process-substitution-fifos

关键指南

Windows 文件路径要求

强制要求：在 Windows 上始终使用反斜杠表示文件路径

在 Windows 上使用 Edit 或 Write 工具时，必须在文件路径中使用反斜杠（\），而不是正斜杠（/）。

---

进程替换与 FIFO（2025）

概述

掌握在 bash 中使用进程替换、命名管道（FIFO）和高效数据流技术实现进程间通信的高级模式。这些模式无需临时文件即可构建强大的数据管道。

进程替换基础

输入进程替换 <(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!

输出进程替换 >(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])

组合输入与输出替换

#!/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)
}

命名管道（FIFO）

创建和使用 FIFO

#!/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

双向通信

#!/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"

生产者-消费者模式

#!/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"

使用文件描述符操作 FIFO

#!/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>&-

协进程（Bash 4+）

协进程的基本用法

#!/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"

命名协进程

#!/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

协进程池模式

#!/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

高级进程替换 / FIFO 模式

多源比较、tee 式流拆分、由进程替换驱动的差异比较、流式转换、临时 FIFO，以及需要大量清理操作的管道组合等详细示例，见 references/advanced-patterns.md。如果基本的 <(...)、>(...)、命名管道或协进程示例无法满足需求，请加载该参考文档。

Bash 5.3 Shell 内替换

无派生进程的命令替换

#!/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"

性能关键型管道

#!/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"
}

管道中的错误处理

检测管道错误

#!/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"
}

管道失败时进行清理

#!/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
}

最佳实践

FIFO 命名约定

#!/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"
}

防止死锁

#!/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

超时模式

#!/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
}

资源

• Bash 参考手册 - 进程替换
• Bash 参考手册 - 协进程
• Bash 常见问题 - 进程替换
• 命名管道（FIFO）

---

掌握进程替换和 FIFO，无需临时文件即可实现高效的进程间通信。