Why should i add a dot and space to execute this oracle provided script?

Question

When multiple oracle databases are installed in one machine, we need to set the environment variables like ORACLE_HOME, ORACLE_SID before logging into the respective database. For this, oracle provides as script called oraenv.
If we try to execute oraenv like
oraenv   #without a dot and space before the filenamethe file seem to get executed but none of the env variables actually get set.
To execute this script correctly we have to put a dot and a space before the filename like
. oraenvWhat is difference between these two way of executing scipts?
The orenv code
# $Header: oraenv.sh.pp 19-jul-2001.14:02:07 jboyce Exp $ oraenv.sh.pp Copyr (c) 1991 Oracle
# usage: . oraenv
# This routine is used to condition a user's environment for access to
# an ORACLE database. It will prompt for the value of the new SID unless
# the variable ORAENV_ASK is set to NO, in which case it will use the
# current value of ORACLE_SID.
# The character "*" denotes a null SID.
# NOTE:          Due to constraints of the shell in regard to environment
# -----          variables, the command MUST be prefaced with ".". If it
#          is not, then no permanent change in the user's environment
#          can take place.
case ${ORACLE_TRACE:-""} in
    T)  set -x ;;
esac
# Determine how to suppress newline with echo command.
N=
C=
if echo "\c" | grep c >/dev/null 2>&1; then
    N='-n'
else
    C='\c'
fi
# Set minimum environment variables
# ensure that OLDHOME is non-null
if [ ${ORACLE_HOME:-0} = 0 ]; then
    OLDHOME=$PATH
else
    OLDHOME=$ORACLE_HOME
fi
case ${ORAENV_ASK:-""} in                       #ORAENV_ASK suppresses prompt when set
    NO)     NEWSID="$ORACLE_SID" ;;
    *)     case "$ORACLE_SID" in
         "")     ORASID=$LOGNAME ;;
         *)     ORASID=$ORACLE_SID ;;
     esac
     echo $N "ORACLE_SID = [$ORASID] ? $C"
     read NEWSID
     case "$NEWSID" in
         "")          ORACLE_SID="$ORASID" ;;
         *)             ORACLE_SID="$NEWSID" ;;          
     esac ;;
esac
export ORACLE_SID
ORAHOME=`dbhome "$ORACLE_SID"`
case $? in
    0)     ORACLE_HOME=$ORAHOME ;;
    *)     echo $N "ORACLE_HOME = [$ORAHOME] ? $C"
     read NEWHOME
     case "$NEWHOME" in
         "")     ORACLE_HOME=$ORAHOME ;;
         *)     ORACLE_HOME=$NEWHOME ;;
     esac ;;
esac
export ORACLE_HOME
# Put new ORACLE_HOME in path and remove old one
case "$OLDHOME" in
    "")     OLDHOME=$PATH ;;     #This makes it so that null OLDHOME can't match
esac                    #anything in next case statement
case "$PATH" in
    *$OLDHOME/bin*)     PATH=`echo $PATH | \
                   sed "s;$OLDHOME/bin;$ORACLE_HOME/bin;g"` ;;
    *$ORACLE_HOME/bin*)     ;;
    *:)               PATH=${PATH}$ORACLE_HOME/bin: ;;
    "")               PATH=$ORACLE_HOME/bin ;;
    *)               PATH=$PATH:$ORACLE_HOME/bin ;;
esac
export PATH
# Install any "custom" code here
# Locate "osh" and exec it if found
ULIMIT=`LANG=C ulimit 2>/dev/null`
if [ $? = 0 -a "$ULIMIT" != "unlimited" ] ; then
  if [ "$ULIMIT" -lt 2113674 ] ; then
    if [ -f $ORACLE_HOME/bin/osh ] ; then
     exec $ORACLE_HOME/bin/osh
    else
     for D in `echo $PATH | tr : " "`
     do
         if [ -f $D/osh ] ; then
          exec $D/osh
         fi
     done
    fi
  fi
fi

Answer 1

The dot is used to "source" a file. It is commonly used to change your environment variables without forcing you to log out and back in again for the changes to take effect. Check out "source command for Solaris 10" or something similiar for more info.