The Unix shell

There are many types of shells in Unix. The most popular are bash and tcsh. Personally, I prefer the first one, but the École Polytechnique installs the second by default in the didactical computer rooms. In any case one can start either of them by simply opening a terminal and typing the corresponding name. These notes are based on the bash shell, but most notions will work on either.

Basic commands:

• cd directoryName: change working directory
• pwd: print the working directory
• cat fileName: display the (text) file fileName to standard output
• mv file position: move file to a new position: e.g. mv /etc/hosts . moves the file hosts from the directory /etc to the current working directory (.)
• cp file position: same as mv, but copy the file
• rm file: remove file
• rmdir directory: remove an empty directory
• grep string file(s): look for a string in a set of files: e.g. grep -Hi complex * looks in all files in the current directory (*) for the string complex ignoring upper/lower case ( -i) and displays the name of the file (-H) as well as the line where the match occurs.

Most Unix commands can be ``chained'': the output of a command is read as the input of the next.

• By default, unix tools send their output messages to the standard output stream (stdout) and their error messages to the standard error stream (stderr)
• Both streams can be redirected. E.g., to redirect both stdout and stderr, use:
  sh -c 'command options arguments > outFile 2>&1'
• The output stream of a command can become the input stream of the next command in a chain: e.g. find ~ $\vert$ grep $\backslash$ .cxx finds all files with extension .cxx in all subdirectories of the home directory; the first command (find) sends a recursive list across subdirectories of the home directory (denoted by ~) to stdout. This stream is transformed by the pipe character (|) in the standard input (stdin) stream of the following command (grep), which filters out all lines not containing .cxx.