oracle command

1) export ORACLE_SID=xxxx

Setting ORACLE_SID sets an environment variable so that if you just type "sqlplus" on your database server, it will make a local connection to the database using the SID defined in /etc/oratab (or /var/opt/oracle/oratab on Solaris - where ever it is, or the service name as defined on Windows)

2)  !ps =eaf | grep -1 pmon

Done at the Unix shell, I would think that this is going to give you an error.  Done from within sqlplus (or any other process) it will shell out to the OS (that's what the ! does) and return a process list (do man ps to see what the options e, a and f do for ps) and then pipe the output to grep where it is pattern matching for the string pmon.

(I will also assume that =eaf is a typo, it should be -eaf)

Since every database should have pmon as a mandatory process, it can give you an idea of what databases you have running.  Presence of a pmon process does not mean that your database is operational though, just be aware.  Databases started but not mounted, or mounted but not open will still have a pmon process.

The -1 passed to grep shows the one line either side in the text you're matching.  -2 would show two lines either side.

For example I have databases dev, dev2 and tst on a server:

root      2099     1  0 Apr03 ?        00:00:00 /usr/sbin/mcelog --daemon
oracle    2133     1  0 Jul23 ?        00:02:40 ora_pmon_dev
oracle    2135     1  0 Jul23 ?        00:02:38 ora_psp0_dev
--
oracle    2208     1  0 Jul23 ?        00:00:08 ora_q001_dev
oracle    2252     1  0 Jul23 ?        00:02:32 ora_pmon_dev2
oracle    2254     1  0 Jul23 ?        00:02:34 ora_psp0_dev2
--
oracle    2337     1  0 Jul23 ?        00:00:28 ora_smco_dev
oracle    2381     1  0 Jul23 ?        00:02:36 ora_pmon_tst
oracle    2383     1  0 Jul23 ?        00:02:37 ora_psp0_tst
--
root      7159  6741  0 09:06 pts/0    00:00:00 ps -eaf
root      7160  6741  0 09:06 pts/0    00:00:00 grep -1 pmon
gdm      28845  3419  0 Apr21 ?        00:04:44 /usr/libexec/gdm-simple-greeter

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It found 3 pmon processes, the grep for pmon and returned the 1 line either side of what it found in the process list.  Not very useful in my mind for the output of a process list, but there you go.

3) select <text> from dual;

The DUAL table is a special 1 row / 1 column table.  Tom Kyte and the Oracle documentation explain what it is in detail:

https://asktom.oracle.com/pls/asktom/f?p=100:11:0::::P11_QUESTION_ID:1562813956388
http://docs.oracle.com/cd/E11882_01/server.112/e40540/datadict.htm#CNCPT1219

4) . oraenv
        -> after that we need to input the sid of a named sid

Oraenv is a script that is provided by Oracle to set up your shell environment to point to a particular database.  It sets appropriate environment variables so you can just type sqlplus or rman or whatever and connect directly to the database  you specified.

You can even do this:

export ORACLE_SID=orcl
export ORAENV_ASK=NO
. oraenv

Then it will take the value assigned to ORACLE_SID and set the environment without further prompting because you set ASK to no.

That is particularly useful for setting the environment in a crontab script, for example.

Steve,

very god answer.

from that link:
"
DUAL exists solely as a means to have a 1 row table
we can reliably select from.  Thats all."

then I dont understnad why we need that and it seems that the rest of the talbe is not reliably ? or dual is a old style Oracle stuff hasn't been remove yet ?

it seems if we do this :

 . oraenv

we don't even need this:

export ORACLE_SID=orcl

right?

also, what is :

1) after we do .oraenv and set an environment,

we can do this

crsctl stat res -t

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Yes, if you do ". oraenv", you don't need to export ORACLE_SID (unless, as I mentioned, you're trying to script it in which case you can set ORACLE_SID and ORAENV_ASK and then run . oraenv).

crsctl apparently is the main controlling program for Oracle Clusterware.  I have never used that product so can't comment any further on what crsctl is doing with any experience backing me.

However, the crsctl utility is documented here: http://docs.oracle.com/cd/E11882_01/rac.112/e16794/crsref.htm#CWADD91142

tks,

but any more reasonable explanation about the dual table one why 1 row is the return and how important Is it?

"crsctl apparently is the main controlling program for Oracle Clusterware.  I have never used that product so can't comment any further on what crsctl is doing with any experience backing me."

then what program you use for that ?