Linux systems have several ways to run jobs unattended at some later time -- either once or many times. Scheduling jobs in the future is handy for system maintenance, for sending yourself reminders, and for making more practical and efficient use of your CPU by running jobs when users are away.
Linux systems have several ways to run jobs unattended at some later time — either once or many times. Scheduling jobs in the future is handy for system maintenance, for sending yourself reminders, and for making more practical and efficient use of your CPU by running jobs when users are away.
As usual though, there are gotchas: if you want to run multiple commands or run commands that need a particular setup (including environment variables, a tty, or a certain current directory), or want to avoid system overload, you need to know a few tricks. This month, let’s dig into job scheduling systems, discuss the potential problems, and find some answers.
Catching some Zs
An easy way to delay execution is with the sleep command. sleep simply waits for some number of seconds (for instance, sleep 60 waits sixty seconds) and then exits.
For example, let’s say you’re going to lunch and you have three big files named a, b, and c to print on your office laser printer. However, you don’t want to tie up the printer constantly. Here’s how you could run one lpr print job, wait ten minutes, run the next job, and so on:
% lpr a; sleep 600; lpr b; sleep 600; lpr c
The ; (semicolon) is the shell’s command separator. With it, you can string several commands together at a single prompt. Commands run in the order you type them, and you won’t get another shell prompt until the last command (lpr c) finishes.
If that’s a problem, you can run the command sequence in a separate xterm window. Or — especially if you have just one console where you need to run commands while sleep is sleeping — run the command in the background with the shell’s & (ampersand) operator. Be sure to use a subshell (the shell’s ( ) operators) to put the entire sequence into the background together.
For instance, to pop open an xmessage window four hours from now with the message, “Meet Jim,” type:
% (sleep 14400; xmessage -near Meet Jim) &
The shell silently runs sleep for 14,400 seconds, and then runs xmessage.
Using sleep has advantages over cron and at (we’ll look at cron and at momentarily). For instance, since sleep is typically used in a shell window (like xterm), it can open other windows. Other techniques can’t open windows because they aren’t run from your terminal under X.
at: Doing It at a Later Time
The at utility queues one or more commands to run at some time in the future. A daemon (or a special cron job) runs jobs submitted by at.
Using at, you can schedule jobs to run minutes or months into the future. Typically, you put the job time on the command line, and enter the commands to run on at‘s standard input. Type man at for details (there are several versions of at, and their command line syntaxes vary.)
Here’s a simple example. Let’s say you need two things done overnight: copy a huge file with wget, and email yourself a note that’ll be waiting for you when you come into work. To run a job less than 24 hours in the future, just give the time: for example, 3:04am or 0304 for 3:04 in the morning. (If your system is down overnight, any jobs missed should be run soon after the system restarts.) Type the command lines from the terminal, which is at‘s standard input. End input with CTRL-D at the start of a line. (Your at may read input silently instead of prompting.)
At 3:04 AM, the commands are run by a shell. (at often runs a Bourne-type shell like bash, but your at system may use csh. If you want to discover which shell at uses, have at run a command like ps $$ | mail yourname to email the shell’s name to yourself.)
To run several commands, or to make a job repeat, put your job in a file. For example, the Bourne shell script cruncher.at (the .at suffix isn’t required), shown in Listing One, uses an if statement to check the system load average, taken from the file /proc/loadavg (however, using the uptime command would be more portable). If the one-minute load average is less than 1, a program named cruncher is run. Otherwise, cruncher.at is re-submitted to run 10 minutes later. Let’s submit the job.
Listing One: cruncher.at: If the load is low, run a big job, otherwise delay
loadavg=`sed ‘s/\..*//’ /proc/loadavg`
if [ "$loadavg" -lt 1 ]
at now + 10 min < cruncher.at
Although some versions of at accept filenames on the command line, all versions can read jobs from standard input. So (as you see in the cruncher.at script file) we’ll use the shell’s < (less than) operator to redirect the file to at‘s standard input:
$ at 0234 < cruncher.at
job 7 at 2003-01-16 02:34
The job is scheduled to run at 2:34 AM or a little later (many systems run at jobs every five minutes or more). You can check the job queue with atq or at -q.
If a job produces output, the at daemon mails it to you. But, if you expect output, you should probably pipe the output to an email program, redirect it to a file, or discard the output by redirecting it to /dev/null (the Linux “bit bucket”). That way, you only get email from the at daemon if something goes wrong.
Also note that redirecting the output of at itself (when you submit the job) won’t redirect output of the job as it runs! That’s because at simply submits the job to a queue. You have to redirect the output of each individual command in the queued job (or in Bourne-like shells, use exec to redirect all output to a file).
Listing Two: Redirecting the output of an at job
exec >> cruncher.at.log 2>&1
echo cruncher.at: starting at `date`
loadavg=`sed ‘s/\..*//’ /proc/loadavg`
if [ "$loadavg" -lt 1 ]
at now + 10 min < cruncher.at
For instance, the first line of our next version of cruncher.at, shown in Listing Two, appends all output (both standard output and standard error) from all following commands to the file cruncher.at.log. The second line outputs a message (written to cruncher.at.log) with the current date and time.
at jobs are great for running jobs irregularly or for running jobs (like cruncher.at) that resubmit themselves. To run a job periodically — from once a minute to once a year — it’s easier to use cron (or variant anacron, described later).
There are several versions of cron. All versions use a daemon to runs jobs, and each includes a master crontab file that lists jobs run for root, the superuser. While system administrators maintain root’s crontab, many versions of cron also provide a personal crontab file for each user — albeit with a slightly different format.
Here’s a simple personal crontab entry. At 1:09 AM every morning, run the script $HOME/bin/ci_crontab:
9 1 * * * bin/ci_crontab
Here, the first five entries (9, 1, *, *, and * are minutes, hours, day of month, month, and day of week, respectively, and the last entry is the command to run.
Each of the first five fields can have a specific value (like 9 for nine minutes past the hour), a set of values separated by a comma (like 11,12 for November and December), or a range of values separated by a hyphen (like 18-23 for 6 PM through 11 PM). An asterisk (*) is a placeholder that means “any.” (This is the format of crontab entries on Red Hat. Run man crontab on your system to see if your crontab format differs).
The ci_crontab script (available at http://www.linux-mag.com/downloads/2003-02/power/ci_crontab.txt) compares the current crontab file to the previous version stored in an RCS archive. If the file has changed, ci_crontab runs ci to check it into RCS. (crontab files can be easily lost or overwritten; this script makes it easy to recover old crontab entries.)
You may be wondering about the relative pathname to the script, bin/ci_crontab. That’s a good lead-in to the tangly topic of cron‘s execution environment. (The next section also generally applies to anacron.)
Exploring the Environment
The cron daemon, which reads crontab files and runs jobs from them, doesn’t run from your terminal. So, cron doesn’t have the same environment as your shell. Environment variables and other settings like the current directory, PATH (a list of directories with program files), and TZ (your timezone), are not defined. There’s at least one exception: each cron job starts in your home directory, or $HOME. So, files you read or write without a pathname will be in your home directory.
This is why our sample entry uses the relative pathname bin/ci_crontab: the ci_crontab script is in the $HOME/bin directory, which isn’t in cron‘s PATH. (You can set environment variables within a crontab file, but it may just be simpler to use a pathname.)
cron passes each crontab entry to a different instance of (typically) the Bourne shell. Within each entry, you can set its shell’s environment — just as you can type a series of environment-setting commands at the shell prompt on a terminal. Separate multiple commands with semicolons (;) or conditional operators. Let’s look at some examples.
19 16 14 1 * (env; id; set) > cron-stuff
This first example uses a subshell to run three commands on January 14 at 4:19 PM: env shows cron‘s environment variables, id displays its UID and GID, and set lists the settings of all shell variables. The modifier “> cron-stuff” redirects all output to the file cron-stuff in the home directory. (This first example is handy to run just once: save its output and then delete the entry from your crontab.)
Let’s look at a more complex example, shown in Listing Three. Listing Three (which must be entered as a single crontab line) runs three commands at 11:03 PM local time, Sunday through Thursday (the day of the week is specified by 0-4, where 0=Sunday, 1=Monday, and so on). Why times like 4:19 PM and 11:03 PM, instead of “normal” times like 4:30 PM or 11:00 PM? See the sidebar, “Cron Job Overload.”
Listing Three: Setting environment variables in cron jobs
The first command sets the timezone enviroment variable to Japan for the remainder of the crontab entry.
The second command uses shell command substitution to set a shell variable named day with the current year, month, and date (in Japan), like 20030114. This version of cron requires literal percent signs (%) to be escaped (\%). We’ll soon see why that’s necessary.
The third command changes the current directory to /tokyo. If it succeeds, cd returns “true” (a status code of 0), and the shell’s conditional operator && executes /tokyo/crunch. (If /tokyo weren’t accessible, cd would fail, and the rest of the command would be skipped.) Because cron‘s PATH doesn’t include the current directory (.), the pathname must be ./crunch. The crunch output is written to a file like /tokyo/logs/crunch.20030114.
And one more example:
17 4 * * * ed – /prj/log%1,$g/^NOTE:/d%w
This entry is like typing the following on a terminal:
$ ed – /prj/log
Here, the entry uses the % (percent) operator, which some crontab files support, to pass an editing script to the standard input of the ed editor. The percent operator also lets you embed newline characters in a crontab entry — which is hard to do otherwise because crontab entries must be on a single line. Any text after the first % is fed to the standard input of the command, with each following % replaced by a newline.
With techniques like these, you can write entire scripts in single-line crontab entries. But you may want to put more-complex entries into a script file, possibly in a library directory full of cron scripts.
Power Tip: Remembered Directories
If you repeatedly cd between the same few directories, you don’t have to type
over and over. Many shells support the command
, which means “go back to the previous directory.”
To change between several directories over and over, don’t use cd. Instead, use the shell’s directory stack, a first-in-first-out list, and the commands pushd and popd, to “push” (add) and “pop” (remove) directories, respectively.
With an argument, pushd adds your current directory to the top of the directory stack and then changes to the directory you specified. With no argument, pushd swaps your current directory with the directory on the top of the stack.
The popd command removes the top directory from the stack and changes to that directory. (pushd and popd also accept arguments like
to specify the nth directory on the stack.)
Finally, dirs shows your current directory and the current state of the stack. Here’s a brief example:
There’s much more to directory stacks! See a good reference about shells — and next month’s Power Tip.
anacron: Doing It When You Can
Your Linux machine may not be up 24 hours a day, 7 days a week. For example, if your Linux machine is a laptop, you may only turn it on from time to time. Because cron jobs are tied to particular times of the day (or week or year), anacron, a variant of cron, might be a better choice for scheduling jobs on these machines.
Let’s take a look at a slightly-edited anacrontab file from a Red Hat system:
Each job — here, each entry in the anacrontab — has a name and a corresponding timestamp file. When anacron starts, it looks for jobs that haven’t been executed in the last n days, where n is the time period listed in the first field of an entry. If a job needs running, anacron delays the number of minutes in the second field, passes the command to a shell, and then updates the timestamp named in the third field. Delays can give the system time to start other processes — including, in the case of anacron, other anacrontab jobs.
In the listing shown above, the first entry sets the search path. The other three entries run jobs. For example, the last entry checks the month timestamp. If the job hasn’t run in the past 30 days (the 30 field), then anacron delays 15 minutes (the 15) before running the job (run-parts /etc/cron.monthly).
In this example, run-parts is a simple shell script that looks in a directory like /etc/cron.monthly for executable programs and runs them all. This is a handy way to avoid cluttering the anacrontab with lots of entries. (You can use the same technique in a crontab.)
anacron considers each entry in the anacrontab unless you specify certain jobs (here, day, week, or month) on its command line. You can use shell-type wildcards to specify jobs.
For instance, your anacrontab could have entries named news.daily, news.weekly, http.daily, http.weekly, and so on. Running anacron ‘*.daily’ (the quotes are necessary to prevent the shell from expanding *) would check the jobs news.daily and http.daily. The command line anacron ‘news.*’ would run jobs whose names start with news..
anacron runs all jobs that it should and then exits. So two places to start it are in your system startup files (/etc/rc*) or periodically (once a day, or more) from either your personal or system-wide crontab file.
Looking Back and Ahead
We’ve looked at several ways to delay and repeat execution: using sleep from a terminal, at to run jobs at irregular intervals, cron to run jobs periodically, and anacron to supplement cron on systems that aren’t up 24×7.
Next time, we’ll explore the dark corners of file transfers.
Cron job overload
Do you punch in “even” cook times, like 1:00, on your microwave oven? Maybe this is the same habit that makes a lot of us run all of our at and cron jobs at the top of the hour — sometimes overloading a system hourly.
Just as you can add some spice to your microwaving by warming your coffee for 57 seconds, you can choose “odd” times like 4:09 AM for your cron jobs, unless the job really needs to run exactly on the hour. Assigning different times to each job levels the load and reduces the chance that your CPU-intensive job runs at the same time as someone else’s.
Jerry Peek is a freelance writer and instructor who has used Unix and Linux for over 20 years. He’s happy to hear from readers at email@example.com.
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