Sunday, January 02, 2011

ksh93 backtraces

It is often times handy to get a backtrace to help you understand how a program got to an error condition. Unfortunately, shell scripting languages tend to not provide an easy mechanism to get a backtrace. The following examines how it can be accomplished with ksh93.

#! /bin/ksh

function backtrace {
        typeset -a stack 
        # Use "set -u" and an undefined variable access in a subshell
        # to figure out how we got here.  Each token of the result is
        # stored as an element in an indexed array named "stack".
        set -A stack $(exec 2>&1; set -u; unset __unset__; echo $__unset__)

        # Trim the last entries in stack array until we find the one that
        # matches the name of this function.
        typeset i=0
        for (( i = ${#stack[@]} - 1; i >= 0; i-- )); do
                [[ "${stack[i]}" == "${}:" ]] && break

        # Print the name of the function that called this one, stripping off
        # the [lineno] and appending any arguments provided to this function.
        print -u2 "${stack[i-1]/\[[0-9]*\]} $*"
        # Print the backtrace.
        for (( i--; i >= 0; i-- )); do
                print -u2 "\t${stack[i]%:}"

# A couple functions to illustrate the output
function a {
        b "$@"

function b {
        c "$@"

function c {
        # Trigger a backtrace and exit if no arguments were passed
        (( $# == 0 )) && backtrace "missing arg" && exit 1
        print -- "$@"

a "$@"
A couple example runs:
$ ./backtrace.ksh hello world!
hello world!
$ ./backtrace.ksh
c: missing arg

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