为什么 xargs 的退出代码会因“-I”选项的存在而不同？

Question

阅读 xargs man page 后，我无法理解以下 xargs 调用的退出代码的区别。

（当我遇到这种行为时，最初的目的是结合 find 和 grep 来检查所有给定文件中是否存在表达式）

复制：

（如果使用 zsh 强制创建文件，则使用 >>!）

# Create the input files.
echo "a" >> 1.txt   
echo "ab" >> 2.txt

# The end goal is to check for a pattern (in this case simply 'b') inside
# ALL the files returned by a find search. 
find .  -name "1.txt" -o -name "2.txt" | xargs -I {}  grep -q "b"  {}
echo $?
  123  # Works as expected since 'b' is not present in 1.txt

find .  -name "1.txt" -o -name "2.txt" | xargs grep -q "b"
echo $?
  0   # Am more puzzled by why the behaviour is inconsistent

手册页上的 EXIT_STATUS 部分说：

xargs exits with the following status:
0 if it succeeds
123 if any invocation of the command exited with status 1-125
124 if the command exited with status 255
125 if the command is killed by a signal
126 if the command cannot be run
127 if the command is not found
1 if some other error occurred.

我会认为，无论是否使用-I，123 if any invocation of the command exited with status 1-125 都应该适用？

您能否分享任何见解来解释这个难题？

Answer 1

下面是 -I 选项与 xargs 的效果的证据，它借助显示调用次数的包装脚本：

cat ./grep.sh
#/bin/bash

echo "I am being invoked at $(date +%Y%m%d_%H-%M-%S)"
grep $@

（实际调用的命令，在这种情况下grep 并不重要）

现在使用包装脚本执行与问题中相同的命令：

❯ find .  -name "1.txt" -o -name "2.txt" | xargs -I {}  ./grep.sh -q "b" {}
I am being invoked at 20190410_09-46-29
I am being invoked at 20190410_09-46-30

❯ find .  -name "1.txt" -o -name "2.txt" | xargs ./grep.sh -q "b"
I am being invoked at 20190410_09-46-53

---

我刚刚发现了一个对回答这个问题的类似问题的答案的评论（完全归功于 https://superuser.com/users/49184/daniel-andersson 他的智慧）：

https://superuser.com/questions/557203/xargs-i-behaviour#comment678705_557230

此外，未加引号的空格不会终止输入项；相反，分隔符是换行符。 ——这是理解行为的核心。如果没有 -I，xargs 只会将输入视为单个字段，因为换行符不是字段分隔符。使用 -I，突然换行符成为字段分隔符，因此 xargs 看到三个字段（它迭代）。这是一个非常微妙的观点，但在引用的手册页中进行了解释。

-I replace-str
   Replace occurrences of replace-str in the initial-arguments
   with names read from standard input.  Also, unquoted blanks do
   not terminate input items; instead the separator is the
   newline character.  Implies -x and -L 1.

基于此，

find .  -name "1.txt" -o -name "2.txt"  
#returns
# ./1.txt 
# ./2.txt

xargs -I {}  grep -q "b"  {}
# interprets the above as two separate lines since,
# with -I option the newline is now a *field separator*. 
# So this results in TWO invocations of grep and since one of them fails, 
# the overall output is 123 as documented in the EXIT_STATUS section

xargs grep -q "b"
# interprets the above as a single input field,
# so a single grep invocation which returns a successful exit code of 0 since the pattern was found in one of the files.