Bash scripts and scheduling cheat sheet

The chapter on one page: what to remember, the commands to have at your fingertips, and flashcards for quick revision.

The chapter in eight sentences

A script is a file of commands with a #!/bin/bash first line and the execute bit set; run it with ./script, bash script, by name through PATH, or source it into the current shell.
Inside a script, $1, $2 … are arguments, $# their number, "$@" all of them (quote it!), $0 the script's name and $? the last exit status.
Variables are set name=value with no spaces and read $name; $(command) substitutes output, $((…)) calculates, ${v:-default} supplies a default.
Exit status 0 means success; scripts should print errors to standard error (>&2) and use exit 1 (error) or exit 2 (usage).
[ … ] tests files, numbers and strings; if / elif / else, case, for and while make decisions and repeat; while IFS=: read processes files line by line.
cron schedules by five fields (minute hour day-of-month month day-of-week); personal jobs via crontab -e, system jobs in /etc/cron.d with a user column.
cron jobs get a minimal environment: use full paths, redirect output, escape % as \%, and read /var/log/cron.
A systemd timer is a .timer plus a .service (Type=oneshot) with OnCalendar and Persistent=true, enabled with systemctl enable --now NAME.timer; at runs a job once.

Item	Detail
First line	`#!/bin/bash`
Make runnable	`chmod +x script.sh` · run `./script.sh`
Debug	`bash -x script.sh`
Arguments	`$1` `$2` · `$#` · `"$@"` · `$0`
Last status	`$?` · `exit 0` · `exit 2`
Default value	`${1:-default}`
Substitute / calculate	`$(cmd)` · `$((a + b))`
Errors to stderr	`echo "msg" >&2`

Item	Detail
Files	`[ -f F ]` `-d` `-e` `-r` `-x` `-s`
Numbers	`[ "$n" -eq\|-ne\|-gt\|-lt\|-ge\|-le 3 ]`
Strings	`[ "$s" = x ]` `!=` `-z` `-n`
Combine	`!` · `&&` · `\|\|`
Branch	`if …; then …; elif …; else …; fi`
Choose	`case "$1" in a) …;; *) …;; esac`
Loop	`for x in LIST; do …; done` · `while […]; do …; done`
Read lines	`while IFS=: read -r a b c; do …; done < file`

Item	Detail
crontab	`crontab -e` / `-l` / `-r` (`-u USER` as root)
System jobs	`/etc/cron.d/NAME`: `m h dom mon dow USER command`
Log	`/var/log/cron`
Traps	full paths · redirect output · `\%`
Timer pair	`NAME.service` (`Type=oneshot`) + `NAME.timer` (`OnCalendar`, `Persistent`)
Check	`systemd-analyze calendar EXPR` · `systemctl list-timers`
Once	`at TIME` · `atq` · `atrm N`

What makes a file a script you can run?: A shebang line (#!/bin/bash) and the execute bit (chmod +x).
Four ways to run a script?: ./script (needs x) · bash script · name via PATH · source script (current shell)
Arguments and status?: $1 $2 … · $# count · "$@" all · $0 name · $? last exit status
Default value for a variable?: ${1:-default}
Test a file, a number, a string?: [ -f F ] · [ "$n" -gt 3 ] · [ "$s" = "x" ]
if / elif / else / fi shape?: if [ … ]; then …; elif [ … ]; then …; else …; fi
Loops?: for x in LIST; do …; done · while [ … ]; do …; done
Choose between patterns?: case "$1" in start) …;; stop) …;; *) …;; esac
Fields of a crontab line?: minute hour day-of-month month day-of-week [user] command
Edit, list, remove your crontab; system jobs?: crontab -e / -l / -r · files in /etc/cron.d with a user column
Gotchas in cron?: Minimal PATH (use full paths); escape % as \%; check /var/log/cron
Timer pair?: NAME.service (Type=oneshot, ExecStart) + NAME.timer (OnCalendar, Persistent); enable --now the timer; systemctl list-timers