Process substitution (<() and >()) in Bash Scripting - Step-by-Step Execution

Concept Flow - Process substitution (<() and >())

Start command needing file input/output

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Use <() or >() to create temp FIFO or /dev/fd

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Run inner command inside <() or >()

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Shell connects FIFO or file descriptor to outer command

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Outer command reads from or writes to this stream

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Process substitution ends when commands finish

Process substitution lets a command use the output or input of another command as if it were a file, using special temporary streams.

Execution Sample

Bash Scripting

diff <(echo "apple") <(echo "orange")

Compares outputs of two echo commands as if they were files using process substitution.

Execution Table

Step	Action	Command/Subshell	File Descriptor/FIFO Created	Outer Command Input	Output Produced
1	Start diff command	diff	None yet	Waiting for inputs	None
2	Create process substitution for first <()	echo "apple"	/dev/fd/63 (or FIFO)	diff reads from /dev/fd/63	Outputs 'apple'
3	Create process substitution for second <()	echo "orange"	/dev/fd/62 (or FIFO)	diff reads from /dev/fd/62	Outputs 'orange'
4	diff compares inputs	diff	Reads from both descriptors	Reads 'apple' and 'orange'	Outputs diff lines
5	diff outputs result	diff	No new descriptors	N/A	1c1 < apple --- > orange
6	Commands finish	All	Descriptors closed	No input	Process substitution ends

💡 Process substitution ends after both inner commands finish and diff completes comparison.

Variable Tracker

Variable	Start	After Step 2	After Step 3	After Step 4	Final
/dev/fd descriptors	None	/dev/fd/63 created for first echo	/dev/fd/62 created for second echo	Both descriptors open for diff	Descriptors closed
diff input streams	None	Reads from /dev/fd/63	Reads from /dev/fd/62	Reads both streams	No input
diff output	None	None	None	Diff result lines	Output complete

Key Moments - 3 Insights

Why does diff treat <(echo "apple") like a file?

Because process substitution creates a temporary file descriptor or FIFO that looks like a file, so diff can read from it as if it were a normal file (see execution_table steps 2 and 3).

What happens if the inner command in <() runs slowly?

The outer command (diff) waits for input from the file descriptor until the inner command produces output or finishes, so the process substitution streams data as it becomes available (see execution_table step 4).

Can process substitution be used for output redirection with >()?

Yes, >() creates a writable file descriptor that the outer command writes to, and the inner command reads from that stream (not shown here but works similarly to <()).

Visual Quiz - 3 Questions

Look at the execution_table at step 3, what is created for the second process substitution?

A /dev/fd/62 or a FIFO

B /dev/fd/63 or a FIFO

C A regular file on disk

D No file descriptor is created

💡 Hint

Check the 'File Descriptor/FIFO Created' column at step 3 in the execution_table.

At which step does diff start reading both inputs simultaneously?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for when diff reads from both descriptors in the 'Outer Command Input' column.

If the first echo command output changed to "banana", what would change in the execution_table output?

A The process substitution would fail

B The file descriptor numbers would change

C The diff output lines would show 'banana' instead of 'apple'

D No change in output

💡 Hint

Check the 'Output Produced' column at step 5 and consider what diff compares.

Concept Snapshot

Process substitution lets commands use outputs or inputs of other commands as files.
Syntax: <(command) for input, >(command) for output.
Shell creates temporary file descriptors or FIFOs.
Outer command reads/writes these like files.
Useful for commands needing file arguments but you want to use command output directly.

Full Transcript

Process substitution in bash scripting allows a command to use the output or input of another command as if it were a file. When you write diff <(echo "apple") <(echo "orange"), the shell runs the echo commands inside temporary file descriptors or FIFOs. These act like files for diff to read. The shell connects these streams so diff can compare the outputs. The process substitution ends when all commands finish. This lets you avoid creating real temporary files and makes scripts cleaner and faster.