This section covers some very basic commands to efficiently work with a Linux system. They should be available on all standard installations. Although many modern Linux variants offer an advanced GUI, it is advisable to be able to control a Linux system from command line.

• ls lists files in current directory. Use ls -al to list all files (including hidden = starting with .) in a long listing.
• mkdir creates a directory. Specify the directory name after the command.
• rm removes files and directories. If directories need to be removed, the -r option must be specified. To force operation, add -f. The last parameters are the files and directories that are to be deleted. Make sure that there are no wrong directories listed! A call to rm -rf / as root is not a good thing.
• mv moves a file, where path is relative to current directory: mv [path]filename [path]filname
• file reveals what content type a file is. Use it to test if a file is text, image, binary or whatever.
• find and rm can be used in combination to delete files found by find. e.g. this will recursively remove all files with extension .txt in the current directory: find . -name \*.txt -print0 | xargs -0 rm
  As an alternative, it is possible to use some shell features: rm $(find -name \*.txt)
• sudo starts a process with super user (=root) capabilities. To start a root shell, use sudo su. If sudo is not installed, use su instead. It requires not the current user's password but the root password.
• less helps to look at what a file stores.
• cat reads a file and prints it to stdout (the console if invoked as-is.)
• locate locates all files that contain a given string.
• grep searches input for matching lines. To search all files in the current directory (and sub directories), use grep -r . -e “text”.
• man shows the manual for some command. Sometimes info shows a more detailed documentation.
• dmesg prints kernel messages.

• Use wget to download files from HTTP and FTP. The tool allows recursive downloading, i.e. also fetch outgoing links from some HTML page. To restrict the files downloaded to a certain type (by file suffix), use the -A option:

  wget -r http://some.host/path/to/file.html -A pdf

  downloads all PDF's referenced by the HTML file.

The output of one command can be piped into the input of another command. This is done by the pipe operator |. To search kernel messages for some string, use dmesg | grep string.

Additionally, the STDOUT (1) and STDERR (2) streams can also be redirected. To redirect the STDERR output to STDOUT, use: command 2>&1 …. This can be used in combination with piping.