> ## 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 Output The `>(...)` operator in Bash represents **output process substitution**. It allows a command to write to another process as if it were writing to a physical file. When Bash encounters this operator, it executes the enclosed command in a background subshell, connects its standard input (`stdin`) to a pipe, and replaces the entire `>(...)` expression with the file path to that pipe's write file descriptor. *Note: Process substitution is a shell extension found in Bash, Zsh, and Ksh. It is not POSIX `sh` compliant and will cause syntax errors if executed in standard `#!/bin/sh` scripts.* ## Syntax and Usage ```bash theme={"dark"} # tee writes to stdout and to the file path provided by >(...) echo "hello" | tee >(cat > output.txt) ``` *Note: There must be absolutely no space between the `>` and the `(`. A space changes the parser's interpretation to standard output redirection followed by a syntax error.* Because `>(...)` strictly yields a file path string, developers often mistakenly assume it will automatically capture a command's standard output. To redirect `stdout` into the substituted process, it must be explicitly combined with the standard output redirection operator (`>`), resulting in the `> >(...)` idiom: ```bash theme={"dark"} # Redirecting standard output (>) into the process substitution (>(...)) echo "hello" > >(sed 's/hello/world/' > output.txt) ``` ## Execution Mechanics When a command utilizing output process substitution is invoked, the shell performs the following sequence: 1. **Allocation:** Bash creates an anonymous pipe using the `pipe()` system call. 2. **Process Creation:** Bash spawns the enclosed command asynchronously. The `stdin` of this new process is attached to the read end of the pipe. 3. **Expansion:** Bash performs process substitution as one of its standard shell expansions (prior to execution). It replaces the literal text `>(...)` in the original command line with the constructed string `/dev/fd/` (where `` is the allocated write file descriptor). 4. **Execution:** The parent command executes, treating `/dev/fd/` as a standard file argument. As the parent command writes to this path, the kernel buffers the data and streams it directly into the `stdin` of the enclosed process. ## Technical Characteristics * **Asynchrony:** The process inside the parentheses runs asynchronously. The parent shell does not inherently wait for the `>(...)` process to terminate before moving to the next command in a script. * **Subshell Isolation:** Because the enclosed command runs in a subshell, any state changes (such as variable assignments, `cd`, or `shopt` modifications) made inside the parentheses are destroyed once the process terminates. They do not affect the parent shell. * **File-like Abstraction:** Unlike standard piping (`|`), which connects the `stdout` of one command directly to the `stdin` of another, process substitution provides a literal file path argument. This is structurally required for commands that strictly expect a file path parameter rather than reading from standard input. * **Directionality:** The `>` indicates the flow of data *into* the process. It is the inverse of `<(...)` (input process substitution), which provides a file path for the parent command to *read* from.

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