Lots of people who watch Youtube videos like to download them for viewing it offline later. On a Linux box, the best tool you can find is a small command line utility called "youtube-dl". This is available as package in fedora/debian/ubuntu/suse repositories. Even if this tool is not in your repository, you can download it directly from the project website. This script requires python, so make sure you have it. Unzip the downloaded archive and copy it to system path (or run from there itself - if you wish).
To download a video, you need to get its Youtube URL which is the URL that you see in the address bar of your browser when you play the video. It will look like this: http://www.youtube.com/watch?v=RANDOM_STRING. The RANDOM_STRING will contain alpha numeric characters and dash.
Let us see the actual use of the tool.
safeer@penguinpower:~$youtube-dl http://www.youtube.com/watch?v=QTw6iexyrNU
It will throw some output about the URL and then start downloading, showing the current download speed and ETA. When finished, it will report "Video data saved to QTw6iexyrNU.flv". The file will be saved in the current directory. If you want to save to another location or change the name of the file, use option "-o".
The default format for the saved video will be "flv" but if you use the -b option ( for best qulity ) the format will be mp4. So change your filename accordingly when using -o and -b together.
There are many other useful options including the option to give username and password for videos that can be accessed only by logged in Yuotube users. Use "youtube-dl -h" to see all the options.
One thing that you should remember about Youtube URLs is that if the url contains one or more "&" symbols, you need to take only the part on the left hand side of the first "&". For eg, if you got this URL "http://www.youtube.com/watch?v=QTw6iexyrNU&feature=related" from your browser, then the actual URL is "http://www.youtube.com/watch?v=QTw6iexyrNU". The rest are arguments to the Youtube website which will not be used by youtube-dl.
This tool has a windows GUI which can be found here.
Now for the people who want to do things their way, following method may be interesting. Open your browser and play the video from Youtube and wait for it to finish. Once the video is played to the full length, open a terminal keeping the Youtube page open. Then run this command on terminal.
safeer@penguinpower:~$ for i in $(find /tmp -type f -user $(whoami) 2>/dev/null);do [[ $(file $i) =~ "Flash" ]] && echo $i;done
/tmp/FlashRXaVqD
This file is the video you just watched, if you want to cross check you can run this file in some media player like vlc.
safeer@penguinpower:~$ vlc /tmp/FlashRXaVqD&
And it will play the video. Now save the file with a name and flv extension.
safeer@penguinpower:~$mv /tmp/FlashRXaVqD ~/chess-play.flv
Not a big deal, but it will save your bandwidth, if you want to first play the video and then only download it.
Chrome is developing a browser extension for Youtube downloading, I haven't tested it yet. Have a look into it if you are interested.
Disclaimer: Downloading Youtube content is subject to copy right and content policies of the concerned parties, the author or this blog will not be responsible for such issues.
Wednesday, January 6, 2010
Friday, January 1, 2010
Typescript - Capturing your shell activity
We know about screen capturing and recording our screen activity. There are hundreds of applications for this purpose. But what about your shell activity? Of course you can use the same screen capturing tools if you are working on the GUI window. But what if you are on a remote shell? What if you want to reuse or copy the results/commands as a backup? Obviously the graphical tools will not be of much help. This is where typescripts come into play.
A typescript is a record of whatever is printed in your terminal. That includes stdin,stdout and stderr, which is basically the commands that you type and the output that these commands generate. The command which creates the typescript is "script". You can just type the command "script" and this will start recording the shell activity to a file by the name "typescript" which will be created in the directory from which the command "script" was run. Alternatively you can specify a separate file as an argument to it. When you run the script command it actually opens a new shell, and keeps recording the shell's activity until you exit that shell and return to the parent shell from where you ran the "script"command. Let us check the usage.
safeer@penguinpower:~$ script
Script started, file is typescript
safeer@penguinpower:~$ ls MyDocuments/
Linux Windows
safeer@penguinpower:~$ mkdir MyDocuments/Cisco
safeer@penguinpower:~$ ls MyDocuments/
Cisco Linux Windows
safeer@penguinpower:~$ rm MyDocuments/Cisco
rm: cannot remove `MyDocuments/Cisco': Is a directory
safeer@penguinpower:~$ exit
exit
Script done, file is typescript
Let us examine the contents of typescript.
safeer@penguinpower:~$ cat typescript
Script started on Sunday 03 January 2010 02:05:01 AM IST
safeer@penguinpower:~$ ls MyDocuments/
Linux Windows
safeer@penguinpower:~$ mkdir MyDocuments/Cisco
safeer@penguinpower:~$ ls MyDocuments/
Cisco Linux Windows
safeer@penguinpower:~$ rm MyDocuments/Cisco
rm: cannot remove `MyDocuments/Cisco': Is a directory
safeer@penguinpower:~$ exit
exit
Script done on Sunday 03 January 2010 02:05:55 AM IST
As you can see all that is printed into the terminal is recorded in the typescript. A few useful options to use with the script:
-a : Append to the end of the type script file - if it exists already. Default behavior is to overwrite the file.
-q: Quiet mode, removes the End timestamp from the typescript.
-t: This is an interesting option, it prints the output timing data to standard error. This data can be used to replay the typescript with real-time input output delays. We will see the usage later.
You can specify an alternate typescript file as the last argument.
safeer@penguinpower:~$ script myscript
Script started, file is myscript
safeer@penguinpower:~$ echo HI
HI
safeer@penguinpower:~$ exit
exit
Script done, file is myscript
safeer@penguinpower:~$ cat myscript
Script started on Sunday 03 January 2010 02:10:28 AM IST
safeer@penguinpower:~$ echo HI
HI
safeer@penguinpower:~$ exit
exit
Script done on Sunday 03 January 2010 02:10:44 AM IST
To gather the timing information, we car try using the -t option.
safeer@penguinpower:~$ script -t myscript 2>myscript.time
Script started, file is myscript
safeer@penguinpower:~$ ls MyDocuments/
Cisco/ Linux/ Windows/
safeer@penguinpower:~$ ls MyDocuments/Cisco/
safeer@penguinpower:~$ touch MyDocuments/Cisco/gns3
safeer@penguinpower:~$ ls MyDocuments/Cisco/
gns3
safeer@penguinpower:~$ exit
exit
Script done, file is myscript
Content of time file will look like this:
safeer@penguinpower:~$ head -2 myscript.time
0.500888 27
0.253943 23
The first field indicates how much time elapsed since the previous output. The second field indicates how many characters were output this time. With this information and the typescript file we can use the command "scriptreplay" to replay our terminal activity with the same timing with which you typed on the terminal and the output was displayed. It will give you a feeling of watching a video of your terminal activity.
The usage is like this:
First get the type script to one file and the timing information obtained with "-t" option ( printed to stderr) to another file.
safeer@penguinpower:~$ script -t myscript 2>myscript.time
Now replay the typescript with the format "scriptreplay [timing file] [typescript file] ".
If no typescript filename is given, the name "typescript" is assumed.
safeer@penguinpower:~$ scriptreplay myscript.time myscript
And this will play the typescript with timing.
A typescript is a record of whatever is printed in your terminal. That includes stdin,stdout and stderr, which is basically the commands that you type and the output that these commands generate. The command which creates the typescript is "script". You can just type the command "script" and this will start recording the shell activity to a file by the name "typescript" which will be created in the directory from which the command "script" was run. Alternatively you can specify a separate file as an argument to it. When you run the script command it actually opens a new shell, and keeps recording the shell's activity until you exit that shell and return to the parent shell from where you ran the "script"command. Let us check the usage.
safeer@penguinpower:~$ script
Script started, file is typescript
safeer@penguinpower:~$ ls MyDocuments/
Linux Windows
safeer@penguinpower:~$ mkdir MyDocuments/Cisco
safeer@penguinpower:~$ ls MyDocuments/
Cisco Linux Windows
safeer@penguinpower:~$ rm MyDocuments/Cisco
rm: cannot remove `MyDocuments/Cisco': Is a directory
safeer@penguinpower:~$ exit
exit
Script done, file is typescript
Let us examine the contents of typescript.
safeer@penguinpower:~$ cat typescript
Script started on Sunday 03 January 2010 02:05:01 AM IST
safeer@penguinpower:~$ ls MyDocuments/
Linux Windows
safeer@penguinpower:~$ mkdir MyDocuments/Cisco
safeer@penguinpower:~$ ls MyDocuments/
Cisco Linux Windows
safeer@penguinpower:~$ rm MyDocuments/Cisco
rm: cannot remove `MyDocuments/Cisco': Is a directory
safeer@penguinpower:~$ exit
exit
Script done on Sunday 03 January 2010 02:05:55 AM IST
As you can see all that is printed into the terminal is recorded in the typescript. A few useful options to use with the script:
-a : Append to the end of the type script file - if it exists already. Default behavior is to overwrite the file.
-q: Quiet mode, removes the End timestamp from the typescript.
-t: This is an interesting option, it prints the output timing data to standard error. This data can be used to replay the typescript with real-time input output delays. We will see the usage later.
You can specify an alternate typescript file as the last argument.
safeer@penguinpower:~$ script myscript
Script started, file is myscript
safeer@penguinpower:~$ echo HI
HI
safeer@penguinpower:~$ exit
exit
Script done, file is myscript
safeer@penguinpower:~$ cat myscript
Script started on Sunday 03 January 2010 02:10:28 AM IST
safeer@penguinpower:~$ echo HI
HI
safeer@penguinpower:~$ exit
exit
Script done on Sunday 03 January 2010 02:10:44 AM IST
To gather the timing information, we car try using the -t option.
safeer@penguinpower:~$ script -t myscript 2>myscript.time
Script started, file is myscript
safeer@penguinpower:~$ ls MyDocuments/
Cisco/ Linux/ Windows/
safeer@penguinpower:~$ ls MyDocuments/Cisco/
safeer@penguinpower:~$ touch MyDocuments/Cisco/gns3
safeer@penguinpower:~$ ls MyDocuments/Cisco/
gns3
safeer@penguinpower:~$ exit
exit
Script done, file is myscript
Content of time file will look like this:
safeer@penguinpower:~$ head -2 myscript.time
0.500888 27
0.253943 23
The first field indicates how much time elapsed since the previous output. The second field indicates how many characters were output this time. With this information and the typescript file we can use the command "scriptreplay" to replay our terminal activity with the same timing with which you typed on the terminal and the output was displayed. It will give you a feeling of watching a video of your terminal activity.
The usage is like this:
First get the type script to one file and the timing information obtained with "-t" option ( printed to stderr) to another file.
safeer@penguinpower:~$ script -t myscript 2>myscript.time
Now replay the typescript with the format "scriptreplay [timing file] [typescript file] ".
If no typescript filename is given, the name "typescript" is assumed.
safeer@penguinpower:~$ scriptreplay myscript.time myscript
And this will play the typescript with timing.
Friday, December 18, 2009
Changing MAC address in linux
There are occasions when you may need to change your MAC (Media Access Control) address temporarily. This can be used:
1. While connected to a public wifi hot spot - it keeps your original MAC address anonymous.
2. To bypass a router/switch which has MAC address filtering.
3. To obtain a dhcp ip lease that is allocated based on MAC address.
And remember! MAC address is something that is soldered into your network adapter. You cant change it permanently; in other words, the change that you make is not persistent over reboots.
First let us find out the current MAC address of the Ethernet adapter in my machine
safeer@penguinpower:~$ ifconfig eth0|awk '/HWaddr/{print $1" "$5 }'
eth0 00:13:e4:5c:93:0a
Now I am going to change the MAC address of my Ethernet interface.
First, bring down the interface
safeer@penguinpower:~$ sudo ifconfig eth0 down
Then change the MAC addres to whatever you want
safeer@penguinpower:~$ sudo ifconfig eth0 hw ether 00:22:4d:b7:36:01
Bring up the interface
safeer@penguinpower:~$ sudo ifconfig eth0 up
Verify MAC address has changed.
safeer@penguinpower:~$ ifconfig eth0|awk '/HWaddr/{print $1" "$5 }'
eth0 00:22:4d:b7:36:01
The syntax of the command is "ifconfig [interface name] hw [encapsulation type] [MAC address] "
Every argument is self explanatory except the encapsulation type. This is "ether" (Ethernet) for both our Ethernet and wireless interfaces. For finding out encapsulation type for other interface types see ifconfig man pages. To see the encapsulation type of your network interfaces use the following command
safeer@penguinpower:~$ ifconfig -a|awk '/HWaddr/{split($3,enc,":"); print $1" "enc[2];}'
eth0 Ethernet
wlan0 Ethernet
1. While connected to a public wifi hot spot - it keeps your original MAC address anonymous.
2. To bypass a router/switch which has MAC address filtering.
3. To obtain a dhcp ip lease that is allocated based on MAC address.
And remember! MAC address is something that is soldered into your network adapter. You cant change it permanently; in other words, the change that you make is not persistent over reboots.
First let us find out the current MAC address of the Ethernet adapter in my machine
safeer@penguinpower:~$ ifconfig eth0|awk '/HWaddr/{print $1" "$5 }'
eth0 00:13:e4:5c:93:0a
Now I am going to change the MAC address of my Ethernet interface.
First, bring down the interface
safeer@penguinpower:~$ sudo ifconfig eth0 down
Then change the MAC addres to whatever you want
safeer@penguinpower:~$ sudo ifconfig eth0 hw ether 00:22:4d:b7:36:01
Bring up the interface
safeer@penguinpower:~$ sudo ifconfig eth0 up
Verify MAC address has changed.
safeer@penguinpower:~$ ifconfig eth0|awk '/HWaddr/{print $1" "$5 }'
eth0 00:22:4d:b7:36:01
The syntax of the command is "ifconfig
Every argument is self explanatory except the encapsulation type. This is "ether" (Ethernet) for both our Ethernet and wireless interfaces. For finding out encapsulation type for other interface types see ifconfig man pages. To see the encapsulation type of your network interfaces use the following command
safeer@penguinpower:~$ ifconfig -a|awk '/HWaddr/{split($3,enc,":"); print $1" "enc[2];}'
eth0 Ethernet
wlan0 Ethernet
Friday, December 11, 2009
GPRS connection in Linux using wvdial
If you have a mobile with GPRS enabled you can use it to connect your Linux Box to Internet. The program that we can use for this is wvdial. Most of the popular distributions will have it in its repository, if not, get it from here.
I am going to connect using:
Nokia E71
GSM based GPRS connection from Vodafone (India)
Kubuntu 9.04
WvDial 1.60
First connect your mobile phone with USB cable to your PC/Laptop. In the case of Nokia, four options will pop-up on the phone. PC Suite, Media Transfer, Mass Storage and Connect to PC to Web. Though I chose Connect PC to Web, it was not detected as a modem. So changed the mode to PC Suite on the phone, and then a modem was detected at /dev/ttyACM0. I got this message on dmesg - "ttyACM0: USB ACM device". Now we need to cerate a configuration file for wvdial. and save it to "/etc/wvdial-vodafone-gprs.conf" for the time being.
The file should start with:
[Dialer Defaults]
Add the basic settings for the modem.
Modem = /dev/ttyACM0
Modem Type = USB Modem
Baud = 460800
For any gprs connection the most important detail you need is the APN ( Access Point Name ) of the service provider, in the case of Vodafone, it is "portalnmms". This should go in the modem initialization commands as below:
Init1 = ATZ
Init2 = AT+CGDCONT=1,"IP","portalnmms"
Note that the APN is part of the Init2 command. Now we need to add the dialing phone number, username and password. This is same across all GSM providers (CDMA phones have different values).
Phone = *99#
Username = 0
Password = 0
Dirty Secret: GSM don't check the username and password for authentication, it just needs the entries to be present in the configuration - with some junk values.
Save the file and dial with wvdial command using /etc/wvdial-vodafone-gprs.conf as an argument:
safeer@penguinpower:~$ sudo wvdial -C /etc/wvdial-vodafone-gprs.conf
You will get a lot of output, look for the following lines ( the IP's shown may differ)
local IP address 10.16.25.127
remote IP address 10.6.6.6
primary DNS address 10.11.230.2
secondary DNS address 202.144.13.50
This indicate the modem is connected. At this point you will be able to contact your DNS server and resolve names. But you need to set proxy for your browser to start browsing. For Vodafone the proxy and the port are 10.10.1.100 and 9401. Set this on your browser and you can start browsing. When you are done, close the connection by pressing Control+C to terminate wvdial command.
If your connection is CDMA, the phone number will be #777 instead of *99#, and the username and password will be your mobile number ( without country code ). Also the Init2 string will be "ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0" (no APN name).
A few more things to note, depending on your phone, the modem device location may change, it can be /dev/ttyUSB0, /dev/ttySL0 etc... always check dmesg to see how it is detected - use command dmesg|tail after connecting your usb cable.
For good measure, you can add the following settings as well to the wvdial configuration:
Dial Command = ATDT
Stupid Mode = 1
ISDN = 0
New PPPD = yes
To find out your provider's GPRS settings, you can visit Manoj Tivari's blog . I got the Vodafone proxy details from this web site.
I am going to connect using:
Nokia E71
GSM based GPRS connection from Vodafone (India)
Kubuntu 9.04
WvDial 1.60
First connect your mobile phone with USB cable to your PC/Laptop. In the case of Nokia, four options will pop-up on the phone. PC Suite, Media Transfer, Mass Storage and Connect to PC to Web. Though I chose Connect PC to Web, it was not detected as a modem. So changed the mode to PC Suite on the phone, and then a modem was detected at /dev/ttyACM0. I got this message on dmesg - "ttyACM0: USB ACM device". Now we need to cerate a configuration file for wvdial. and save it to "/etc/wvdial-vodafone-gprs.conf" for the time being.
The file should start with:
[Dialer Defaults]
Add the basic settings for the modem.
Modem = /dev/ttyACM0
Modem Type = USB Modem
Baud = 460800
For any gprs connection the most important detail you need is the APN ( Access Point Name ) of the service provider, in the case of Vodafone, it is "portalnmms". This should go in the modem initialization commands as below:
Init1 = ATZ
Init2 = AT+CGDCONT=1,"IP","portalnmms"
Note that the APN is part of the Init2 command. Now we need to add the dialing phone number, username and password. This is same across all GSM providers (CDMA phones have different values).
Phone = *99#
Username = 0
Password = 0
Dirty Secret: GSM don't check the username and password for authentication, it just needs the entries to be present in the configuration - with some junk values.
Save the file and dial with wvdial command using /etc/wvdial-vodafone-gprs.conf as an argument:
safeer@penguinpower:~$ sudo wvdial -C /etc/wvdial-vodafone-gprs.conf
You will get a lot of output, look for the following lines ( the IP's shown may differ)
local IP address 10.16.25.127
remote IP address 10.6.6.6
primary DNS address 10.11.230.2
secondary DNS address 202.144.13.50
This indicate the modem is connected. At this point you will be able to contact your DNS server and resolve names. But you need to set proxy for your browser to start browsing. For Vodafone the proxy and the port are 10.10.1.100 and 9401. Set this on your browser and you can start browsing. When you are done, close the connection by pressing Control+C to terminate wvdial command.
If your connection is CDMA, the phone number will be #777 instead of *99#, and the username and password will be your mobile number ( without country code ). Also the Init2 string will be "ATQ0 V1 E1 S0=0 &C1 &D2 +FCLASS=0" (no APN name).
A few more things to note, depending on your phone, the modem device location may change, it can be /dev/ttyUSB0, /dev/ttySL0 etc... always check dmesg to see how it is detected - use command dmesg|tail after connecting your usb cable.
For good measure, you can add the following settings as well to the wvdial configuration:
Dial Command = ATDT
Stupid Mode = 1
ISDN = 0
New PPPD = yes
To find out your provider's GPRS settings, you can visit Manoj Tivari's blog . I got the Vodafone proxy details from this web site.
Monday, November 16, 2009
Truncating a file
There are situations when you want to adjust the size of a file rather than deleting it. Especially when the file is being used by a process and you want to reduce its size without interrupting the process. Removing a file and creating another file by the same name will not work in such cases as the inode number of the file will change. This happens mainly with log files, where you want to reduce the size of the log - to save space -but don't want to stop the process that is writing to the log.
BSD has an excellent tool for this - the "truncate" command. This command can be used either to reduce/increase the size of a file. Let us consider a live example.
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
476M mysql-slowquery.log
To reduce the size to 300MB
safeer->vps1.safeer.in ~ $ truncate -s 300M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
300M mysql-slowquery.log
Instead of specifying the exact size, if you just want to free up a 100Mb of disk space, you can do it this way:
safeer->vps1.safeer.in ~ $ truncate -s -100M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
200M mysql-slowquery.log
When you truncate a file by this method, the data is lost from the end of the file.
Truncate also has an option to extend a file's size, but it do not allocate any space for that. So it is not so useful.
safeer->vps1.safeer.in ~ $ truncate -s +400M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
200M mysql-slowquery.log
Another way of specifying size for truncation is to give a reference file. Truncate will calculate the size of the referenced file and truncate the given file to its length.
safeer->vps1.safeer.in ~ $ du -hs httpd.log mysql-slowquery.log
153M httpd.log
200M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ truncate -r httpd.log mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs httpd.log mysql-slowquery.log
153M httpd.log
153M mysql-slowquery.log
truncate do not change the inode of the file.
safeer->vps1.safeer.in ~ $ ls -i mysql-slowquery.log
48980316 mysql-slowquery.log
safeer->vps1.safeer.in ~ $ truncate -s -132M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ ls -i mysql-slowquery.log
48980316 mysql-slowquery.log
As you can see, this is a very useful command, but unfortunately we don't have a similar tool in Linux. So we can use any of the following method to emulate some truncate features.
To empty a file (truncate -s 0 )
safeer@penguinpower:~$ du -hs mysql-slowquery.log
1.1G mysql-slowquery.log
safeer@penguinpower:~$ >mysql-slowquery.log
safeer@penguinpower:~$ du -hs mysql-slowquery.log
0 mysql-slowquery.log
For resizing to an exact size you can use the dd command.
safeer@penguinpower:~$ du -hs mysql-slowquery.log
747M mysql-slowquery.log
safeer@penguinpower:~$ dd if=/dev/null seek=705 bs=1M of=mysql-slowquery.log
0+0 records in
0+0 records out
0 bytes (0 B) copied, 5.1892e-05 s, 0.0 kB/s
safeer@penguinpower:~$ du -hs mysql-slowquery.log
706M mysql-slowquery.log
Here dd argument bs specifies the unit of file size where as seek will specify the number of units to which you want to resize the file. Technically, copying /dev/null to a file will erase the content in the destination file. But with the "seek" option we specify that "seek value" X "block size" data from the beginning of the file should be preserved and the rest of the data should be erased. So in the above example, file was reduced to ~705MB (705 * 1M).
The following commands are equivalent.
truncate -s 0 mysql-slowquery.log
>mysql-slowquery.log
cat /dev/null > mysql-slowquery.log
dd if=/dev/null of=mysql-slowquery.log
BSD has an excellent tool for this - the "truncate" command. This command can be used either to reduce/increase the size of a file. Let us consider a live example.
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
476M mysql-slowquery.log
To reduce the size to 300MB
safeer->vps1.safeer.in ~ $ truncate -s 300M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
300M mysql-slowquery.log
Instead of specifying the exact size, if you just want to free up a 100Mb of disk space, you can do it this way:
safeer->vps1.safeer.in ~ $ truncate -s -100M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
200M mysql-slowquery.log
When you truncate a file by this method, the data is lost from the end of the file.
Truncate also has an option to extend a file's size, but it do not allocate any space for that. So it is not so useful.
safeer->vps1.safeer.in ~ $ truncate -s +400M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs mysql-slowquery.log
200M mysql-slowquery.log
Another way of specifying size for truncation is to give a reference file. Truncate will calculate the size of the referenced file and truncate the given file to its length.
safeer->vps1.safeer.in ~ $ du -hs httpd.log mysql-slowquery.log
153M httpd.log
200M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ truncate -r httpd.log mysql-slowquery.log
safeer->vps1.safeer.in ~ $ du -hs httpd.log mysql-slowquery.log
153M httpd.log
153M mysql-slowquery.log
truncate do not change the inode of the file.
safeer->vps1.safeer.in ~ $ ls -i mysql-slowquery.log
48980316 mysql-slowquery.log
safeer->vps1.safeer.in ~ $ truncate -s -132M mysql-slowquery.log
safeer->vps1.safeer.in ~ $ ls -i mysql-slowquery.log
48980316 mysql-slowquery.log
As you can see, this is a very useful command, but unfortunately we don't have a similar tool in Linux. So we can use any of the following method to emulate some truncate features.
To empty a file (truncate -s 0 )
safeer@penguinpower:~$ du -hs mysql-slowquery.log
1.1G mysql-slowquery.log
safeer@penguinpower:~$ >mysql-slowquery.log
safeer@penguinpower:~$ du -hs mysql-slowquery.log
0 mysql-slowquery.log
For resizing to an exact size you can use the dd command.
safeer@penguinpower:~$ du -hs mysql-slowquery.log
747M mysql-slowquery.log
safeer@penguinpower:~$ dd if=/dev/null seek=705 bs=1M of=mysql-slowquery.log
0+0 records in
0+0 records out
0 bytes (0 B) copied, 5.1892e-05 s, 0.0 kB/s
safeer@penguinpower:~$ du -hs mysql-slowquery.log
706M mysql-slowquery.log
Here dd argument bs specifies the unit of file size where as seek will specify the number of units to which you want to resize the file. Technically, copying /dev/null to a file will erase the content in the destination file. But with the "seek" option we specify that "seek value" X "block size" data from the beginning of the file should be preserved and the rest of the data should be erased. So in the above example, file was reduced to ~705MB (705 * 1M).
The following commands are equivalent.
truncate -s 0 mysql-slowquery.log
>mysql-slowquery.log
cat /dev/null > mysql-slowquery.log
dd if=/dev/null of=mysql-slowquery.log
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