In Linux, tasks can be configured to run
automatically within a specified period of time, on a specified date, or when
the system load average is below a specified number. Red Hat Linux comes
preconfigured to run important system tasks to keep the system updated. For
example, the slocate database used by the locate command is updated daily. A
system administrator can use automated tasks to perform periodic backups,
monitor the system, run custom scripts, and more.
Red Hat Linux comes with four automated
tasks utilities: cron, anacron, at, and batch.
Cron is a daemon that can be used to
schedule the execution of recurring tasks according to a combination of the
time, day of the month, month, day of the week, and week. Cron assumes that the
system is on continuously. If the system is not on when a task is scheduled, it
is not executed. To configure tasks based on time periods instead of exact
times Anacron.
To use the cron service, you must have
the vixie-cron RPM package installed, and the crond service must be running. To
determine if the package is installed, use the rpm -q vixie-cron command. To
determine if the service is running, use the command /sbin/service crond
status.
01
* * * * root run-parts /etc/cron.hourly
02
4 * * * root run-parts /etc/cron.daily
22
4 * * 0 root run-parts /etc/cron.weekly
42 4 1 * * root run-parts
/etc/cron.monthly
Each line in the /etc/crontab file
represents a task and has the format:
minute hour
day month dayofweek
command
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·
minute — any integer from 0 to 59
·
hour — any integer from 0 to 23
·
day — any integer from 1 to 31 (must be
a valid day if a month is specified)
·
month — any integer from 1 to 12 (or the
short name of the month such as jan, feb, and so on)
·
day of week — any integer from 0 to 7,
where 0 or 7 represents Sunday (or the short name of the week such as sun, mon,
and so on)
·
command — the command to execute (The
command can either be a command such as ls /proc >> /tmp/proc or the
command to execute a custom script that you wrote.)
For any of the above values, an asterisk
(*) can be used to specify all valid values. For example, an asterisk for the
month value means execute the command every month within the constraints of the
other values.
A hyphen (-) between integers specifies
a range of integers. For example, 1-4 means the integers 1, 2, 3, and 4.
Anacron
Anacron is a task scheduler similar to
cron except that it does not require the system to run continuously. It can be
used to run the daily, weekly, and monthly jobs usually run by cron.
To use the Anacron service, you must
have the anacron RPM package installed and the anacron service must be running.
To determine if the package is installed, use the rpm -q anacron command. To
determine if the service is running, use the command /sbin/service anacron
status.
Anacron tasks are listed in the
configuration file /etc/anacrontab. Each line in the configuration file
corresponds to a task and has the format:
period delay
job-identifier command
|
·
period — frequency (in days) to execute
the command
·
delay — delay time in minutes
·
job-identifier — description of the
task, used in Anacron messages and as the name of the job's timestamp file, can
contain any non-blank characters (except slashes)
·
command — command to execute
For each task, Anacron determines if the
task has been executed within the period specified in the period field of the
configuration file. If it has not been executed within the given period,
Anacron executes the command specified in the command field after waiting the
number of minutes specified in the delay field.
After the task is completed, Anacron
records the date in a timestamp file in the /var/spool/anacron directory. Only
the date is used (not the time), and the value of the job-identifier is used as
the filename for the timestamp file.
Environment variables such as SHELL and
PATH can be defined at the top of /etc/anacrontab as with the cron
configuration file.
The default configuration file looks
similar to the following:
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#
See anacron(8) and anacrontab(5) for details.
SHELL=/bin/sh
PATH=/usr/local/sbin:/usr/local/bin:/sbin:/bin:/usr/sbin:/usr/bin
#
These entries are useful for a Red Hat Linux system.
1 5
cron.daily run-parts
/etc/cron.daily
7 10
cron.weekly
run-parts /etc/cron.weekly
30 15
cron.monthly
run-parts /etc/cron.monthly
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At and Batch
While cron and anacron are used to
schedule recurring tasks, the at command is used to schedule a one-time task at
a specific time. The batch command is used to schedule a one-time task to be
executed when the systems load average drops below 0.8.
To use at or batch you must have the at
RPM package installed, and the atd service must be running. To determine if the
package is installed, use the rpm -q at command. To determine if the service is
running, use the command /sbin/service atd status.
To schedule a one-time job at a specific
time, type the command at time,
where time is the time to
execute the command.
The argument time can be one of the following:
·
HH:MM format — For example, 04:00
specifies 4:00AM. If the time is already past, it is executed at the specified
time the next day.
·
Midnight — Specifies 12:00AM.
·
Noon — Specifies 12:00PM.
·
Teatime — Specifies 4:00PM.
·
Month-name day year format — For
example, January 15 2002 specifies the 15th day of January in the year 2002.
The year is optional.
·
MMDDYY, MM/DD/YY, or MM.DD.YY formats —
For example, 011502 for the 15th day of January in the year 2002.
·
Now + time — time is in minutes, hours,
days, or weeks. For example, now + 5 days specifies that the command should be
executed at the same time in five days.
The time must be specified first,
followed by the optional date. For more information about the time format, read
the /usr/share/doc/at-<version>/timespec
text file.
After typing the at command with the
time argument, the at> prompt is displayed. Type the command to execute,
press [Enter], and type Ctrl-D. More than one command can be specified by
typing each command followed by the [Enter] key. After typing all the commands,
press [Enter] to go to a blank line and type Ctrl-D. Alternatively, a shell
script can be entered at the prompt, pressing [Enter] after each line in the
script, and typing Ctrl-D on a blank line to exit. If a script is entered, the
shell used is the shell set in the user's SHELL environment, the user's login
shell, or /bin/sh (whichever is found first).
If the set of commands or script tries
to display information to standard out, the output is emailed to the user.
Use the command atq to view pending
jobs. Usage of the at command can be restricted.
To execute a one-time task when the load
average is below 0.8, use the batch command.
After typing the batch command, the
at> prompt is displayed. Type the command to execute, press [Enter], and
type Ctrl-D. More than one command can be specified by typing each command
followed by the [Enter] key. After typing all the commands, press [Enter] to go
to a blank line and type Ctrl-D. Alternatively, a shell script can be entered
at the prompt, pressing [Enter] after each line in the script, and typing
Ctrl-D on a blank line to exit. If a script is entered, the shell used is the
shell set in the user's SHELL environment, the user's login shell, or /bin/sh
(whichever is found first). As soon as the load average is below 0.8, the set
of commands or script is executed.
If the set of commands or script tries
to display information to standard out, the output is emailed to the user.
Use the command atq to view pending
jobs. Usage of the batch command can be restricted.
To view pending at and batch jobs, use
the atq command. It displays a list of pending jobs, with each job on a line.
Each line is in the format job number, date, hour, job class, and username.
Users can only view their own jobs. If the root user executes the atq command,
all jobs for all users are displayed.
/ Root file system. Should just contain /bin, /sbin, /dev,
/root,/lib, and /etc.
/usr Programmes and source code.
/var Variable data, such as spools, man pages, news and mail
queues, database data.
/boot Boot kernels.
/home User data and "stuff".
/tmp Temporary file locations.
Test operators
String test operators - comparisons use
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Operator
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Meaning
|
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string1 = string2
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string1 is equal to string2
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string1 != string2
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string1 is NOT equal to string2
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string1
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string1 is NOT NULL or not
defined
|
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-n string1
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string1 is NOT NULL and does exist
|
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-z string1
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string1 is NULL and does exist
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Shell also test for file and directory types
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Test
|
Meaning
|
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-s
file
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Non empty file
|
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-f
file
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Is File exist or normal file and not a directory
|
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-d
dir
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Is Directory exist and not a file
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-w
file
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Is writeable file
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-r file
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Is read-only file
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-x
file
|
Is file is executable
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/dev/null - Use to send unwanted output of program
This is special Linux file which is used
to send any unwanted output from program/command.
Syntax:
command > /dev/null
Syntax:
command > /dev/null
Example:
$ ls > /dev/null
$ ls > /dev/null
File Manipulation
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Ls
|
list contents of current directory.
Examples:
ls file.1 lists presence of file.1 ls -l lists presence of file.1 with expanded information ls -a lists contents of current directory, including files that start with "." |
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jot file.1
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opens jot editor; reads file.1 into jot for editing
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cat file.1
|
show contents of file.1 on screen
|
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more file.1
|
show contents of file.1 on screen, but doesn't
scroll past top of screen
|
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head file.1
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show first 20 lines of file.1 on screen
|
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tail file.1
|
show last 20 lines of file.1 on screen
|
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diff file.1 file.2
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Shows differences between file.1 and file.2
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|
Wc
|
counts lines, words, and characters in
file file.1. Variations:
wc -l counts lines in file file.1 wc -w counts words in file file.1 wc –m counts characters in file file.1 |
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cp file.1 file.2
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copies file.1 to file.2.
If file.2 exists, it is
overwritten. Original file.1
is not removed
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mv file.1 file.2
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moves file.1 to file.2.
If file.2 exists, it is
overwritten. Original file.1
is removed
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rm file.1
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remove file.1. No utilities exist to retrieve it
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grep PATTERN file.1
|
print all lines that contain PATTERN
in file.1
|
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lpr file.1
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print file.1 on default laser printer
|
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Lpr
|
Show list of available printers and
questions to direct your printing of a file to a particular printer
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|
du –sk
|
print size (in kilobytes) of current
directory ("."). Variations:
du -sk * prints sizes (in kilobytes) of all files and sub-directories du –sk file.1 prints size (in kilobytes) of file.1 |
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compress file.1
|
reduce the size of file.1 and replace it with a file
named file.1.Z. This
compressed file is NOT a (readable) ASCII file.
|
|
uncompress file.1.Z
|
restore the compressed file file.1.Z
|
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find . -name file.1 –print
|
find the loation of file.1 in the directory and
sub-directories. Variations:
find . -name "*pattern" -print finds all files that end with pattern. find . -name file.1 -exec rm '{}' ';' removes all files named file.1. |
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grep pattern file.1
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print only the lines of file.1 that contain pattern
|
|
sort file.1
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displays contents of file.1 sorted alphabetically.
Variations:
sort -n file.1 displays contents of file.1 sorted numerically sort -r file.1 displays contents of file.1 sorted alphabetically in reverse order. |
Directory Manipulation
|
Pwd
|
show
the directory that you are in (present working directory)
|
|
cd
dir.1
|
change
directory to dir.1
|
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mkdir
dir.1
|
make
new directory dir.1
|
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rmdir
dir.1
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remove
EMPTY directory dir.1
|
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rm
-r dir.1
|
remove
directory dir.2 AND its
contents
|
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cp
-r dir.1 dir.2
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copy
dir.1 (and its contents) to dir.2
|
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mv
file.1 dir.1
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move
file.1 to dir.1
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Ls
|
show
contents of current directory. Variations:
ls dir.1 shows contents of dir.1 ls -d dir.1 shows PRESENCE of dir.1 |
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du
-sk dir.1
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show
sum of size (in kilobytes) of dir.1
and its contents
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tar
-cvf dir.1.tar dir.1
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store
an image of dir.1 and it's
contents in file file.1
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Process Control
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command1&
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execute command1 in background
|
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ps –ef
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print expanded list of all processes
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kill pid1
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remove process pid1
|
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<control-c>
|
interrupt current process
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<control-z>
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suspend current process
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Jobs
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display background and suspended
processes
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kill %1
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remove suspended process #1
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Top
|
display the current, most
computer-intensive commands
|
|
Osview
|
display the operating system
statitistics
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