To debug a bash script and to know what the environment is where a script is behaving it is convenient to log. This is what I added to my e-mail prosessing script yesterday. It is simple and of course every one else could think of it. Probably it is invented hundreds of times. Here's my solution.

#!/bin/bash
###################################################################################################
# Log
# Script to demonstrate logging
#
# author: Martien van den Akker
# (C) march 2010
# Darwin-IT Professionals
###################################################################################################
#Declarations
TRUE=1
FALSE=0
#Logging variables
LOG_ENABLED=$TRUE
#LOG_ENABLED=$FALSE
LOG_DIR=/tmp/log
LOG_FILENAME=$LOG_DIR/routemq.log


#Check log dir and create it if it does not exist.
check_logdir(){
if [ "$LOG_ENABLED" -eq $TRUE ]; then
if [ -d $LOG_DIR ]; then
#log a seperation line
log
else
mkdir $LOG_DIR
fi
fi
}

#Log
log(){
if [ "$LOG_ENABLED" -eq $TRUE ]; then
TEXT="$1 ""$2"
echo $TEXT >>$LOG_FILENAME;
fi
}
# First check logdir
check_logdir
# Log Arguments

log "Number of arguments: " $#
log "First argument: " $1
log "Second argument: " $1
log "End of script"

The script starts with a call to check_logdir(). This function checks if the LOG_DIR exists. If it exists it adds a seperation line. This is because the if has to have a command in the then section. But it is also convenient because you have a seperation line between script calls.
Then there is the log function. The log function accepts two parameters. One is the prompt, the other is a string to be concatenated to the prompt. Handy for listing parameter-values.
But you could also do a logging of only one line. For example the last line.

The logging can be enabled or disabled by commenting/uncommenting the proper line of:

LOG_ENABLED=$TRUE
#LOG_ENABLED=$FALSE