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!
The problem is that the parsing of the pipeline (the |
in your example) happens before the variable $JOB_COMMAND
is expanded. In Bash, parsing the line happens first, then variables are expanded, split based on $IFS
, and substituted into the commands. So in your command, the | egrep
| egrep
and everything following it are being passed as literal arguments to your elastic-mapreduce
command; the error about --output
is probably due to the -o
argument.
As the Bash FAQ points out, you shouldn't be storing commands in variables, you should use shell functions instead.
job_command () {
/var/elastic-mapreduce/elastic-mapreduce --create \
--name 'Extrabux-MapReduce' \
--num-instances 16 --instance-type c1.medium --key-pair extrabux-keypair \
--log-uri s3n://extrabux-log/ \
--bootstrap-action "s3://extrabux-scripts/startup.sh" | \
egrep -o 'j-[a-zA-Z0-9]+'
}
And now you can call it as follows:
JOBID=$(job_command)
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:
eval $JOB_COMMAND
Pipes don't work that way. Split it into two commands. Oh, and BASH FAQ entry #50 .
如果,它将更具可读性(和功能性)
RESULT=`$JOB_COMMAND $JOB_ARGS | egrep ....`
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