这个 shell 脚本行有什么问题？
我有一个名为 $JOB_COMMAND 的 shell 变量，它具有启动映射缩减作业的命令，如下所示：
user@server:/scriptpath# echo $JOB_COMMAND
/var/elastic-mapreduce/elastic-mapreduce --create
--name 'Extrabux-MapReduce'
--num-instances 16 --instance-type c1.medium --key-pair extrabux-keypair
--log-uri s3n://extrabux-log/
--bootstrap-action "s3://extrabux-scripts/startup.sh" |
egrep -o 'j-[a-zA-Z0-9]+'
显然，对于问题的要点来说，不需要很多这些选项，但在某些情况下，内容会影响答案，它会被包含在内。
现在，当我尝试从变量运行命令时，我得到以下结果：
user@server:/scriptpath# $JOB_COMMAND
Error: --output must follow one of --streaming
但是，如果我像这样复制并粘贴回显命令的结果，我会得到以下结果：
user@server:/scriptpath# /var/elastic-mapreduce/elastic-mapreduce --create
--name 'Extrabux-MapReduce'
--num-instances 16 --instance-type c1.medium --key-pair extrabux-keypair
--log-uri s3n://extrabux-log/
--bootstrap-action "s3://extrabux-scripts/startup.sh" |
egrep -o 'j-[a-zA-Z0-9]+'
j-3FV0MT3H7G21S
如您所见，命令运行并成功吐出作业 ID
这是我想做的，将作业 ID 放入另一个 shell var 中：
user@server:/scriptpath# JOBID=`$JOB_COMMAND`
知道为什么在 var 内运行该命令会中断吗？ 仅供参考，我也尝试过
user@server:/scriptpath# JOBID=$($JOB_COMMAND)
，这也给了我错误，
谢谢您的帮助！
I have a shell var called $JOB_COMMAND that has the command to launch a map reduce job like so:
user@server:/scriptpath# echo $JOB_COMMAND
/var/elastic-mapreduce/elastic-mapreduce --create
--name 'Extrabux-MapReduce'
--num-instances 16 --instance-type c1.medium --key-pair extrabux-keypair
--log-uri s3n://extrabux-log/
--bootstrap-action "s3://extrabux-scripts/startup.sh" |
egrep -o 'j-[a-zA-Z0-9]+'
Obviously lots of those options are not needed for the point of the question, but in some off chance the content affects the answer, it's included.
Now when I try to run the command from the variable, I get this:
user@server:/scriptpath# $JOB_COMMAND
Error: --output must follow one of --streaming
However if I copy and paste the result of echoing the command like so I get the following:
user@server:/scriptpath# /var/elastic-mapreduce/elastic-mapreduce --create
--name 'Extrabux-MapReduce'
--num-instances 16 --instance-type c1.medium --key-pair extrabux-keypair
--log-uri s3n://extrabux-log/
--bootstrap-action "s3://extrabux-scripts/startup.sh" |
egrep -o 'j-[a-zA-Z0-9]+'
j-3FV0MT3H7G21S
As you can see, the command runs and successfully spits out a job id
Here is what I would like to do, get the job ID into another shell var:
user@server:/scriptpath# JOBID=`$JOB_COMMAND`
Any idea why running the command when it is inside a var is breaking?
FYI, I have also tried
user@server:/scriptpath# JOBID=$($JOB_COMMAND)
and that also gives me the error
Thanks for your help!
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评论（3）
问题在于管道的解析（示例中的
|）发生在变量$JOB_COMMAND扩展之前。在 Bash 中，首先解析该行，然后扩展变量，根据$IFS分割变量，并替换到命令中。所以在你的命令中，| egrep及其后面的所有内容都作为文字参数传递给您的elastic-mapreduce命令；关于--output的错误可能是由于-o参数造成的。正如 Bash FAQ 指出的那样，您不应该将命令存储在变量中，您应该使用 < a href="http://www.gnu.org/software/bash/manual/bashref.html#Shell-Functions" rel="nofollow">shell 函数 代替。
现在您可以按如下方式调用它：
使用 shell 函数还允许您将参数传递到命令中，您可能会发现这很有用。
如果出于某种原因您确实想将命令存储在变量中，则需要使用
eval，但不建议这样做：The problem is that the parsing of the pipeline (the
|in your example) happens before the variable$JOB_COMMANDis expanded. In Bash, parsing the line happens first, then variables are expanded, split based on$IFS, and substituted into the commands. So in your command, the| egrepand everything following it are being passed as literal arguments to yourelastic-mapreducecommand; the error about--outputis probably due to the-oargument.As the Bash FAQ points out, you shouldn't be storing commands in variables, you should use shell functions instead.
And now you can call it as follows:
Using shell functions would also allow you to pass parameters in to your command, which you may find to be useful.
If you really want to store a command in a variable for some reason, you will need to use
eval, but this is not recommended:管道不是这样工作的。将其分成两个命令。哦，还有 BASH 常见问题解答条目 #50。
Pipes don't work that way. Split it into two commands. Oh, and BASH FAQ entry #50.
如果
Will be much readable (and functional) if