Run a command as user "x" in a shell script

Ever need to run a command as a specific user? Many Oracle commands require you to become the “oracle” user to make use of them.

To do this in a script, simply define your Oracle variables and use the “su” command. Here’s some example code:

su – $ORA_OWNER -c “$ORA_HOME/bin/lsnrctl start listener_rmdyd1”
su – $ORA_OWNER -c “$ORA_HOME/bin/lsnrctl start listener_hbstgu1”

Remove OSS Java

The open source variant of Java is usually installed by default. If you don’t want/need it, remove it with the following commands:

$ rpm -e java-1.4.2-gcj-compat-
$ rpm -e gcc-java

Record your actions

Just run the “script” command before you start typing anything. It will record all the actions you take at the command line in a readable file called ‘typescript’ in your current directory.

Clear a log file without deleting it

Need to clear out a log file to get rid of the “noise” in it? I do that often when tracking down a problem. However, deleting it usually means restarting the service that spawned it, which is not always possible (on a production server). The solution is to use redirection and the so-called “bit bucket” to get this done.

You clear the contents of a file (by sending it null data), but keep the file itself. Here’s how:

Lets say we are not happy that the “lastlog” file has grown to 19 megabytes in size.

# ls -ltr lastlog
-r——– 1 root root 19M Feb 6 12:34 lastlog

So we fill up the lastlog file with nothingness from /dev/null:
# cat /dev/null > lastlog

Ahhh, now lastlog is a good size!
# ls -ltr lastlog
-r——– 1 root root 0 Feb 6 12:34 lastlog

Semaphore. Defined.

Semaphores can best be described as counters which are used to provide synchronization between processes or between threads within a process for shared resources like shared memories.