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To schedule a command on a Linux VPS, place five time fields followed by the command in a user crontab: minute hour day-of-month month day-of-week command. For example, */5 * * * * /bin/sh /opt/test.sh runs a script every five minutes. You edit that table with crontab -e, verify it with crontab -l, and should use absolute paths plus explicit logging so the job behaves predictably without an interactive shell.
What cron and crontab do on a VPS
cron is the scheduler daemon; a crontab is the table of schedules it reads. Each user can have a personal crontab, and the daemon checks the entries as time advances. When an entry matches the current date and time, cron starts the command with the permissions of the crontab owner.
This is useful for recurring, unattended work such as backups, report generation, cache cleanup, certificate checks, or maintenance scripts. A VPS job does not run inside your SSH session, so it has a different environment: there is no interactive terminal, aliases may not exist, and the working directory is not the directory you happened to be in when you saved the file.
The five-field crontab syntax
A normal user crontab line has this order:
| Field | Allowed values | Meaning |
|---|---|---|
| Minute | 0–59 | Minute within the hour |
| Hour | 0–23 | Hour in 24-hour notation |
| Day of month | 1–31 | Calendar day |
| Month | 1–12 or jan–dec |
Calendar month |
| Day of week | 0–7 or sun–sat |
Weekday; Sunday is both 0 and 7 |
The fields are separated by spaces or tabs. The command starts after the fifth field and continues to the end of the line. Asterisks mean “every value” in that field.
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Reading a line from left to right
In 30 2 * * *, 30 is minute 30, 2 is 02:00, and each asterisk allows every day of the month, every month, and every weekday. Add a command to make a complete entry:
30 2 * * * /usr/local/bin/nightly-backupThis requests a run at 02:30 every day. The schedule uses the VPS host’s local time zone, so confirm the server’s clock and time-zone configuration before relying on a business-hour schedule.
Operators: wildcards, lists, ranges and steps
Each time field accepts combinations of four basic forms.
- Wildcard (
*): every value.*in the hour field allows every hour. - List (
,): selected values.20,40in the minute field runs at minutes 20 and 40. - Range (
-): an inclusive span.1-10means values 1 through 10; names such asmon-wedandjan-marcan be used where supported by the field. - Step (
/): every specified interval across a field or range.*/30in the minute field means minute 0 and 30 of each hour.
These forms can be combined. 0-30/10 in the minute field selects 0, 10, 20 and 30. A list such as 9-17,20 is read as all values from 9 through 17 plus 20; keep complex expressions readable by using comments on their own lines.
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The day-of-month and day-of-week rule
The two day fields have a behavior that surprises many administrators. If both day-of-month and day-of-week are restricted (neither is *), cron runs the command when either field matches, not only when both match.
For example:
30 4 1,15 * 5 /usr/local/bin/reportThis runs at 04:30 on the 1st and 15th of the month and on every Friday. To express “only when both conditions are true,” put the logic in a wrapper script that checks the date, or use a schedule design that leaves one day field as * and performs the additional test in the command.
Practical cron schedules for a Linux VPS
| Goal | Crontab entry | What it does |
|---|---|---|
| Every five minutes | */5 * * * * /bin/sh /opt/test.sh >> /var/log/test.log 2>&1 |
Runs at five-minute intervals and appends standard output and errors to a log. |
| Daily at 10:00 | 0 10 * * * /bin/sh /opt/test.sh |
Runs at 10:00 every day. |
| Hourly from 10:00 through 18:00 | 0 10-18 * * * /bin/sh /opt/test.sh |
Runs on the hour at 10:00, 11:00, through 18:00. |
| Sundays at 22:00 | 0 22 * * 0 /usr/bin/apt-get update |
Runs when the day-of-week value is Sunday (0; 7 is also Sunday). |
| At midnight daily | 0 0 * * * /usr/local/bin/cleanup |
Runs at 00:00 each day. |
The paths in these examples are intentional. Cron’s default PATH can be minimal, and a script that works interactively as python, node, or apt-get may fail when cron cannot locate that executable. Use the full path (find it with command -v program in your shell) or define a suitable PATH in the crontab.
Special strings for common schedules
Eight shortcuts replace all five time fields:
| String | Equivalent schedule |
|---|---|
@reboot |
Run once when the cron service starts the user’s jobs after system startup. |
@yearly or @annually |
0 0 1 1 * — midnight on January 1. |
@monthly |
0 0 1 * * — midnight on the first day of each month. |
@weekly |
0 0 * * 0 — midnight at the start of Sunday. |
@daily or @midnight |
0 0 * * * — midnight every day. |
@hourly |
0 * * * * — the start of every hour. |
For example:
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Editing and managing a user’s crontab
Use the account that should own and run the job. On a VPS, that distinction matters: a root crontab can change the whole system, while an application account usually has safer, narrower permissions.
- Open the editor: run
crontab -e. On the first run, the system may ask you to select an editor. - Add one complete line: enter the five fields, command path, and any redirection. Put a comment on a separate line if you need to document the schedule.
- Save and exit: the crontab command validates and installs the file when the editor closes.
- Verify the installed table: run
crontab -land check spacing, paths, and the account shown by your shell prompt.
To manage another user’s table, an administrator can specify -u user, for example sudo crontab -u appuser -e or sudo crontab -u appuser -l. crontab -r removes the current user’s entire crontab; use it carefully because it does not remove just one selected line.
Comments, variables and the non-interactive environment
Blank lines and leading spaces or tabs are ignored. A line whose first non-space character is # is a comment. Keep comments on their own lines: an inline comment after a command is a known portability trap because the comment text can become part of the command rather than being discarded.
You can set environment variables before scheduled entries using NAME=value; spaces around the equals sign are optional. A practical header might look like this:
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SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin
MAILTO=admin@example.net
# Run the import at 03:15 every day
15 3 * * * /usr/local/bin/import-dataThe documented defaults include SHELL=/bin/sh; LOGNAME and HOME come from the crontab owner’s password entry. If your script expects Bash features, call Bash explicitly (for example, /bin/bash /opt/job.sh) instead of assuming that /bin/sh is Bash. Likewise, use absolute paths for files because the job’s current directory is not a dependable project directory.
Capturing output and diagnosing failures
Cron may mail command output when local mail is configured. Setting MAILTO makes the intended recipient explicit; an empty value, MAILTO=, disables mail for entries below it. For VPS operations, a logfile is usually easier to inspect:
*/5 * * * * /usr/local/bin/health-check >> /var/log/health-check.log 2>&1>> appends standard output, while 2>&1 sends standard error to the same destination. Ensure the crontab owner can write the directory and file; a root-owned log path will fail for an unprivileged account. Rotate growing logs so a frequent job does not consume the VPS disk.
When a job never runs
- Run
crontab -las the expected account and confirm the line is installed. - Check each field against the server’s actual local time and time zone. Remember that the hour field is 0–23.
- Execute the exact command manually with its full paths. Fix permissions, interpreter paths, missing environment variables, and working-directory assumptions.
- Inspect the redirected logfile or local cron/system logs for a permission, syntax, or “command not found” error.
- Temporarily schedule a harmless test every five minutes, such as writing the date to a file, then remove it after verification.
When it runs more or less often than expected
Look for a step expression that includes minute 0, and check whether both day fields are restricted: the OR rule can add Fridays (or another weekday) to the dates you listed. A daylight-saving transition can also create a skipped or repeated local clock time; avoid ambiguous times for critical work and make the command safe to run twice.
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User crontab versus the system-wide crontab
A personal crontab contains five time fields followed directly by the command. System-wide files such as /etc/crontab insert a user field between day-of-week and command:
M H DOM MON DOW USER COMMANDFor example, the system-wide form must identify which account should execute the command; copying that line into crontab -e would shift the fields and produce an incorrect schedule. System-wide files are appropriate when an administrator owns the schedule, while a user’s crontab avoids granting unnecessary privileges.
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Cron is compact and widely available, making it a good fit for straightforward recurring commands. Systemd timers are a commonly cited alternative when you need service dependencies, explicit unit configuration, or persistence behavior for jobs that were missed while a machine was powered off. Their syntax and operational model differ, so treat a migration as a separate design decision rather than translating fields mechanically. Whichever scheduler you use, make the task idempotent, log its result, and define what should happen after a failure.
Quick Recap
A safe deployment checklist
- Confirm the VPS time zone and the account that should run the task.
- Write the five fields in minute, hour, day-of-month, month, day-of-week order.
- Use lists, ranges, and steps deliberately; test a complex expression on paper.
- Use absolute paths to interpreters, scripts, input files, and output files.
- Set required variables such as
PATH,SHELL, or application configuration explicitly. - Redirect output to a writable, rotated logfile or configure
MAILTO. - Test the exact command manually, then install with
crontab -e. - Verify with
crontab -l, observe one run, and remove temporary test entries. - Review permissions and avoid putting secrets directly in a world-readable script or command line.
Quick reference
| Task | Command or pattern |
|---|---|
| Edit current user’s jobs | crontab -e |
| List current user’s jobs | crontab -l |
| Remove current user’s entire table | crontab -r |
| Edit another user’s jobs (administrator) | crontab -u user -e |
| Every five minutes | */5 * * * * command |
| Every day at midnight | 0 0 * * * or @daily |
| At startup | @reboot command |
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