Bash process substitution: comparison and fan‑out without temporary files

Bash process substitution: comparison and fan‑out without temporary files

You want to compare two sorted directory listings, or grep a pattern from a command’s output, without writing intermediate files to disk. Bash process substitution solves this by turning command output into a filename, so utilities can consume pipeline output directly. Traditional pipelines chain commands, but utilities like diff, comm, and grep -f expect file arguments, not pipe input.

How process substitution works

When bash sees <(command) or >(command), it spawns a subshell, creates a FIFO or /dev/fd/N entry pointing to the subshell’s stdout or stdin, and passes that path as the argument. The enclosing command sees a filename, reads or writes it, and data moves through the kernel pipe without touching disk.

Reading output as a file: the <()

Commands that accept a file operand can consume command output directly.

diff <(sort file1) <(sort file2)

Here, sort output becomes a temporary filename that diff reads as if it were a regular file. The differing lines are reported, and bash cleans up the FIFO after the command finishes.

Expected check: run the command in bash; the line present in only one file appears in the output. If you run the same command under /bin/sh, you’ll get a syntax error because process substitution is bash‑specific.

Writing fan‑out with the >() form

When you need to fan out a stream to multiple commands, >(...) gives you a writable path. Writes to that path are fed to the subshell’s stdin.

grep pattern >(wc -l) >(grep -c another)

Each >(...) creates a separate pipe. The grep pattern output fans out, and both subshells process it independently. No tee file is needed.

Portability and verification

• Confirm bash is running: bash --version (major version 4+).
• Verify process substitution works: diff <(printf 'a b') <(printf 'a c') in bash should report the differing line.
• Inspect the file descriptor: while the command runs, ls /dev/fd will show an entry pointing to the subshell.
• Confirm non‑POSIX behavior: run the same diff under /bin/sh; it will fail, demonstrating the limitation.

When to avoid it

If your script must run under sh, dash, or minimal shells that don’t expose /dev/fd, fall back to explicit temp files (mktemp) or a tee‑based pipeline. Process substitution also introduces subtle ordering risks if you read from multiple <() paths sequentially without ensuring they’ve both completed.

Actionable closing

Before relying on process substitution, open a bash shell, run the verification commands above, and confirm /dev/fd entries appear as expected. If your deployment environment uses a different shell by default, substitute a mktemp + cat pattern instead. When bash is guaranteed, the pattern reduces clutter, avoids cleanup, and makes pipelines easier to read.