Suse & OpenSuse Tips » command

Automatically dating your tar backups

admin — Wed, 06 May 2009 05:38:01 +0000

Normally, we use the tar command coupled with some compression program such as gzip to create a backup of our directories. I.e.

# tar -cf /var/log/backup/tux.tar /home/tux

to backup the home directory of the user tux into the directory /var/log/backup.

However, if we are going to do this on a regular basis, we will be overwriting the file tux.tar. This would mean that we will only have one snapshot of the user tux home directory.

To keep copies of backups from different times, you will need to use different filenames, preferably with a date indicator. You can easily do this by,

# tar –cf /var/log/backup/tux-`date +%F`.tar /home/tux

Of course, if you backup a few times within the day, then you may want to use a different option from the +%F format option.

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Automatically dating your tar backups

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Short and quick tutorial for the command “touch”

admin — Wed, 06 May 2009 05:19:12 +0000

The touch command is typically used for 2 purposes, to create a new file and to modify the access and modification times of the file. If the file already exist, it will update the both the access and modification times of the file and if it does not exist, it will create a new file. The latter is probably the most common usage of the command.

For example, to update the access and modification time stamp on a file to the current time;

# touch helloworld.c

You can also use a specific time by using the –t option or just modify either the access or modification time only, using the –a and –m options respectively.

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Short and quick tutorial for the command “touch”

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Using command output with backquotes

admin — Wed, 06 May 2009 03:43:32 +0000

The backquote (`), which is quite alien to most Windows users, is the key just below the top left hand side ESC key on your keyboard. This is a useful key to use when you want to use the standard output of a UNIX/LINUX command as part of another command.

For example, if you want to output the results of an addition.

# expr 10 + 42
# 52

If you would like to make it fancier,

# echo “The sum is `expr 10 + 42`”
The sum is 52

Note the backquotes (in red).

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Using command output with backquotes

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Using redirections: 2>&1 (Part 2 of 2)

admin — Mon, 20 Apr 2009 05:21:45 +0000

If you go through enough shell scripts, you will notice the frequent use of 2>&1. What does it mean and how it works?

We have briefly introduced standard in (stdin) and standard out (stdout). In the command line interface and also in shell programming, these are represented as follows:

0 Standard In (STDIN)
1 Standard Out (STDOUT)
2 Standard Error (STDERR)

The 2>&1 basically means that to redirect standard error (STDERR) to standard out (STDOUT). We use tar to archive the home directory of the user susetips and redirect the verbose output to a file bak.log as an example;

# tar cvf susetips.bak /home/susetips >> bak.log
tar: Removing leading ‘/’ from member names

The command above basically kept a listing of the files that were backed up in the file bak.log. This file can be kept as a reference to know what files were actually backed up. However, there’s a message from the tar command that still appeared on the terminal. This message is a warning/error message that outputs to STDERR, which in this case is the terminal. The >> redirection just puts the STDOUT to the file but not the STDERR. To redirect the STDERR to the same place as the STDOUT;

# tar cvf susetips.bak /home/susetips >> bak.log 2>&1
#

Where the 2 represented STDERR, > means redirect to and &1 is the variable name for STDOUT. You can check the content of the file bak.log by using the cat command.

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Using redirections: 2>&1 (Part 2 of 2)

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Using redirections; , >> (Part 1 of 2)

admin — Mon, 20 Apr 2009 04:51:16 +0000

Redirections can be sometimes confusing to learn and utilize effectively. We will try to explain it in the simplest possible way that we think is effective. Please feel free to suggests a better approach to explaining it.

Firstly, the 3 basic redirections.

< ; this less than symbol is to instruct the shell to read from a file instead of the standard input (stdin) which is generally the keyboard. As an example, assuming that you have a file 2compute and using the command bc (basic calculator), you can redirect the content of 2compute for bc.

# cat 2compute
1 + 1
2 / 2
# bc < 2compute
2
1
#

> ; the greater than symbol is to redirect the standard out (stdout) to a file instead of the screen/monitor (depending on what stdout is defined as). As an example, the ls command which list the content of your directory will display it on your terminal. We can redirect the output into a file (we use mydirectory as an example)

# ls
bin Desktop Distribution Documents susetips
# ls > mydirectory
# cat mydirectory
bin
Desktop
Distribution
Documents
mydirectory
susetips

>> ; similarly to the > symbol, using double greater than will append to the specified file as oppose to overwriting the file (if it exists). Following our previous example, if you use the single >, it will overwrite the old file. Using the double >, it will append to it.

# cat mydirectory
bin
Desktop
Distribution
Documents
mydirectory
susetips
# ls > mydirectory
# cat mydirectory
bin
Desktop
Distribution
Documents
mydirectory
susetips
#ls >> mydirectory
# cat mydirectory
bin
Desktop
Distribution
Documents
mydirectory
susetips
bin
Desktop
Distribution
Documents
mydirectory
susetips
#

Next, the puzzling 2>&1

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Using redirections; <, >, >> (Part 1 of 2)

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Using the “date” command and it’s % options

admin — Fri, 17 Apr 2009 07:03:51 +0000

Well, you can’t really get a date with this command but it is a very useful command to know for administrative tasks as well as for automatic documentation through scripting.

However, the date command’s options often baffles a lot of people (well, in my administering SuSE Linux class anyway) especially when they read about the formatting using the % symbol on the man pages.

Let me briefly extract what the man pages say;

NAME
    date – print or set the system date and time

SYNOPSIS
    date [OPTION] … [+FORMAT]
    date [-u|—utc|—universal] [MMDDhhmm[[CC]YY][.ss]]

DESCRIPTION
    … blah blah …

    FORMAT controls the output. The only valid option
    for the second form specifies Coordinated
    Universal Time. Interpreted sequences are:

    %%    a literal %
    …
    %F    full date; same as %Y-%m-%d

I have seen many different attempts at trying to understand how to use the %F, e.g.

# date %F

# date –F

However, it is actually quite clear but somehow it is not the usual UNIX/LINUX command option format. To get the date in the format 2009-04-17 (today’s date), you simply do

# date +%F
2009-04-17

It becomes obvious after you read the man page again.

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Using the “date” command and it’s % options

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Short and quick tutorial for the command “tar”

admin — Thu, 16 Apr 2009 10:00:46 +0000

We have recently posted about tarball and the use of tar coupled with compression programs to create it. Here, we will go through three (3) short tutorial on using tar.

To compress a list of files

To compress (the c in the options) a list of files into a specific file (the f in the options) whilst having the tar program to provide progress of it (verbose mode, the v in the option), we use

# tar cvf .tar

Leading / will be removed from the archived listing as this will prevent accidental replacement of system files if the file is uncompressed by root.

To uncompress a file

Similarly, we use the same command options where we replace the c with x (for eXtract).

# tar xvf tarfile.tar

This will extract all the files from the archive and recreate the directories and files in the location where the user is in. Note that, it is good practice to create a new directory, change directory into it before you extract from a tar archive. This is to prevent overwriting of existing files, unless that is the intention.

To view content of a file

Sometimes, we just want to check if we have a specific file in an archive. For example, if we are accessing a tape drive (at /dev/st0), we can list (the t in the option) the content using

# tar tvf /dev/st0

More often than not, the listing will be too long for us. We can pipe the output to grep.

# tar tvf /dev/st0 | grep

Hope this is short enough. Leave a comment if you want to ask questions pertaining to tar.

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Short and quick tutorial for the command “tar”

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What are .Z, .gz and .bz2 files?

admin — Wed, 15 Apr 2009 07:04:24 +0000

As part of a tarball distribution and also other compressed files, you will come across files with the .Z, .gz and .bz2 extensions. These are basically files that have been compressed using different applications, much like the .zip files predominantly found in the Windows environment.

compress for .Z files

The UNIX command compress is used to create and extract files with the .Z extension. Unfortunately, this is not available on SuSE. If you need to uncompress .Z files that you downloaded, you can use the command zcat.

gzip and gunzip for .gz files

This is the more common compression program and algorithm that’s used with Linux.

# gzip
# gunzip .gz

bzip2 and bunzip2 for .bz2 files

Known to have a more superior compression ratio than gzip but less frequently used.

# bzip2
# bunzip2 .bz2

zip and unzip for .zip files

Being in a world where Windows seems to be the norm, we can’t avoid receiving file that’s compressed with WinZIP or other tools that create .zip file. Luckily for us, we have zip and unzip for these files.

# zip .ZIP
# unzip .ZIP

Please comment if you have any further questions.

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What are .Z, .gz and .bz2 files?

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Short and quick tutorial for the command “find”

admin — Mon, 06 Apr 2009 06:30:15 +0000

We can learn about commands in Linux using the manpages, for example, to learn how to use the find command, we can just do

# man 1 find

However, this means we need to read the long manual and generally most of us are somewhat impatient. We will give short and quick tutorials on the common usage of the commands.

The find command is to search for files, directories, pipes, sockets, devices (anything that is defined as files in Linux) in the system. The quickest tutorial is

# find / –name

The ‘/’ tells the command to search from the root directory, another common variable here is ‘.’, which means to search from this directory onwards.

The ‘-name ’ specifies the file that you want to look for, e.g. proposal.txt as the filename.

That’s the shortest tutorial we can think of for “find”. For more advance stuff, “man 1 find”.

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Short and quick tutorial for the command “find”

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What’s the difference between “su” and “su –”?

admin — Fri, 06 Mar 2009 11:54:13 +0000

We get asked this question quite often and maybe it is good to give a simple explanation. “su” by itself means that you would like to assume the super user identity. You can also assume the identity of another user using “su ”.

Then, many people actually ask why do we usually type “su –” instead of just “su”? The main difference is that the one with the “-“ will run the to-be-assumed userid’s startup scripts which will basically mean that you will have the environment settings (such as $PATH) as though you are logged in as the new userid. Without the “-“, you will have the environment settings of the current userid (probably your userid).

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What’s the difference between “su” and “su –”?

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Differential backup script sample