bash comparison

Reference: http://www.tldp.org/LDP/abs/html/comparison-ops.html

 

 

 

#+ 
http://tldp.org/LDP/Bash-Beginners-Guide/html/sect_07_02.html

 

ntp client

# Server is ntpd

But client is $ntpdate

we need to install ntpdate

# – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – – –

Change  Server Time and Location

NTP server install on linux

I installed NTP server on Ubuntu Server 10.04 using:

The NTP daemon seems working and listening on the 123 port.

However, I was not able to get the time from another machine:

Remove  previous version

 


If you have ntpd installed on another machine, you can use your first server in the servers list of your ntp.conf file, or can synchronize one time with the -q option, as follows:

 

How to replace a string in shell script ?

 

Ways To Search For Files Using The Terminal

Today we will look at some of the common ways to search for files in Linux using the Terminal.

1) find : To search for files on the command line you can use the command “find”. The following is syntax for the “find” command:

find path criteria action

“path” – The section of the files system to search (the specific directories and all the sub directories). If nothing is specified the file system below the current directory is used.

“criteria” – The file properties.

“action” – Options that influence conditions or control the search as a whole, ie,
“–print”

 

 

2007-11-22-153941_1280x800_scrot

 

2) locate : The command “locate” is an alternative to the command “find -name”. The command find must search through the selected part of the file system, a process that can be quite slow. On the other hand, locate searches through a database previously created for this purpose (/var/lib/locatedb), making it much faster. The database is automatically created and updated daily. But change made after the update has been performed are not taken into account by locate, unless the database is updated manually using the command updatedb.

 

2007-11-22-154054_1280x800_scrot

 

3) whereis : The command “whereis” returns the binaries (option -b), manual pages (option -m), and the source code (option -s) of the specific command. If no options is used all the information is returned, if the information is available. This command is faster than “find” but is less thorough.

2007-11-22-154226_1280x800_scrot

 

4) which : The “which” command searches all paths listed in the variable PATH for the specific command and returns the full path of the command. the command is specifically useful if several version of a command exist in different directories and you want to know which version is executed when entered without specifying a path.

2007-11-22-154254_1280x800_scrot

 

5) type : The “type” command can be used to find out what kind of command is executed when command is entered – a shell built in command or an external command. The option -a delivers all instances of a command bearing this name in the file system.

2007-11-22-154354_1280x800_scrot

cron job, crontab -e, -l using

Cron Job  Timing ,

Advanced Crontab

The Crontabs discussed above are user crontabs. Each of the above crontabs is associated with a user, even the system crontab which is associated with the root user. There are two other types of crontab.

Firstly, as mentioned above anacron uses the run-parts command and /etc/cron.hourly/etc/cron.weekly, and /etc/cron.monthly directories. However anacron itself is invoked from the /etc/crontab file. This file could be used for other cron commands, but probably shouldn’t be. Here’s an example line from a ficticious /etc/crontab:

This would run Rusty’s command script as user rusty from his home directory. However, it is not usual to add commands to this file. While an experienced user should know about it, it is not recommended that you add anything to /etc/crontab. Apart from anything else, this could cause problem if the /etc/crontab file is affected by updates! Rusty could lose his command.

The second type of crontab is to be found in /etc/cron.d. Within the directory are small named crontabs. The directory is often used by packages, and the small crontabs allows a user to be associated with the commands in them.

Instead of adding a line to /etc/crontab which Rusty knows is not a good idea, Rusty might well add a file to /etc/cron.d with the name rusty, containing his cron line above. This would not be affected by updates but is a well known location.

When would you use these alternate crontab locations? Well, on a single user machine or a shared machine such as a school or college server, auser crontab would be the way to go. But in a large IT department, where several people might look after a server, then /etc/cron.d is probably the best place to install crontabs – it’s a central point and saves searching for them!

You may not need to look at /etc/crontab or /etc/cron.d, let alone edit them by hand. But an experienced user should perhaps know about them and that the packages that he/she installs may use these locations for their crontabs.

string

meaning

@reboot

Run once, at startup.

@yearly

Run once a year, “0 0 1 1 *”.

@annually

(same as @yearly)

@monthly

Run once a month, “0 0 1 * *”.

@weekly

Run once a week, “0 0 * * 0”.

@daily

Run once a day, “0 0 * * *”.

@midnight

(same as @daily)

@hourly

Run once an hour, “0 * * * *”.

Using Cron

To use cron for tasks meant to run only for your user profile, add entries to your own user’s crontab file. Start the crontab editor from a terminal window:

crontab -e

Edit the crontab using the format described in the next sections. Save your changes. (Exiting without saving will leave your crontab unchanged.)

Note that a great source of information about the format can be found at:

man 5 crontab

Commands that normally run with administrative privileges (i.e. they are generally run using sudo) should be added to the root user’s crontab (instead of the user’s crontab):

sudo crontab -e

Crontab Sections

Each of the sections is separated by a space, with the final section having one or more spaces in it. No spaces are allowed within Sections 1-5, only between them. Sections 1-5 are used to indicate when and how often you want the task to be executed. This is how a cron job is laid out:

minute (0-59), hour (0-23, 0 = midnight), day (1-31), month (1-12), weekday (0-6, 0 = Sunday), command

The above example will run /usr/bin/somedirectory/somecommand at 4:01am on January 1st plus every Monday in January. An asterisk (*) can be used so that every instance (every hour, every weekday, every month, etc.) of a time period is used. Code:

The above example will run /usr/bin/somedirectory/somecommand at 4:01am on every day of every month.

Comma-separated values can be used to run more than one instance of a particular command within a time period. Dash-separated values can be used to run a command continuously. Code:

The above example will run /usr/bin/somedirectory/somecommand at 01 and 31 past the hours of 4:00am and 5:00am on the 1st through the 15th of every January and June.

The “/usr/bin/somedirectory/somecommand” text in the above examples indicates the task which will be run at the specified times. It is recommended that you use the full path to the desired commands as shown in the above examples. Enter which somecommand in the terminal to find the full path to somecommand. The crontab will begin running as soon as it is properly edited and saved.

You may want to run a script some number of times per time unit. For example if you want to run it every 10 minutes use the following crontab entry (runs on minutes divisible by 10: 0, 10, 20, 30, etc.)

which is also equivalent to the more cumbersome

Crontab Options

  • The -l option causes the current crontab to be displayed on standard output.
  • The -r option causes the current crontab to be removed.
  • The -e option is used to edit the current crontab using the editor specified by the EDITOR environment variable.

After you exit from the editor, the modified crontab will be checked for accuracy and, if there are no errors, installed automatically. The file is stored in /var/spool/cron/crontabs but should only be edited via the crontab command.

Enable User Level Cron

If the /etc/cron.allow file exists, then users must be listed in it in order to be allowed to run the crontab command. If the /etc/cron.allow file does not exist but the /etc/cron.deny file does, then users must not be listed in the /etc/cron.deny file in order to run crontab.

In the case where neither file exists, the default on current Ubuntu (and Debian, but not some other Linux and UNIX systems) is to allow all users to run jobs with crontab.

No cron.allow or cron.deny files exist in a standard Ubuntu install, so all users should have cron available by default, until one of those files is created. If a blank cron.deny file has been created, that will change to the standard behavior users of other operating systems might expect: cron only available to root or users in cron.allow.

Note, userids on your system which do not appear in /etc/shadow will NOT have operational crontabs, if you desire to enter a user in /etc/passwd, but NOT /etc/shadow that user’s crontab will never run. Place an entry in /etc/shadow for the user with a * for the password crypt,ie:

Further Considerations

Crontab commands are generally stored in the crontab file belonging to your user account (and executed with your user’s level of permissions). If you want to regularly run a command requiring administrative permissions, edit the root crontab file:

Depending on the commands being run, you may need to expand the root users PATH variable by putting the following line at the top of their crontab file:

It is sensible to test that your cron jobs work as intended. One method for doing this is to set up the job to run a couple of minutes in the future and then check the results before finalising the timing. You may also find it useful to put the commands into script files that log their success or failure, for example:

For more information, see the man pages for cron and crontab (man is detailed on the Basic Commands page). If your machine is regularly switched off, you may also be interested in at and anacron, which provide other approaches to scheduled tasks. For example, anacron offers simple system-wide directories for running commands hourly, daily, weekly, and monthly. Scripts to be executed in said times can be placed in/etc/cron.hourly//etc/cron.daily//etc/cron.weekly/, and /etc/cron.monthly/. All scripts in each directory are run as root, and a specific order to running the scripts can be specified by prefixing the scripts’ filenames with numbers (see the man page for run-parts for more details). Although the directories contain periods in their names, run-parts will not accept a file name containing a period and will fail silently when encountering them.

/etc/init.d/cron