Shell Programming and Scripting

19 2 * * *  > /path/to/lockfile && echo 'startSA' > /path/to/lockfile
20 2 * * *  /path/to/SA >   /path/to/logfile
21 2 * * *  /path/to/SB >> /path/to/logfile
22 2 * * *  /path/to/SC >> /path/to/logfile

export start_parm='startSA'
while true
do
   grep -q "$start_parm" /path/to/lockfile
   [ $?  -eq 0 ]  && break
   sleep 1
done
# script code goes here
# ....
# ....
#
# last line of code:
echo 'startSB' > /path/to/lockfile
exit

export start_parm='startSB'
while true
do
   grep -q "$start_parm" /path/to/lockfile
   [ $?  -eq 0 ]  && break
   sleep 1
done
# script code goes here
# ....
# ....
#
# last line of code:
echo 'startSC' > /path/to/lockfile
exit

export start_parm='startSC'
while true
do
   grep -q "$start_parm" /path/to/lockfile
   [ $?  -eq 0 ]  && break
   sleep 1
done
# script code goes here
# ....
# ....
#
# last line of code:
echo 'endALL' > /path/to/lockfile
exit

#!/bin/ksh
# The following cd command must specify an absolute pathname to run it from cron.
cd '/directory/where/this/script/should/run'
IAm=${0##*/}
Lock="$IAm.lock"
set -C  # Generate error if redirection target exists.
# Create lock file, if it does not exist...
if ! date "+$IAm started %x at %X with PID $$" > "$Lock"
then    # Exit if the lock file already existed...
        printf "%s: Error: Another copy of this script is already running.\n" \
                "$IAm" >&2
        cat "$Lock" >&2
        exit 1
fi
trap 'rm -f "$Lock"' 0    # Set trap to remove the lock file on exit.
set +C
./SB&           # Start SB
./SA&           # Start SA
SApid=$!        # Get pid of SA
wait $SApid     # wait for SA to finish
./SC& ./SD&     # Start SC and SD
wait            # wait for SB, SC, and SD to finish