为何Linux Shell的diff命令加-B选项仍显示空行相关输出？

问题现象 #

根据diff手册，-B（--ignore-blank-lines）选项应忽略仅涉及空行的变更，但实际使用中存在场景不生效：当非空行与空行发生替换时，该选项无法过滤对应的差异输出。

以下是可复现问题的完整脚本：

#!/bin/bash

echo -e 'a\nb\nc' > file_a1
echo -e 'a\n\nb'  > file_a2

echo
echo file_a1
cat    file_a1
# a
# b
# c

echo
echo file_a2
cat    file_a2
# a
#
# b

echo
echo regular diff
diff    file_a1 file_a2
# 1a2
# >        <- 空行相关输出
# 3d3
# < c

echo
echo diff -B : 空行被忽略，符合预期
diff -B file_a1 file_a2
# 3d3
# < c

echo -e 'a\nb' > file_b1
echo -e '\nb'  > file_b2

echo
echo file_b1
cat    file_b1
# a
# b

echo
echo file_b2
cat    file_b2
#
# b

echo
echo regular diff
diff    file_b1 file_b2
# 1c1
# < a
# ---
# >        <- 空行相关输出

echo
echo diff -B : 输出与无-B时一致，选项未生效
diff -B file_b1 file_b2
# 1c1
# < a
# ---
# >        <- 空行相关输出

echo
echo diff版本
diff --version
# diff (GNU diffutils) 3.9
# Copyright (C) 2023 Free Software Foundation, Inc.
# License GPLv3+: GNU GPL version 3 or later <https://gnu.org/licenses/gpl.html>.
# This is free software: you are free to change and redistribute it.
# There is NO WARRANTY, to the extent permitted by law.
# 
# Written by Paul Eggert, Mike Haertel, David Hayes,
# Richard Stallman, and Len Tower.

原因分析 #

GNU diff的-B选项仅会忽略单纯添加或删除空行的独立差异块。当空行与非空行的变更合并在同一个差异块中（比如示例中第一行的a被替换为空行），该选项不会过滤这个混合变更的块——因为这不属于"仅涉及空行"的变更。

解决方案 #

若需要在替换场景下也忽略空行影响，可通过以下方式处理：

1. 预处理文件删除空行后再对比：

diff <(sed '/^$/d' file_b1) <(sed '/^$/d' file_b2)

2. 结合空格忽略选项增强效果：

diff -B --ignore-space-change file_b1 file_b2

内容的提问来源于stack exchange，提问作者Alex Escodro