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Crontab line builder

flock · logging · timeout · nice · random delay · % escaped for you

Put in a schedule and the command you would type in a terminal. The builder writes the crontab line a careful admin would — no overlapping runs, output captured, a time limit — escapes the characters cron treats specially, and explains every piece.

Schedule

Command (as you would type it in a shell) User (system crontab only) Comment line (optional)

Safety options

Prevent overlapping runs (flock)

Output

Time limit

Run in directory

Lower CPU priority (nice)

Random start delay, up to (s)

Run with bash (SHELL=/bin/bash)

Environment

MAILTO

CRON_TZ (cronie only)

Set PATH

Your crontab lines

What each piece does

Piece	What it does

When it runs

Server time zone

How to build a crontab line

Enter the schedule, or click a preset. Anything the parser accepts works, including @daily and @reboot.
Type the command exactly as you would run it in a terminal — do not escape anything yourself.
Tick the safety options you want. Overlap protection and a log file are on by default because they prevent the two most common cron incidents: runs piling up, and failures nobody sees.
Add environment lines if the job needs them — PATH for programs outside /usr/bin, MAILTO for where output goes.
Copy the lines into crontab -e (leave User empty) or into a file in /etc/cron.d/ (fill it in). Keep a newline after the last line.

Why every % becomes \%

In a crontab, % in the command part is not a percent sign. Cron turns the first unescaped % into a newline and sends everything after it to the command's standard input. So tar czf /backup/$(date +%F).tgz /srv — fine in a terminal — reaches the shell as tar czf /backup/$(date +, a syntax error, and the job fails with a confusing message in cron's mail. Written as date +\%F it works. The builder escapes every % in the command, the log path and the $((RANDOM % 300)) of the random delay; a \% you already typed is left alone.

What the options protect against

Option	Adds	Protects against
Overlap lock	flock -n /tmp/job.lock …	A slow run still going when the next one starts, then two, then ten.
Log file	>> /var/log/job.log 2>&1	Errors vanishing into a mailbox nobody reads.
Time limit	timeout 30m …	A hung job holding the lock forever, so no later run can start.
Directory	cd /srv/app && …	Relative paths resolving against the user's home directory.
nice	nice -n 10 …	A batch job slowing down the service on the same machine.
Random delay	sleep $((RANDOM % 300)) && …	Every server in a fleet hitting the same backend in the same second. For a fixed per-host offset instead, use the stagger generator.
PATH	PATH=/usr/local/bin:…	"command not found" because cron's default PATH is minimal.

Before installing a whole file, run it through the crontab checker: it catches unescaped %, a missing final newline and jobs scheduled in the hour that daylight saving time skips.

FAQ

Do I need the user field?

Only in the system crontab (/etc/crontab) and files in /etc/cron.d/, which have a sixth column naming the user to run as. A personal crontab edited with crontab -e has no user field — putting one there makes cron treat the name as the command.

Why did it add SHELL=/bin/bash?

The random delay uses $RANDOM, which is a bash feature; cron runs commands with /bin/sh unless the crontab sets SHELL. The builder also warns when your own command uses bash-only syntax such as [[ … ]], arrays or &> without bash selected.

Does CRON_TZ work on Ubuntu?

No. CRON_TZ is a cronie feature (Fedora, RHEL, Arch). Debian and Ubuntu cron have no per-user time zones; there, TZ only changes the time zone the command sees, not when it runs.

Why wrap the command in sh -c when a lock or time limit is on?

flock, timeout and nice run one program. If your command has a pipe or &&, only the part before it would be locked or timed. Wrapping it in sh -c '…' (or bash -c) makes the whole command one program.