The Shell

The operating system is the code that carries out the system calls. Editors, compilers, assemblers, linkers, and command interpreters definitely are not part of the operating system, even though they are important and useful. At the risk of confusing things somewhat, in this section we will look briefly at the UNIX command interpreter, called the shell. Although it is not part of the operating system, it makes heavy use of many operating system features and thus serves as a good example of how the system calls can be used. It is also the primary interface between a user sitting at his terminal and the operating system, unless the user is using a graphical user interface. Many shells exist, including sh, csh, ksh, and bash. All of them support the functionality described below, which derives from the original shell (sh).

When any user logs in, a shell is started up. The shell has the terminal as standard input and standard output. It starts out by typing the prompt, a character such as a dollar sign, which tells the user that the shell is waiting to accept a command. If the user now types

date

for example, the shell creates a child process and runs the date program as the child. While the child process is running, the shell waits for it to terminate. When the child finishes, the shell types the prompt again and tries to read the next input line.

The user can specify that standard output be redirected to a file, for example,

date >file

Similarly, standard input can be redirected, as in

sort file2

which invokes the sort program with input taken from file1 and output sent to file2.

The output of one program can be used as the input for another program by connecting them with a pipe. Thus

cat file1 file2 file3 | sort >/dev/lp

invokes the cat program to concatenate three files and send the output to sort to arrange all the lines in alphabetical order. The output of sort is redirected to the file /dev/lp , typical1y the printer.

If a user puts an ampersand after a command, the shell does not wait for it to complete. Instead it just gives a prompt immediately. Consequently,

cat file1 file2 file3 | sort >/dev/lp &

starts up the sort as a background job, allowing the user to continue working normally while the sort is going on. The shell has a number of other interesting features, which we do not have space to discuss here. Most books on UNIX discuss the shell at some length (e.g., Kernighan and Pike, 1984; Kochan and Wood 1990; Medinets, 1999; Newham and Rosenblatt, 1998: and Robbins, 1999).