Commands by khayyam (18)

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commandlinefu.com is the place to record those command-line gems that you return to again and again. That way others can gain from your CLI wisdom and you from theirs too. All commands can be commented on, discussed and voted up or down.

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Schedule Nice Background Commands That Won't Die on Logout - Alternative to nohup and at
Check out the usage of 'trap', you may not have seen this one much. This command provides a way to schedule commands at certain times by running them after sleep finishes sleeping. In the example 'sleep 2h' sleeps for 2 hours. What is cool about this command is that it uses the 'trap' builtin bash command to remove the SIGHUP trap that normally exits all processes started by the shell upon logout. The 'trap 1' command then restores the normal SIGHUP behaviour. It also uses the 'nice -n 19' command which causes the sleep process to be run with minimal CPU. Further, it runs all the commands within the 2nd parentheses in the background. This is sweet cuz you can fire off as many of these as you want. Very helpful for shell scripts.

Get all documents (doc,docx,xls,xlsx,pdf,ppt,pptx,...) linked in a webpage

list block devices
Shows all block devices in a tree with descruptions of what they are.

Merge video files together using mencoder (part of mplayer)
Using mplayer's mencoder, you can merge video files together. '-oac' specifies the audio encoding (here copy, to just copy and not compress) '-ovc' specifies the video encoding (same thing).

Clean swap area after using a memory hogging application
When you run a memory intensive application (VirtualBox, large java application, etc) swap area is used as soon as memory becomes insufficient. After you close the program, the data in swap is not put back on memory and that decreases the responsiveness. Swapoff disables the swap area and forces system to put swap data be placed in memory. Since running without a swap area might be detrimental, swapon should be used to activate swap again. Both swapoff and swapon require root privileges.

View user activity per directory.
View all files opened by a user in specified directory. The +D option makes lsof search all sub-directories to complete depth, while ignoring symbolic links.

Save the current directory without leaving it
Save the current directory without having to leave it. When you do decide to leave the current directory, use popd to return to it.

Get all mac address
I prefer the ip command to ifconfig as ifconfig is supposedly going to be deprecated. Certain IP address aliases can only be seen with the ip command (such as the ones applied by RHCS).

function to verify an IP address - can be used at the shell prompt or in a shell script
When processing IP addresses in the shell (or shell script) it is useful to be able to verify that the value of data is an IP address (an not some random string or non-sensible IP address).

MP3 player
I tried a few curses based mp3 players for playing back choir practice songs for my wife. Unfortunately none of the ones I tried were capable of scrubbing a track. Firefox saves the day.


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