Process Substitution & FIFOs (2025)

CRITICAL GUIDELINES

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Process Substitution & FIFOs (2025)

Overview

Master advanced inter-process communication patterns in bash using process substitution, named pipes (FIFOs), and efficient data streaming techniques. These patterns enable powerful data pipelines without temporary files.

Process Substitution Basics

Input Process Substitution <(command)

#!/​usr/​bin/​env bash
set -euo pipefail

# Compare two command outputs
diff <(sort file1.txt) <(sort file2.txt)

# Compare remote and local files
diff <(ssh server 'cat /​etc/​config') /​etc/​config

# Merge sorted files
sort -m <(sort file1.txt) <(sort file2.txt) <(sort file3.txt)

# Read from multiple sources simultaneously
paste <(cut -f1 data.tsv) <(cut -f3 data.tsv)

# Feed command output to programs expecting files
# Many programs require filename arguments, not stdin
wc -l <(grep "error" *.log)

# Process API response with tool expecting file
jq '.items[]' <(curl -s "https://api.example.com/​data")

# Source environment from command output
source <(aws configure export-credentials --format env)

# Feed to while loop without subshell issues
while IFS= read -r line; do
    ((count++))
    process "$line"
done < <(find . -name "*.txt")
echo "Processed $count files"  # Variable survives!

Output Process Substitution >(command)

#!/​usr/​bin/​env bash
set -euo pipefail

# Write to multiple destinations simultaneously (tee alternative)
echo "Log message" | tee >(logger -t myapp) >(mail -s "Alert" [email protected])

# Compress and checksum in one pass
tar cf - /​data | tee >(gzip > backup.tar.gz) >(sha256sum > backup.sha256)

# Send output to multiple processors
generate_data | tee >(processor1 > result1.txt) >(processor2 > result2.txt) > /​dev/​null

# Log and process simultaneously
./​build.sh 2>&1 | tee >(grep -i error > errors.log) >(grep -i warning > warnings.log)

# Real-time filtering with multiple outputs
tail -f /​var/​log/​syslog | tee \
    >(grep --line-buffered "ERROR" >> errors.log) \
    >(grep --line-buffered "WARNING" >> warnings.log) \
    >(grep --line-buffered "CRITICAL" | mail -s "Critical Alert" [email protected])

Combining Input and Output Substitution

#!/​usr/​bin/​env bash
set -euo pipefail

# Transform and compare
diff <(sort input.txt | uniq) <(sort reference.txt | uniq)

# Pipeline with multiple branches
cat data.csv | tee \
    >(awk -F, '{print $1}' > column1.txt) \
    >(awk -F, '{print $2}' > column2.txt) \
    | wc -l

# Complex data flow
process_data() {
    local input="$1"

    # Read from process substitution, write to multiple outputs
    while IFS= read -r line; do
        echo "$line" | tee \
            >(echo "LOG: $line" >> "$log_file") \
            >(process_line "$line" >> results.txt)
    done < <(cat "$input" | filter_input)
}

Named Pipes (FIFOs)

Creating and Using FIFOs

#!/​usr/​bin/​env bash
set -euo pipefail

# Create FIFO
mkfifo my_pipe

# Clean up on exit
trap 'rm -f my_pipe' EXIT

# Writer (in background or separate terminal)
echo "Hello from writer" > my_pipe &

# Reader (blocks until data available)
cat < my_pipe

# With timeout (using read)
if read -t 5 line < my_pipe; then
    echo "Received: $line"
else
    echo "Timeout waiting for data"
fi

Bidirectional Communication

#!/​usr/​bin/​env bash
set -euo pipefail

# Create two FIFOs for bidirectional communication
REQUEST_PIPE="/​tmp/​request_$$"
RESPONSE_PIPE="/​tmp/​response_$$"

mkfifo "$REQUEST_PIPE" "$RESPONSE_PIPE"
trap 'rm -f "$REQUEST_PIPE" "$RESPONSE_PIPE"' EXIT

# Server process
server() {
    while true; do
        if read -r request < "$REQUEST_PIPE"; then
            case "$request" in
                "QUIT")
                    echo "BYE" > "$RESPONSE_PIPE"
                    break
                    ;;
                "TIME")
                    date > "$RESPONSE_PIPE"
                    ;;
                "UPTIME")
                    uptime > "$RESPONSE_PIPE"
                    ;;
                *)
                    echo "UNKNOWN: $request" > "$RESPONSE_PIPE"
                    ;;
            esac
        fi
    done
}

# Client function
send_request() {
    local request="$1"
    echo "$request" > "$REQUEST_PIPE"
    cat < "$RESPONSE_PIPE"
}

# Start server in background
server &
SERVER_PID=$!

# Send requests
send_request "TIME"
send_request "UPTIME"
send_request "QUIT"

wait "$SERVER_PID"

Producer-Consumer Pattern

#!/​usr/​bin/​env bash
set -euo pipefail

WORK_QUEUE="/​tmp/​work_queue_$$"
mkfifo "$WORK_QUEUE"
trap 'rm -f "$WORK_QUEUE"' EXIT

# Producer
producer() {
    local item
    for item in {1..100}; do
        echo "TASK:$item"
    done
    echo "DONE"
}

# Consumer (can have multiple)
consumer() {
    local id="$1"
    while read -r item; do
        [[ "$item" == "DONE" ]] && break
        echo "Consumer $id processing: $item"
        sleep 0.1  # Simulate work
    done
}

# Start consumers (they'll block waiting for data)
consumer 1 < "$WORK_QUEUE" &
consumer 2 < "$WORK_QUEUE" &
consumer 3 < "$WORK_QUEUE" &

# Start producer
producer > "$WORK_QUEUE"

wait
echo "All work complete"

FIFO with File Descriptors

#!/​usr/​bin/​env bash
set -euo pipefail

FIFO="/​tmp/​fd_fifo_$$"
mkfifo "$FIFO"
trap 'rm -f "$FIFO"' EXIT

# Open FIFO for read/​write on FD 3
# Opening for both prevents blocking on open
exec 3<>"$FIFO"

# Write to FIFO via FD
echo "Message 1" >&3
echo "Message 2" >&3

# Read from FIFO via FD
read -r msg1 <&3
read -r msg2 <&3
echo "Got: $msg1, $msg2"

# Close FD
exec 3>&-

Coprocess (Bash 4+)

Basic Coprocess Usage

#!/​usr/​bin/​env bash
set -euo pipefail

# Start coprocess (bidirectional pipe)
coproc BC { bc -l; }

# Send data to coprocess
echo "scale=10; 355/​113" >&"${BC[1]}"

# Read result
read -r result <&"${BC[0]}"
echo "Pi approximation: $result"

# More calculations
echo "sqrt(2)" >&"${BC[1]}"
read -r sqrt2 <&"${BC[0]}"
echo "Square root of 2: $sqrt2"

# Close write end to signal EOF
exec {BC[1]}>&-

# Wait for coprocess to finish
wait "$BC_PID"

Named Coprocess

#!/​usr/​bin/​env bash
set -euo pipefail

# Named coprocess for Python interpreter
coproc PYTHON { python3 -u -c "
import sys
for line in sys.stdin:
    exec(line.strip())
"; }

# Send Python commands
echo "print('Hello from Python')" >&"${PYTHON[1]}"
read -r output <&"${PYTHON[0]}"
echo "Python said: $output"

echo "print(2**100)" >&"${PYTHON[1]}"
read -r big_num <&"${PYTHON[0]}"
echo "2^100 = $big_num"

# Cleanup
exec {PYTHON[1]}>&-
wait "$PYTHON_PID" 2>/​dev/​null || true

Coprocess Pool Pattern

#!/​usr/​bin/​env bash
set -euo pipefail

# Create pool of worker coprocesses
declare -A WORKERS
declare -A WORKER_PIDS

start_workers() {
    local count="$1"
    local i

    for ((i=0; i<count; i++)); do
        # Each worker runs a processing loop
        coproc "WORKER_$i" {
            while IFS= read -r task; do
                [[ "$task" == "QUIT" ]] && exit 0
                # Simulate work
                sleep 0.1
                echo "DONE:$task"
            done
        }

        # Store FDs dynamically
        local -n write_fd="WORKER_${i}[1]"
        local -n read_fd="WORKER_${i}[0]"
        local -n pid="WORKER_${i}_PID"

        WORKERS["$i,in"]="$write_fd"
        WORKERS["$i,out"]="$read_fd"
        WORKER_PIDS["$i"]="$pid"
    done
}

# Note: Coprocess pool management is complex
# Consider GNU Parallel for production workloads

Advanced Process Substitution / FIFO Patterns

Detailed recipes for multi-source comparison, tee-style stream splitting, process-substitution-driven diffing, streaming transforms, temporary FIFOs, and cleanup-heavy pipeline compositions live in references/​advanced-patterns.md. Load that reference when basic <(...), >(...), named pipes, or coprocess examples are not enough.

Bash 5.3 In-Shell Substitution

No-Fork Command Substitution

#!/​usr/​bin/​env bash
# Requires Bash 5.3+
set -euo pipefail

# Traditional: forks subshell
result=$(echo "hello")

# Bash 5.3: No fork, runs in current shell
result=${ echo "hello"; }

# Significant for variable modifications
counter=0
# Traditional - counter stays 0 (subshell)
result=$(counter=$((counter + 1)); echo "$counter")
echo "Counter: $counter"  # Still 0

# Bash 5.3 - counter is modified (same shell)
result=${ counter=$((counter + 1)); echo "$counter"; }
echo "Counter: $counter"  # Now 1

# REPLY variable syntax (even more concise)
${ REPLY="computed value"; }
echo "$REPLY"

# Or using ${| } syntax
${| REPLY=$(expensive_computation); }
echo "Result: $REPLY"

Performance-Critical Pipelines

#!/​usr/​bin/​env bash
# Requires Bash 5.3+
set -euo pipefail

# Build result without forks
build_path() {
    local parts=("$@")
    local result=""

    for part in "${parts[@]}"; do
        # No fork for each concatenation
        result=${ printf '%s/%s' "$result" "$part"; }
    done

    echo "${result#/​}"
}

# Accumulate values efficiently
accumulate() {
    local -n arr="$1"
    local sum=0

    for val in "${arr[@]}"; do
        # In-shell arithmetic capture
        sum=${ echo $((sum + val)); }
    done

    echo "$sum"
}

Error Handling in Pipelines

Pipeline Error Detection

#!/​usr/​bin/​env bash
set -euo pipefail

# Check all pipeline stages
run_pipeline() {
    local result

    # pipefail ensures we catch errors in any stage
    if ! result=$(stage1 | stage2 | stage3); then
        echo "Pipeline failed" >&2
        return 1
    fi

    echo "$result"
}

# PIPESTATUS for detailed error info
run_with_status() {
    cmd1 | cmd2 | cmd3

    local -a status=("${PIPESTATUS[@]}")

    for i in "${!status[@]}"; do
        if [[ "${status[$i]}" -ne 0 ]]; then
            echo "Stage $i failed with status ${status[$i]}" >&2
        fi
    done

    # Return highest exit status
    local max=0
    for s in "${status[@]}"; do
        ((s > max)) && max="$s"
    done
    return "$max"
}

Cleanup on Pipeline Failure

#!/​usr/​bin/​env bash
set -euo pipefail

# Track resources for cleanup
declare -a CLEANUP_PIDS=()
declare -a CLEANUP_FILES=()

cleanup() {
    local pid file

    for pid in "${CLEANUP_PIDS[@]}"; do
        kill "$pid" 2>/​dev/​null || true
    done

    for file in "${CLEANUP_FILES[@]}"; do
        rm -f "$file" 2>/​dev/​null || true
    done
}

trap cleanup EXIT

# Register cleanup
register_pid() { CLEANUP_PIDS+=("$1"); }
register_file() { CLEANUP_FILES+=("$1"); }

# Example usage
run_safe_pipeline() {
    local fifo="/​tmp/​pipeline_$$"
    mkfifo "$fifo"
    register_file "$fifo"

    producer > "$fifo" &
    register_pid "$!"

    consumer < "$fifo" &
    register_pid "$!"

    wait
}

Best Practices

FIFO Naming Convention

#!/​usr/​bin/​env bash
set -euo pipefail

# Include PID and descriptive name
create_fifo() {
    local name="$1"
    local fifo="/​tmp/​${name}_$$_$(date +%s)"
    mkfifo -m 600 "$fifo"  # Restrictive permissions
    echo "$fifo"
}

# Use tmpdir for security
create_secure_fifo() {
    local name="$1"
    local tmpdir
    tmpdir=$(mktemp -d)
    local fifo="$tmpdir/​$name"
    mkfifo -m 600 "$fifo"
    echo "$fifo"
}

Preventing Deadlocks

#!/​usr/​bin/​env bash
set -euo pipefail

# ✗ DEADLOCK - writer blocks, reader never starts
# mkfifo pipe
# echo "data" > pipe  # Blocks forever

# ✓ SAFE - open both ends or use background
mkfifo pipe
trap 'rm -f pipe' EXIT

# Option 1: Background writer
echo "data" > pipe &
cat < pipe

# Option 2: Open for read/​write
exec 3<>pipe
echo "data" >&3
read -r data <&3
exec 3>&-

# Option 3: Non-blocking open (requires careful handling)
exec 3<pipe &
exec 4>pipe
echo "data" >&4
read -r data <&3

Timeout Patterns

#!/​usr/​bin/​env bash
set -euo pipefail

# Read with timeout
read_with_timeout() {
    local fifo="$1"
    local timeout="$2"
    local result

    if read -t "$timeout" -r result < "$fifo"; then
        echo "$result"
        return 0
    else
        echo "Timeout after ${timeout}s" >&2
        return 1
    fi
}

# Write with timeout (using timeout command)
write_with_timeout() {
    local fifo="$1"
    local timeout="$2"
    local data="$3"

    if timeout "$timeout" bash -c "echo '$data' > '$fifo'"; then
        return 0
    else
        echo "Write timeout after ${timeout}s" >&2
        return 1
    fi
}

Resources

• Bash Reference - Process Substitution
• Bash Reference - Coprocesses
• BashFAQ - Process Substitution
• Named Pipes (FIFOs)

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Master process substitution and FIFOs for efficient inter-process communication without temporary files.