Friday, April 2, 2010

Using tar with ssh effectively

We know that tar could be used to take backup. We may like to transfer backup to some remote box. Also some time,it is needed to transfer file with directory structure in the remote box.
Lets say we have to transfer many files in the remote box while keeping file permissions and directory structure intact.
We may choose to tar the source directory ,compress it and scp to destination box. We will need to create a v big temp file only to be transferred to the destination box and and removed from source box. Once the file is scp'd to destination box, we will extract the tar'd file to create files in the same directory structure.

Steps :-

(In Src box)

1) Tar the directory in the source file ( tar -cvzf tarfilename.tar.gz /path/of/dir )
2) transfer file to destination box (scp tarfilename.tar.gz user@remote-host)
3) Delete the file ( rm tarfilename.tar.gz)

(In Dest Box)

4) Untar the file (tar -xvzf tarfilename.tar.gz)
5) Remote the file (rm tarfilename.tar.gz)

We can accomplish all these steps in a single command.

Tar has the great ability to send data to stdout/stdin using the "-" (dash) as a filename in the command line. So using that we can string together pipes to send the data to a remote server

tar -cvzf - /dir/to/be/transferred/ | ssh remoteuser@remotehost 'tar -C /path/of/dir -xvzf -'


What this does is pretty simple: it creates a compressed tar file of the /dir/to/be/transferred/ and sends it to stdout (-). We catch stdout with the pipe (|) and then call ssh to connect to a remote server where we execute the remote command tar. Using the remote tar we run a decompress extraction of stdin (-). tar -c changes directory

Great way to transfer a bunch of files securely, as well maintain ownership and permissions.
Its the example of PUSH method.
Following example will show the reverse method.

Lets say we have a backup file in remote host and we want to restore it in local box.
(PULL Method)

ssh user@remote-host "cat /path/to/backup/backupfile.tar.bz2" |tar jpxf -

In this command, we are cat'ing zipped file and output is being redirected to local tar using pipe. So local tar command will replace -(dash) with zipped file and will untar it.

Many other scenarios could be found in

Monday, March 29, 2010

Run the last command as ROOT

There are many privileged commands which you run as non-prev user only to get error or moy desired output. Then you think like sudo and type the same command again.
We can type sudo !! to execute the last command as PRIV user

Sunday, March 28, 2010

How to know the status of all the running services

There are many commands like netstat -plant,ps -aux but when you want to know all the services which are running presently into your RHEL box, service --status-all command is very handy . It shows all the running services into your box.

Quit from shell without saving into history

There are many instances when we want to quit from shell without saving any command in history. We might have run by mistake some rookie command and you dont want to disclose it to others.

kill -9 $$ will do the needful as $$ will provide the PID of the current shell.

Saturday, March 27, 2010

Keyboard shortcuts for shell

Some useful and more often used keyboard short cuts in bash shell under Mac

1)CTRL + A Move to the start of the line.

2)CTRL + E Move to the end of the line.( e for END)

3)CTRL + W Delete word backwards

4)CTRL + U Delete whole line backwards.(Delete from the cursor to the beginning of the line,If cursor is at the end of the line, it will delete the whole line) It can also be used to delete whole line if you are at the end of the command and you wish not to execute it.

5)CTRL + k Delete from the cursor to the end of the line.(Delete WHOLE LINE,if the cursor is in the starting,it will delete whole line)
Place the cursor in the right place and press Either U or K to delete rest of the line

6)CTRL + R to Search history. Press escape to verify and enter to execute

7)CTRL + L To clear line leaving the last line on the screen.