3. Preferred shell options

The following are proposed as a standard set of options to enable in all scripts, except where there is a clear and documented need to do otherwise:

• set -e / -o errexit: exit immediately when a command fails. This reduces the verbosity of error handling, similar to using a limited form of exceptions rather than C-like code.

• set -u / -o nounset: exit immediately when using an undefined variable. This helps to catch the use of undefined or mistyped variables, similar to using “implicit none” in Fortran.

• set -o pipefail: consider a pipeline failed if any of its commands fail, rather than ignoring errors in all but the last command.

• shopt -s inherit_errexit: this disables the Bash peculiarity where “set -e” gets reset within $(...) command substitutions.

These can be combined for brevity:

set -eu -o pipefail
shopt -s inherit_errexit

The following are worthy of consideration in cases where they are appropriate:

• shopt -s extglob: allow various extended globbing syntaxes.

• shopt -s failglob: exit immediately if a glob matches no filenames.

• shopt -s nullglob: globs that match no files expand to an empty string rather than themselves.

Finally, where verbose tracing of the script’s execution is desired, “set -x / -o xtrace” should be used.

More rarely, e.g. when tracking down hard-to-find parsing errors, “set -v / -o verbose” may also be useful to output a trace as the script is read rather than as it is executed, but this should not normally be left on in production scripts.

“set -a / set -o allexport” may be used sparingly and locally, if a whole list of variables are to be exported, but should always be reset immediately afterwards to avoid polluting the environment with unnecessary variables from the rest of the script.