Periodic Processes

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Periodic Processes

• Chapter 9

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Introduction

• The key to staying in control of your system is
  to automate as many tasks as possible.
• For example, and adduser program can add new
  users faster than you can, with a smaller chance
  of making mistakes.
• Almost any task can be encoded in a shell, Perl,
  or expect script.
• It is often useful to have a script or command
  executed without any human intervention
• For example, you might want to have a script
  verify (say, every half hour) that your network
  routers and bridges are working correctly and
  have it send you email when a problem is
  discovered.

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1. cron schedule commands

• Under UNIX, periodic execution is handled by the
  cron daemon.
• cron starts when the system boots and remains
  running as long as the system is up.
• cron reads one or more configuration files
  containing lists of command lines and times at
  which they are to be invoked.
• The commands are executed by sh, so almost
  anything you can do from the shell can be done by
  cron.

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1. cron schedule commands

• A cron configuration file is called a crontab,
  short for cron table.
• All crontab files are stored in a single system
  directory, where cron knows where to look for
  them
• The crontab command transfers crontab files to
  and from this directory.
• Typically there is (at most) one crontab file per
  user
• Files are named with the loginname of the user
  they belong to, and cron uses these filenames to
  figure out which UID to use when running the
  commands that each file contains

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1. cron schedule commands

• Cron normally does it work silently, but some
  versions keep a log file.
• The log file grows quickly and is rarely useful
• Leave logging off unless youre debugging a
  specific problem.
• Most crons do not compensate for commands that
  are missed while the system is down.
• Some do not understand daylight savings

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2. The format of crontab files

• All the crontab files on a system share a common
  format.
• Comments are introduced with a in the first
  column
• Each non comment line contains six fields
• minute hour day month weekday command
• The first five are separated by whitespace.
• Whitespace in the command is taken literally

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2. The format of crontab files

• Example 1
• 45 10 1-5
• Means 1045 am Monday through Friday
• Example 2
• 0,30 13 5
• Means every half hour on Friday AND every half
  hour on the 13th,
• It does not mean every half hour on Friday the
  13th.

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2. The format of crontab files

• The command is the sh command line to be executed
• It can be any valid shell command and should not
  be quoted.
• Usually any output produced by a cron command is
  mailed to the owner of the crontab
• Example
• 20 1 find /tmp atime 3 exec rm f
  
• What does it do?

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3. Crontab management

• crontab filename
• Installs filename as your crontab (replaces old
  file)
• crontab -e
• Checks out a copy of your crontab file and
  invokes your editor on it.
• As specified by the EDITOR environment variable
• crontab -l
• Lists the contents of your crontab file to
  standard output
• crontab -r
• Removes your file

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3. Crontab management

• Most systems allow root to supply a username
  argument so that other users crontab files may be
  viewed, edited, or deleted
• crontab r jsmith
• By default, all users can submit crontab files to
  cron
• cron.allow and cron.deny allow you to override
  this policy
• It is important to note that access is controlled
  by crontab (not by cron), so if a user can sneak
  a crontab file into the appropriate directory
  cron will blindly execute it.

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4. Some common uses for cron

• A number of tasks are especially suited for
  invocation by cron.
• These usually make up the bulk of the material in
  roots crontab
• Most UNIX systems often come with some crontab
  entries preinstalled for you.
• So, lets look at some of the tasks

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4. Some common uses for cron

• Cleaning the filesystem
• core files
• Useful for software developers, but for
  administrators they are usually a waste of space.
• NFS
• When a file is deleted on an NFS server (but
  still is used remotely) it will often rename the
  file .nfsxxx
• /tmp and /var/tmp files

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4. Some common uses for cron

• Sample cron lines
• find / -xdev name code atime 7 exec rm f
  
• find / -xdev atime 3 ( name o name
  . o name .CKP o name o name
  .nfs ) exec rm f
• find /var/preserve mtime 14 exec rm f
• cd /tmp find . ! name . ! name lostfound
  type d -mtime 3 exec /bin/rm rf

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4. Some common uses for cron

• Network distribution of configuration files
• If you are running a network of machines, it is
  often convenient to maintain a single
  network-wide version of configuration files
• Examples
• Mail aliases
• NIS

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4. Some common uses for cron

• Rotating log files
• UNIX log files generally grow without bound until
  they are manually reset.
• You could just truncate them at various intervals
• A more conservative strategy is to rotate log
  files by keeping several older versions of each
  one.
• See page 205 for more info on this.

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5. Vendor specifics

• Red Had and FreeBSD use a free version of cron
  called Vixie-cron

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