The Scenario

Imagine you want to compare the contents of two files using a command like diff. Normally, you need to create temporary files or run multiple commands to prepare the data before comparing.

The Problem

Manually creating temporary files is slow and messy. You might forget to delete them, cluttering your system. Also, juggling multiple commands increases the chance of mistakes and wastes time.

The Solution

Process substitution lets you treat the output of a command as if it were a file. This means you can directly compare or process command outputs without making temporary files, making your scripts cleaner and faster.

Before vs After

✗ Before

command1 > temp1.txt
command2 > temp2.txt
diff temp1.txt temp2.txt
rm temp1.txt temp2.txt

✓ After

diff <(command1) <(command2)

What It Enables

You can seamlessly connect commands that expect files, using live command outputs instead, simplifying complex workflows.

Real Life Example

Checking differences between two live system states, like current running processes vs saved snapshots, without creating extra files.

Key Takeaways

Process substitution replaces temporary files with live command outputs.

It makes scripts cleaner and reduces manual cleanup.

It helps connect commands that need file inputs with dynamic data.

Practice

1. What does the process substitution syntax <() do in bash scripting?

easy

A. It runs a command and waits for user input.

B. It redirects the output of a command to a file on disk.

C. It creates a temporary file with command output saved.

D. It runs a command and provides its output as a file input to another command.

Solution

1. Step 1: Understand process substitution <()

   This syntax runs a command and makes its output appear as a file that another command can read.

2. Step 2: Compare with other options

   Unlike redirecting output to a disk file, <() creates a temporary file descriptor for input.

3. Final Answer:

   It runs a command and provides its output as a file input to another command. -> Option D

4. Quick Check:

   Process substitution <() = command output as input file [OK]

Hint: Remember: <() gives command output as a file input [OK]

Common Mistakes:

• Confusing <() with output redirection >
• Thinking it creates a real disk file
• Assuming it waits for user input

2. Which of the following is the correct syntax to use process substitution to compare two files file1 and file2 using diff?

easy

A. diff <(cat file1) <(cat file2)

B. diff >(cat file1) >(cat file2)

C. diff <(file1) <(file2)

D. diff file1 >() file2

Solution

1. Step 1: Identify correct process substitution usage

   Using <() runs commands and provides their output as files to diff.

2. Step 2: Check each option

   diff <(cat file1) <(cat file2) correctly uses cat file1 and cat file2 inside <(). Options B, C, and D misuse syntax or operators.

3. Final Answer:

   diff <(cat file1) <(cat file2) -> Option A

4. Quick Check:

   Correct syntax for process substitution in diff = diff <(cat file1) <(cat file2) [OK]

Hint: Use <() with commands producing input, not >() [OK]

Common Mistakes:

• Using >() instead of <() for input
• Trying to put filenames directly inside <()
• Mixing redirection symbols incorrectly

3. What will be the output of the following bash command?

comm -12 <(sort fileA.txt) <(sort fileB.txt)

Assuming fileA.txt contains:
apple banana carrot
and fileB.txt contains:
banana carrot date

medium

A. apple
banana
carrot
date

B. banana
carrot

C. apple

D. banana
carrot
date
apple

Solution

1. Step 1: Understand comm -12 with process substitution

   The command compares sorted files and outputs lines common to both (options 1 and 2).

2. Step 2: Sort and compare file contents

   Sorted fileA.txt: apple, banana, carrot
   Sorted fileB.txt: banana, carrot, date
   Common lines: banana, carrot

3. Final Answer:

   banana carrot -> Option B

4. Quick Check:

   Common sorted lines = banana and carrot [OK]

Hint: comm -12 shows common lines from sorted inputs [OK]

Common Mistakes:

• Not sorting files before comm
• Expecting all lines combined
• Confusing comm options

4. Identify the error in this script snippet using process substitution:

grep 'error' >(sort > errors_sorted.txt)

medium

A. Using >() where <() is needed for input

B. Missing quotes around 'error'

C. Cannot redirect output inside >()

D. sort command should be outside process substitution

Solution

1. Step 1: Analyze the use of >() in the command

   The syntax >() creates a writable file descriptor for output redirection, but here grep expects a file input, not output.

2. Step 2: Determine correct substitution type

   To provide input from a command, <() should be used. Using >() here is incorrect.

3. Final Answer:

   Using >() where <() is needed for input -> Option A

4. Quick Check:

   Input needs <() not >() [OK]

Hint: Use <() for input, >() for output redirection [OK]

Common Mistakes:

• Confusing input and output process substitution
• Assuming >() can be nested inside commands
• Ignoring command expectations for input/output

5. You want to run a command that reads from a file and writes output to another command simultaneously, without creating temporary files on disk. Which process substitution usage correctly achieves this?

command1 <(producer) >(consumer)

What does this setup do?

hard

A. Runs command1 output to producer and input from consumer.

B. Creates two temporary files for producer and consumer.

C. Runs producer and feeds its output as input to command1, then sends command1 output to consumer.

D. Runs consumer first, then producer.

Solution

1. Step 1: Understand <() and >() roles

   <() provides command output as input file, >() provides a writable file for output redirection.

2. Step 2: Analyze the command flow

   producer output feeds command1 input via <(), and command1 output is sent to consumer via >().

3. Final Answer:

   Runs producer and feeds its output as input to command1, then sends command1 output to consumer. -> Option C

4. Quick Check:

   <() input, >() output redirection [OK]

Hint: Use <() for input, >() for output to chain commands [OK]

Common Mistakes:

• Mixing input and output substitutions
• Assuming temporary disk files are created
• Confusing command execution order