Robust Scripts

Strict Mode

Safer Defaults

Strict shell options turn common scripting mistakes into early failures. Defaults keep missing inputs explicit without weakening the script.

Program

Play the script to watch a missing argument become a default value, then become a filename-safe slug.

strict_mode.sh
Replay: real traced execution (multi-file project)
#!/usr/bin/env bash

set -euo pipefail
input="${1:-draft report}"
safe="${input// /_}"
echo "$safe"

  1. options ← errexit, nounset, pipefail

    3set -euo pipefail4input="${1:-draft report}"
    values this steperrexit, nounset, pipefailoptions

  2. input ← draft report

    3set -euo pipefail4input="${1:-draft report}"5safe="${input// /_}"
    values this stepdraft reportinputunset$1

  3. safe ← draft_report

    4input="${1:-draft report}"5safe="${input// /_}"6echo "$safe"
    values this stepdraft_reportsafedraft reportinput

  4. echo "$safe"

    5safe="${input// /_}"6echo "$safe"
    outputdraft_report
    values this stepdraft_reportsafe

Follow the Script

  1. set -euo pipefail turns on errexit, nounset, and pipefail.
  2. $1 is unset in the traced run.
  3. ${1:-draft report} makes input equal draft report.
  4. ${input// /_} changes spaces to underscores.
  5. The script prints draft_report. | step | value | | --- | --- | | options | errexit, nounset, pipefail | | $1 | unset | | input | draft report | | safe | draft_report | | stdout | draft_report |
strict mode `set -euo pipefail` makes failed commands, unset variables, and failed pipelines visible.
default expansion `${1:-draft report}` supplies a value when the first argument is missing or empty.
substitution `${input// /_}` replaces every space in the value.

Exercise: strict_mode.sh

Reproduce draft_report, then trace how the unset first argument becomes the safe output.