Process substitution (<() and >()) in Bash Scripting - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to use process substitution to compare the contents of two files.

Bash Scripting

diff <([1] file1.txt) <(cat file2.txt)

Drag options to blanks, or click blank then click option'

Als

Bcat

Cecho

Dgrep

Attempts:

3 left

💡 Hint

Common Mistakes

Using commands like ls which list files instead of reading contents.

Using echo which does not read file contents.

The cat command reads the contents of file1.txt and provides it as input via process substitution.

2fill in blank

medium

Complete the code to redirect the output of a command to a file using process substitution.

Bash Scripting

tee >( [1] > output.log ) < input.txt

Drag options to blanks, or click blank then click option'

Als

Becho

Ccat

Dgrep

Attempts:

3 left

💡 Hint

Common Mistakes

Using ls which lists directory contents instead of writing output.

Using echo which does not read from input.

The cat command reads from the process substitution and writes to output.log.

3fill in blank

hard

Fix the error in the code to correctly use process substitution to sort and compare two files.

Bash Scripting

diff <(sort [1]) <(sort file2.txt)

Drag options to blanks, or click blank then click option'

Afile3.txt

Bfile2.txt

C-r

Dfile1.txt

Attempts:

3 left

💡 Hint

Common Mistakes

Using the wrong filename inside the process substitution.

Using options like -r without a filename.

The command sorts file1.txt and file2.txt before comparing them with diff.

4fill in blank

hard

Fill both blanks to create a command that compares the output of two commands using process substitution.

Bash Scripting

diff <([1]) <([2])

Drag options to blanks, or click blank then click option'

Aps aux

Bls -l

Ccat file.txt

Decho Hello

Attempts:

3 left

💡 Hint

Common Mistakes

Using commands that produce no output or static text.

Using the same command twice without reason.

This command compares the list of running processes with the detailed list of files in the current directory.

5fill in blank

hard

Fill all three blanks to create a pipeline that writes the output of a command to a file and also processes it.

Bash Scripting

command | tee >( [1] ) | [2] | [3]

Drag options to blanks, or click blank then click option'

Acat > output.txt

Bgrep error

Cwc -l

Dsort

Attempts:

3 left

💡 Hint

Common Mistakes

Not redirecting output properly to the file.

Using commands in the wrong order.

The pipeline writes the command output to output.txt, filters lines containing 'error', then counts those lines.

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