#!/bin/bash

trap 'echo "VARIABLE-TRACE> \$variable = \"$variable\""' DEBUG
# Echoes the value of $variable after every command.

variable=29

echo "Just initialized \"\$variable\" to $variable."

let "variable *= 3"
echo "Just multiplied \"\$variable\" by 3."

exit $?

#  The "trap 'command1 . . . command2 . . .' DEBUG" construct is
#+ more appropriate in the context of a complex script,
#+ where placing multiple "echo $variable" statements might be
#+ clumsy and time-consuming.

# Thanks, Stephane Chazelas for the pointer.


Output of script:

VARIABLE-TRACE> $variable = ""
VARIABLE-TRACE> $variable = "29"
Just initialized "$variable" to 29.
VARIABLE-TRACE> $variable = "29"
VARIABLE-TRACE> $variable = "87"
Just multiplied "$variable" by 3.
VARIABLE-TRACE> $variable = "87"