Shell scripting and automation

Scripts that behave 5 topics

• Entry check: are you ready to script?
  A forty-five minute check of the command-line skills this course builds on, with a pointer to the Linux from zero topic to revisit for each one.
• Anatomy of a script
  What a shell script is, the shebang and the execute permission, three ways to run it, a header, the habit of quoting variables, and where scripts live.
• Variables, arguments and defaults
  Variables and command substitution, positional arguments and $@, defaults and required values with ${1:-x} and ${1:?msg}, shift, trimming text, and arithmetic.
• Decisions: if, tests and case
  if and elif, tests with [[ ]], numbers and text, guard clauses with errors to standard error, && and ||, and case for many choices.
• Loops: for, while read and until
  for loops over lists and files, while read for the lines of a CSV with IFS, the subshell trap, until and counters, and break and continue.

Doing more 4 topics

• Functions and return values
  Defining and calling functions, status versus value, local variables, using a function in if, and a library of shared functions.
• Arrays and lookups
  Indexed arrays and how to loop over them, reading a file with mapfile, associative arrays for lookups, checking keys, and passing arrays to functions.
• Text tools inside scripts: cut, sed, awk and here-documents
  Choosing between cut, sed, awk and sort for the four text moves, adding up with awk arrays, masking text with sed, and writing messages with here-documents.
• Files and names safely
  Why file names are dangerous, quoting and --, loops with wildcards, find with NUL separators, tidying names without overwriting, and temporary files with mktemp and a…

Scripts that fail well 4 topics

• Exit status and strict mode
  The meaning of exit codes, set -e, set -u and pipefail, what strict mode does not catch, expected failures, and making failure visible.
• Checking input and reporting errors
  Checks at the top of a script (arguments, shape, allowed values, files, size), usage messages and die functions, regular expressions and case for validation, good error…
• Traps, locks and clean-up
  Traps on EXIT, ERR and INT, reporting the failing line, atomic writes with a temporary file and mv, locks with mkdir and flock, and the standard shape of a script that…
• Debugging scripts
  Checking syntax with bash -n, reading the usual error messages, tracing with set -x and PS4, looking at values with declare -p, bisecting to the first bad line, and a…

Automation 4 topics

• Options with getopts
  Positional arguments or options, reading options with getopts and OPTARG, silent mode for unknown options and missing values, shifting to the operands, defaults and…
• Logging and reports
  One log format with levels, where output goes (standard error, a log file, tee, exec and the system log), a verbosity switch, a summary from an EXIT trap, reading the…
• Scheduling properly
  Cron's small environment, testing a line with env -i, crontab lines with a schedule, a lock, a full path and a log, safe changes to the crontab, repeat-safe jobs, and…
• Projects studio: four automation projects
  Four small projects built with the whole module: a backup rotator, an access-log digest, a health check and a new-member onboarding script, each with a story, a reason…

Infrastructure behind your scripts 4 topics

• What runs your script: shell, process, environment
  What runs a shell script: the shell process, fork and exec, the interpreter named by the shebang, the environment, file descriptors and the exit status, with a card for…
• Signals, jobs and process lifetime
  Signals and what a process does with them, the process table and states, background jobs with wait, traps for clean-up, orphans and zombies, and how to read the exit…
• Scheduling at scale: overlap, missed runs and pile-up
  How scheduled jobs go wrong: runs that overlap, runs that are missed, and runs that pile up, with cron's environment, flock, systemd timers and the clock, and a card for…
• Running and watching automation: logs, alerts and safe repeats
  How to run automation so you can trust it: logs with levels, heartbeats that catch silence, idempotent steps, retries with backoff, and log rotation, with a card for…

Interview ready 2 topics

• Interview ready: scripting basics and safety
  The questions people really ask about scripts, quoting, strict mode, functions, file names and exit statuses, with model answers, timed terminal tasks and a mock round.
• Interview ready: automation, debugging and a mock round
  The questions people really ask about cron, locks, options, logging and debugging, with model answers, timed terminal tasks and a mock round.

The report scripts

A tested room-report script that takes a room, checks its data, and loops over every room.

Scheduling and watching sheet

Five cards for the parts that run a script, a crontab line with a lock, full paths and a log, a heartbeat check, and one failure drill worked through.

Why take this course

• Automate the chores you repeat
• Run real scripts in a simulated machine
• Learn the safety habits that matter

You will build

• A report script that takes a room
• A script that loops over data
• A scheduled, logged job

What you can do afterwards

• Write scripts others can trust
• Explain what each line does and why
• Answer scripting interview questions