Bash 脚本 - shell 命令输出重定向
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Bash Scripting - shell command output redirection
提问by RomanM
Can someone help explain the following:
有人可以帮助解释以下内容：
If I type:
如果我输入：
a=`ls -l`
Then the output of the ls command is saved in the variable a
然后ls命令的输出保存在变量中 a
but if I try:
但如果我尝试：
a=`sh ./somefile`
The result is outputed to the shell (stdout) rather than the variable a
结果输出到 shell ( stdout) 而不是变量a
What I expected was the result operation of the shell trying to execute a scrip 'somefile' to be stored in the variable.
我期望的是 shell 尝试执行somefile要存储在变量中的脚本 ' '的结果操作。
Please point out what is wrong with my understanding and a possible way to do this.
请指出我的理解有什么问题以及可能的方法。
Thanks.
谢谢。
EDIT:
编辑：
Just to clarify, the script 'somefile' may or may not exist. If it exsists then I want the output of the script to be stored in 'a'. If not, I want the error message "no such file or dir" stored in 'a'
只是为了澄清，脚本“ somefile”可能存在也可能不存在。如果它存在，那么我希望脚本的输出存储在“ a”中。如果没有，我希望错误消息“没有这样的文件或目录”存储在 ' a'
回答by paxdiablo
I think because the shell probably attaches itself to /dev/tty but I may be wrong. Why wouldn't you just set execute permissions on the script and use:
我认为是因为 shell 可能将自身附加到 /dev/tty 但我可能是错的。为什么不直接在脚本上设置执行权限并使用：
a=`./somefile`
If you want to capture stderr andstdout to a, just use:
如果要将 stderr和stdout捕获到 a，只需使用：
a=`./somefile 2>&1`
To check file is executable first:
首先检查文件是否可执行：
if [[ -x ./somefile ]] ; then
a=$(./somefile 2>&1)
else
a="Couldn't find the darned thing."
fi
and you'll notice I'm switching to the $() method instead of backticks. I prefer $() since you can nest them (e.g., "a=$(expr 1 + $(expr 2 + 3))").
你会注意到我正在切换到 $() 方法而不是反引号。我更喜欢 $() 因为你可以嵌套它们（例如，“ a=$(expr 1 + $(expr 2 + 3))”）。
回答by Bogdan
You can try the new and improved way of doing command substitution, use $() instead of backticks.
您可以尝试新的和改进的命令替换方式，使用 $() 代替反引号。
a=$(sh ./somefile)
If it still doesn't work, check if somefileis not actually stderr'ing.
如果它仍然不起作用，请检查somefile是否实际上不是stderr。
回答by phihag
You are correct, the stdout of ./somefileis stored in the variable a. However, I assume somefile outputs to stderr. You can redirect that with 2>&1directly after ./somefile.
你是对的，标准输出./somefile存储在变量中a。但是，我假设 somefile 输出到 stderr。您可以2>&1直接在./somefile.
