Advanced Bash−Scripting Guide

4.
Advanced Bash−Scripting Guide
sh −x scriptname echoes the result each command, but in an abbreviated manner. This is the
equivalent of inserting set −x or set −o xtrace in the script.
Inserting set −u or set −o nounset in the script runs it, but gives an unbound variable error
message at each attempt to use an undeclared variable.
Using an "assert" function to test a variable or condition at critical points in a script. (This is an idea
borrowed from C.)
Example 30−4. Testing a condition with an "assert"
#!/bin/bash
# assert.sh
assert ()
# If condition false,
{ #+ exit from script with error message.
E_PARAM_ERR=98
E_ASSERT_FAILED=99
if [ −z "$2" ]
then
return $E_PARAM_ERR
fi
# Not enough parameters passed.
# No damage done.
}
lineno=$2
if [ ! $1 ]
then
echo "Assertion failed: \"$1\""
echo "File \"$0\", line $lineno"
exit $E_ASSERT_FAILED
# else
# return
# and continue executing script.
fi
a=5
b=4
condition="$a −lt $b"
# Error message and exit from script.
# Try setting "condition" to something else,
#+ and see what happens.
assert "$condition" $LINENO
# The remainder of the script executes only if the "assert" does not fail.
# Some commands.
# ...
echo "This statement echoes only if the \"assert\" does not fail."
# ...
# Some more commands.
exit 0
5. trapping at exit.
The exit command in a script triggers a signal 0, terminating the process, that is, the script itself. [61]
It is often useful to trap the exit, forcing a "printout" of variables, for example. The trap must be the
Chapter 30. Debugging 321