Shell: Process substitution and named pipes

Need to compare outputs of two commands, or use command output where a file is expected.

Process substitution creates temporary file descriptors from command output:

# Compare output of two commands
diff <(sort file1.txt) <(sort file2.txt)

# Compare two API responses
diff <(curl -s api.example.com/v1/users | jq .) \
     <(curl -s api.example.com/v2/users | jq .)

# Feed multiple inputs to a command
paste <(cut -f1 file1.tsv) <(cut -f3 file2.tsv)

# Use with while read (avoids subshell issue with pipes)
while IFS= read -r line; do
  count=$((count + 1))
done < <(grep -r 'TODO' src/)
echo "Found $count TODOs"  # count is correct!

# Write to multiple destinations
tee >(gzip > backup.gz) >(wc -l > count.txt) < input.txt

Note: <() creates a readable fd, >() creates a writable fd.

Process substitution avoids temporary files and subshell variable scoping issues that occur with pipes.

Shell scripting when you need command output as file input