> ## 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. # Bash Process Substitution Input 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. ```bash theme={"dark"} 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: ```bash theme={"dark"} $ 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.

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