commandlinefu.com is the place to record those command-line gems that you return to again and again.
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Every new command is wrapped in a tweet and posted to Twitter. Following the stream is a great way of staying abreast of the latest commands. For the more discerning, there are Twitter accounts for commands that get a minimum of 3 and 10 votes - that way only the great commands get tweeted.
Use your favourite RSS aggregator to stay in touch with the latest commands. There are feeds mirroring the 3 Twitter streams as well as for virtually every other subset (users, tags, functions,…):
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This command summarizes the disk usage across the files and folders in a given directory, including hidden files and folders beginning with ".", but excluding the directories "." and ".."
It produces a sorted list with the largest files and folders at the bottom of the list
This command is useful for finding out which directories below the current location use the most space. It is summarised by directory and excludes mounted filesystems. Finally it is sorted by size.
This command finds all the files whose status has changed between the ctime of the older and newer .
Very useful if you can see from an ls listing a block of consecutive files you want to move or delete, but can't figure out exactly the time range by date.
On systems where your home directory is shared across different machines, your bash history will be global, rather than being a separate history per machine. This setting in your .bashrc file will ensure that each machine has its own history file.
Write 200 blocks of 512k to a dummy file with dd, timing the result. The is useful as a quick test to compare the performance of different file systems.
Instead of deleting an existing symlink and then re-creating it pointing at the new location, it is possible to perform the same action with this one command.
Interesting discussion on whether this is possible to do atomically here: http://answers.google.com/answers/threadview?id=526119
On a machine behind a firewall, it's possible to pass the proxy server address in as a prefix to wget to avoid having to set it as an environment variable first.