File Management
Networking
Package Management
SSH
Shell/CLI
Troubleshooting

Increase the value for maxdepth to control the number of directory levels you want to remove files under (or remove the parameter completely to remove all zero length files under all sub-folders).

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For example, to find the text "foo" in all files under the current dir (recursively): For more usages of grep to search for strings in files see here

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For example, moving all XML files in a folder and sub-folders to somewhere else (/some/folder): a possibly faster alternative is or (for just the current folder)

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For example, to replace the text "192.168.1.2" with "10.0.0.13" in all files with the name Root under all sub-dirs from the current dir:

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For example, to remove all the CVS folders from a CVS checkout, while leaving the other files and folders intact:

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For example, in a folder containing too many ".log" files to remove via rm, run a command like so:

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A bash script which will convert all uppercase characters in all filenames in current directory to lowercase (can ignore errors for files which are already all lowercase):

	#!/bin/bash
	for file in *
		do newfile=`echo $file|tr [A-Z] [a-z]`
		echo "Moving $file to $newfile"
		mv $file $newfile
	done
	

An alternative is to use the command line rename utility like so

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To find the text BLA in the filename and replace it with the text FOO in all .tgz files: To change the file extension of all .JPG files to .jpg: To strip the spaces out of all filenames:

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For example, to split a large gzipped tar archive with the name too.big.for.cdrom.backup.tgz into CD-ROM sized pieces of around 657MB you would issue the following command: This creates files named xaa, xab, xac etc. - one for each chunk of 657MB. To restore the original file from these pieces, copy all the files to some location and issue the command

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First, run Then run the program, then to get a list of all files that changed.




Running "netstat" without any options shows all local connections and domain sockets. To filter this to just show services listening for incoming TCP and UDP connections: Services that list their foreign address as "0.0.0.0:*" (IPv4) and "[::]:*" (IPv6) are listening locally and on all network interfaces and should be checked that they aren't accessible from the internet (unless intentional).
More info at How to use netstat on Linux/

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To install a package named X: To do a dummy run of an installation a package named X (nothing will be installed, useful to check that an installation won't have any problems): To uninstall a package named X, but leave its configuration files in place: To uninstall a package named X and all its related files: To remove orphaned packaged: Search all installed packages and check whether there are any broken dependencies (useful to run from time to time): To get the latest package information from the source in the apt sources.list: To upgrade to the latest package versions obtained from running the above command: To search for a package: The above command accepts regular expressions as the argument so to search for package starting with X use ^X and ending with X use X$ etc.

To see which package a file belongs to: To do the opposite and see which files a package provides: To improve the performance of APT downloads you can select mirrors by using "netselect-apt". When it is run it will check the mirrors from debian and ping them and save a new sources.list file in the current directory with the fastest mirrors.

To do a dump of all the software currently installed run If you save this to a file you can restore it later (e.g. on a different machine) by running To clean up APT's cache, you can manually remove files from but a better solution is to use APT to do this, first to clean up packages that it considers old: Or to delete them all:

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Certain firewall setups allow one to SSH only onto a single machine (e.g. somehost1.com) and from there one can then SSH onto another machine (e.g. somehost2.com) (i.e. direct SSH to somehost2.com is not possible). In order to SSH to somehost2.com without having to first SSH onto somehost1.com, create a command called something like sssh as follows: If you have set up SSH key access to somehost1.com you can now SSH onto somehost2.com from the machine with direct access only to somehost1.com like so: and only be prompted for somehost2.com's password.

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In a similar vein to the above tip, you can create a very similar command to allow you to SCP to somehost2.com via somehost1.com without having to first SCP onto somehost1.com and from there onto somehost2.com. Create a command (call it sscp) like so: and use it like so:

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To run GUI programs on one machine using another machine one can use X forwarding by connecting using and once logged in running the command to start the GUI app, the GUI window will open on the local machine. X11Forwarding must be enabled in the file (which is usually the base by default). To speed things up -Y (Trusted X11Forwarding) or -C (compression) can be used instead of -X.

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It is worthwhile to periodically check the contents of the file for failed SSH connections. This might indicate someone attempting a brute force attack etc.



To run a number of commands sequentially, put a ";" between each: To run a command only if the one before succeeds, use "&&" between them instead: Similarly to run a command only if the one before it fails, use "||":

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For example, to create a gzipped tar archive with the name foo.tgz of all the contents of the current dir:

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The easiest way to determine how much space your files and directories are taking up is to use the following command: For a quick way to find the biggest ones, try:

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If you set the variables in .bashrc, issue the command:

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Create a new screen session with a certain name: Reattach to a screen by name: Create a new screen: Moving between more than one open screen: To scroll back through program output in a screen: (push ESC again to go back).
To name a screen window/tab:

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!$ represents the last word of previous command in your history.

$* represents all command line arguments passed to a command.

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Use > to take output from a command to a file: This will take the results from cat and put them in a file called results.txt, overwriting results.txt if it exists. To append the output instead of overwriting, one can use >>: Use < to take input into a command: This will look for the string "X" in someFile. One can place a number before ">" to direct standard input/output where 0=standard input, 1=standard output, 2 =standard error output. This can be used, for example, to see results of a command sent to standard out while ignoring any errors. e.g. if we run "find" but want to ignore errors (e.g. permissions errors etc). we can redirect error output to /dev/null leaving us with what we are interested in (the standard output):

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Issue the command to see a list of open sockets and pid/program names:

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Issue the command: (top might also be useful)

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As root, add the required paths to and then run

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Issue the command:

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Issue the command:


The error message is usually something like 'Xlib: connection to ":0.0" refused by server'. This occurs when you try start an X application as a user other than the user which started X so the authentication keys differ. The first thing to try is as the user who started the x session (usually the user you logged in to X as) before you switch to a new user. This can be added to .bashrc to always allow local (i.e. not over a network) X connections by all users. Some Linux distributions allow you to start X applications as root by starting the root shell using instead of