为什么bash参数扩展会导致rsync命令以不同的方式运行？

Question

我正在尝试运行rsync命令,将文件复制到新位置.如果我直接运行rsync命令,在命令行上没有任何参数扩展,rsync就能达到我的预期

$ rsync -amnv --include='lib/***' --include='arm-none-eabi/include/***' \
  --include='arm-none-eabi/lib/***' --include='*/' --exclude='*' \
  /tmp/from/ /tmp/to/

building file list ... done
created directory /tmp/to
./
arm-none-eabi/
arm-none-eabi/include/
arm-none-eabi/include/_ansi.h
...
arm-none-eabi/lib/
arm-none-eabi/lib/aprofile-validation.specs
arm-none-eabi/lib/aprofile-ve.specs
...
lib/
lib/gcc/
lib/gcc/arm-none-eabi/
lib/gcc/arm-none-eabi/4.9.2/
lib/gcc/arm-none-eabi/4.9.2/crtbegin.o
...

sent 49421 bytes  received 6363 bytes  10142.55 bytes/sec
total size is 423195472  speedup is 7586.32 (DRY RUN)

但是,如果我将过滤器参数括在变量中,并使用该变量调用该命令,则会观察到不同的结果. rsync复制了我不期望的许多额外目录:

$ FILTER="--include='lib/***' --include='arm-none-eabi/include/***' \
  --include='arm-none-eabi/lib/***' --include='*/' --exclude='*'"
$ rsync -amnv ${FILTER} /tmp/from/ /tmp/to/

building file list ... done
created directory /tmp/to
./
arm-none-eabi/
arm-none-eabi/bin/
arm-none-eabi/bin/ar
...
arm-none-eabi/include/
arm-none-eabi/include/_ansi.h
arm-none-eabi/include/_syslist.h
...
arm-none-eabi/lib/
arm-none-eabi/lib/aprofile-validation.specs
arm-none-eabi/lib/aprofile-ve.specs
...
bin/
bin/arm-none-eabi-addr2line
bin/arm-none-eabi-ar
...
lib/
lib/gcc/
lib/gcc/arm-none-eabi/
lib/gcc/arm-none-eabi/4.9.2/
lib/gcc/arm-none-eabi/4.9.2/crtbegin.o
...

sent 52471 bytes  received 6843 bytes  16946.86 bytes/sec
total size is 832859156  speedup is 14041.53 (DRY RUN)

如果我echo失败的命令,它会生成成功的确切命令.复制输出并直接运行可以得到预期的结果.

很明显我对bash参数扩展的工作原理缺失了.有人可以解释为什么两个不同的调用会产生不同的结果吗？

Answer 1

shell在扩展变量之前解析引号,因此将引号放在变量的值中并不能达到预期的效果 - 当它们到位时,它们做任何有用的事情都为时已晚.请参阅BashFAQ#50:我正在尝试将命令放在变量中,但复杂的情况总是会失败!更多细节.

在您的情况下,看起来解决此问题的最简单方法是使用数组而不是纯文本变量.这样,在创建数组时会解析引号,每个"单词"都作为单独的数组元素存储,如果正确引用变量(使用双引号和[@]),则数组元素将包含在命令的参数列表中没有任何不必要的解析:

filter=(--include='lib/***' --include='arm-none-eabi/include/***' \
  --include='arm-none-eabi/lib/***' --include='*/' --exclude='*')
rsync -amnv "${filter[@]}" /tmp/from/ /tmp/to/

请注意,数组在bash和zsh中可用,但不是所有其他与POSIX兼容的shell.此外,我降低了filter变量名称 - 建议的做法,以避免与shell的特殊变量(全部为大写)发生冲突.