获取shell命令的确切输出 [英] Get exact output of a shell command

问题描述

bash 手册针对命令替换:

Bash通过执行命令并将命令替换替换为命令的标准输出来执行扩展，并删除所有尾随的换行符.

Bash performs the expansion by executing command and replacing the command substitution with the standard output of the command, with any trailing newlines deleted.

演示-3个字符，第一个换行符:

Demonstration - 3 characters, newlines first:

$ output="$(printf "\n\nx")"; echo -n "$output" | wc -c
3

这里的换行符不是末尾，也不会被删除，因此计数为3.

Here the newlines are not at the end, and do not get removed, so the count is 3.

演示-3个字符，最后一行换行:

Demonstration - 3 characters, newlines last:

$ output="$(printf "x\n\n")"; echo -n "$output" | wc -c
1

此处换行符从末尾删除，因此计数为1.

Here the newlines are removed from the end, so the count is 1.

什么是将命令的二进制干净输出转换为变量的稳健解决方法?

What is a robust work-around to get the binary-clean output of a command into a variable?

Bourne外壳兼容性的奖励点.

Bonus points for Bourne shell compatibility.

推荐答案

以与伯恩兼容"的方式做到这一点的唯一方法是使用外部实用程序.

The only way to do it in a "Bourne compatible" way is to use external utilities.

除了用c写一个，还可以使用 xxd 和 expr (例如):

Beside writting one in c, you can use xxd and expr (for example):

$ output="$(printf "x\n\n"; printf "X")"         # get the output ending in "X".
$ printf '%s' "${output}" | xxd -p               # transform the string to hex.
780a0a58
$ hexstr="$(printf '%s' "${output}" | xxd -p)"   # capture the hex
$ expr "$hexstr" : '\(.*\)..'                    # remove the last two hex ("X").
780a0a
$ hexstr="$(expr "$hexstr" : '\(.*\)..')         # capture the shorter str.
$ printf "$hexstr" | xxd -p -r | wc -c           # convert back to binary.
3

缩短:

$ output="$(printf "x\n\n"; printf "X")"
$ hexstr="$(printf '%s' "${output}" | xxd -p )"
$ expr "$hexstr" : '\(.*\)..' | xxd -p -r | wc -c
3

使用命令 xxd 是因为它具有转换回二进制文件的功能.

The command xxd is being used for its ability to convert back to binary.

请注意，wc将失败，并带有许多UNICODE字符(多字节字符):

Note that wc will fail with many UNICODE characters (multibyte chars):

$ printf "Voilà" | wc -c
6

$ printf "★" | wc -c
3

它将打印字节数，而不是字符数.

It will print the count of bytes, not characters.

在较旧的shell中，变量 $ {#var} 的长度也会失败.

The length of a variable ${#var} will also fail in older shells.

当然，要使其在Bourne shell中运行，您必须使用`…`而不是 $(...).

Of course, to get this to run in a Bourne shell you must use `…` instead of $(…).

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