How to get the SQL Signon that Agent Jobs "Run As" or "Executed as User"

Question

How to get the SQL Signon that Agent Jobs "Run As" or "Executed as User"?
I have an install SQL scripts that creates a Linked Server. I want to put some security on the Linked Server and only grant the Agent Job Signon (the "Run As" or "Executed as User") access to the linked server. I need to retrieve the
Agent Job Signon (something like "NT SERVICE\SQLAgent$FIDEV360BI02").
I could query certain jobs and SUBSTRING the Message column - using some form of the query below, which would return "Executed as user: NT SERVICE\SQLAgent$SSDEVBI02. The step succeeded." But that is pretty imprecise.
use msdb
SELECT [JobName] = JOB.name,
[Step] = HIST.step_id,
[StepName] = HIST.step_name,
[Message] = HIST.message,
[Status] = CASE WHEN HIST.run_status = 0 THEN 'Failed'
WHEN HIST.run_status = 1 THEN 'Succeeded'
WHEN HIST.run_status = 2 THEN 'Retry'
WHEN HIST.run_status = 3 THEN 'Canceled'
END,
[RunDate] = HIST.run_date,
[RunTime] = HIST.run_time,
[Duration] = HIST.run_duration,
[Retries] = HIST.retries_attempted
FROM sysjobs JOB
INNER JOIN sysjobhistory HIST ON HIST.job_id = JOB.job_id
-- CHANGE THIS
-- WHERE JOB.name like '%GroupMaster%' or Job.name like '%etlv%'
ORDER BY HIST.run_date, HIST.run_time

Answer 1

by default all sql jobs are executed as sql server agent account, unless otherwise a proxy is setup.
you can get the proxy information as Olaf mentioned, if the proxy_id is null for the step, it implies that the job step was executed as sql server service account and in such case it will be null
so, if it is null, it ran as sql server agent account.
so, one work around is get the sql server agent service account and if the proxy is null, that means it ran as sql server agent account, so, use isnull function. the disadvantage would be if the sql server agent account was switched, you might not get the
accurate information as the new account will show up though the job really ran as old account, to get this information, you need to  get this from the logmessage column as you mentioned above.
 try this code...
/*from sql 2008r2 sp1, you get the service accounts using tsql,otherwise you have to query the registry keys*/
declare @sqlserveragentaccount varchar(2000)
select @sqlserveragentaccount= service_account
from sys.dm_server_services
where servicename like '%sql%server%agent%'
select message,isnull(name,@sqlserveragentaccount) as AccountName
from sysjobhistory a inner join sysjobsteps b
on a.step_id=b.step_id and a.job_id=b.job_id
left outer join sysproxies c on c.proxy_id=b.proxy_id
Hope it Helps!!