Process substitution (<() and >()) in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

What is the output of this process substitution command?

Consider the following bash command using process substitution. What will it output?

Bash Scripting

diff <(echo -e "apple\nbanana") <(echo -e "apple\nbanana\ncherry")

A 2a3\n> cherry

B 1a2\n> banana

C No output (files are identical)

D Syntax error

💡 Hint

Process substitution creates temporary files from command outputs for diff to compare.

The first input has two lines: apple and banana. The second input has three lines: apple, banana, and cherry. The diff command shows that after line 2 in the first input, line 3 (cherry) is added in the second input.

📝 Syntax

intermediate

Which option correctly uses process substitution to sort two files and compare them?

You want to compare the sorted contents of file1.txt and file2.txt using diff and process substitution. Which command is correct?

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

B diff >(sort file1.txt) >(sort file2.txt)

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

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

💡 Hint

Process substitution uses <() to provide command output as a file input.

Option D correctly uses <() to run sort on each file and pass the outputs as temporary files to diff. Option D uses >() which is for output redirection, not input. Option D tries to pipe a filename, which is invalid. Option D uses unnecessary input redirection inside process substitution.

🔧 Debug

advanced

Why does this process substitution command fail?

You run this command and get an error: 'bash: syntax error near unexpected token `>'': cat >(grep foo > output.txt) Why does it fail?

A Because >() is for output redirection and cannot be used as an argument to cat

B Because grep cannot write to output.txt inside process substitution

C Because cat requires input files, not output redirection

D Because process substitution only works with <(), not >()

💡 Hint

Think about what >() does and how cat expects its arguments.

The >() syntax creates a named pipe for output redirection. Using it as an argument to cat is invalid because cat expects input files or stdin, not output pipes. This causes a syntax error.

🚀 Application

advanced

How to merge two sorted lists using process substitution?

You have two sorted files, list1.txt and list2.txt. You want to merge them into one sorted output using the 'sort -m' command and process substitution. Which command achieves this?

A sort -m list1.txt list2.txt

B sort -m <(cat list1.txt) <(cat list2.txt)

C sort -m >(cat list1.txt) >(cat list2.txt)

D sort -m <(sort list1.txt) <(sort list2.txt)

💡 Hint

Process substitution can provide command outputs as files to commands expecting file arguments.

Option B uses process substitution to provide the contents of each file as inputs to 'sort -m', which merges sorted inputs. Option B merges files directly but does not guarantee they are sorted first. Option B misuses >() which is for output redirection. Option B unnecessarily sorts already sorted files again.

🧠 Conceptual

expert

What is the main difference between <() and >() in bash process substitution?

Choose the best explanation of the difference between <() and >() in bash process substitution.

A <() redirects output to a file; >() reads input from a file

B <() and >() are interchangeable and both provide command output as input

C <() provides the output of a command as a file input; >() provides a writable file to send data into a command

D <() is used only for input files; >() is used only for output files but not with commands

💡 Hint

Think about which one creates a readable file and which one creates a writable file.

<() runs a command and provides its output as a readable file (input) to another command. >() creates a writable file (like a named pipe) that a command can write into. They serve opposite purposes in process substitution.