Skip to main content

Documentation Index

Fetch the complete documentation index at: https://docs.syntblaze.com/llms.txt

Use this file to discover all available pages before exploring further.

The <(...) operator, known as process substitution, dynamically executes a command and exposes its standard output (stdout) as a temporary file path to an enclosing command. It bridges the gap between commands that strictly require file paths as arguments and commands that generate output to stdout. 
outer_command <(inner_command)
Note: There must be no space between the < and the (. A space would cause Bash to interpret the < as a standard input redirection operator.

​
Execution Mechanism

When the Bash parser encounters the <(...) construct, it does not pass the literal string or the direct output to the outer command. Instead, it performs the following sequence of system-level operations:
  1. Descriptor Allocation: Bash creates a named pipe (FIFO) or allocates an anonymous pipe mapped to a file descriptor. On modern POSIX systems, this is typically exposed via the /dev/fd/ directory (e.g., /dev/fd/63).
  2. Asynchronous Execution: The inner_command is spawned in a background subshell.
  3. Stream Redirection: The stdout of the inner_command is connected to the write end of the allocated pipe or file descriptor.
  4. Argument Substitution: Bash replaces the entire <(...) syntax in the original command line with the absolute path to the read end of the pipe.
  5. Cleanup: Once the file descriptors are closed and the outer command terminates, Bash reaps the background process and destroys the temporary file descriptor or FIFO.

​
Visualizing the Substitution

Because Bash replaces the operator with a file path before the outer command executes, passing it to a command like echo reveals the underlying file descriptor rather than the output of the inner command: 
$ echo <(ls)
/dev/fd/63
In this execution context, the outer command receives /dev/fd/63 as a standard positional argument. It opens this path as a file and reads the stream generated by the inner command.

​
Technical Characteristics

  • Concurrency: The inner process and the outer process run asynchronously. The outer command begins execution immediately, reading from the pipe as the inner command writes to it.
  • Unidirectional Stream: The <(...) variant is strictly for reading output from the inner command. It is a read-only stream from the perspective of the outer command.
  • Non-Seekable: Because the underlying mechanism relies on pipes or FIFOs, the resulting “file” is a continuous stream. The outer command cannot perform seek operations (e.g., lseek(), rewinding, or jumping to specific byte offsets) on the provided file path.
  • Scope: The file descriptor is transient and strictly scoped to the execution lifecycle of the outer command.
Master Bash with Deep Grasping Methodology!Learn More