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Overview

One of the most common problems with writing any kind of program is lack of proper error handling.

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So we're going to explore how to add our own error handling mechanisms. This will also force us to better understand shells & sub-shells, exit and result codes, and other communication between execution contexts.

Shells and sub-shells

Every bash program has its own execution environment (sub-shell), which is a child process of its calling parent shell. A new sub-shell is created, runs, and returns when:

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• Input to sub-shells
  • program arguments
  • environment variables
  • file or stream data
• Output from sub-shells
  • exit code
  • standard output
  • file data

Environment variables

In addition to passing arguments to a program, a caller may set environment variables (normal bash shell variables) that can be read in the called environment. However by default, variables in a parent shell are not copied into sub-shells unless they are exported using the export keyword.

# a normal bash variable is not visible to sub-shells
foo=abc
( echo $foo )

# exported bash variables are visible to sub-shells
export foo
( echo $foo )

export bar="def"
( echo $bar )

Script exit codes and function return values

Unlike most other programming languages, bash functions and scripts can only return a single integer between 0 and 255. By convention a return value of 0 means success, and any other return value is an error code.

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exercise 1

On the command line, call exit with various codes in a parentheses sub-shell and check the result in the caller.

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