Chapter 4 - The UNIX Shells

Quiz

Most versions of UNIX provide the chsh command to allow a user to change his or her default shell. If this command is not available, the system administrator must modify your entry in the system password file to change your default (login) shell.

Use the cd command to change your current directory to a new location.

You can continue a shell command on a new line by typing a backslash ( \ ) as the last character of a line. When you hit RETURN or ENTER, the shell will prompt you to continue the command line.

A built-in command is part of the shell itself. A utility is an executable program that is installed (usually in a system directory such as /bin ) which the shell executes.

You can make a script executable by setting the execute bit in the file permission for the file. To make it executable to all users, including the owner and others in the same group as the owner, you would set the execute bit for all three. A script that can be read, written, and executed by the owner, and only read and executed by all other users would have a permission mode of 755.

Filename substitution is also called ìglobbing.î

There are many correct answers to this question. An example would be using ì*.cî to refer to all C program source files in the current directory (e.g. ls ñl *.c ).

Any process executing a shell can itself execute another shell that will perform some task and return to the original shell. This is similar to the parent process and child process paradigm. The first shell is called the parent to the shell it executes (creates). The new shell is the child to the original shell. The term subshell is just another way of describing the child shell.

The kill command is somewhat misleadingly named since what it really does is send a signal to a process. Any signal can be specified but by default, the signal that is sent is a SIGTERM signal which will cause the process to terminate. It was probably named kill because killing a process is the most common use of the command, so in many cases the name makes perfect sense. Consider the case, however, where you send a SIGUSR1 (user defined) signal to a running process because it will trap the signal, do some special processing, and then return to what it was doing before the signal was sent. In this case, you really arenít killing the process, you are simply sending it a message.

The most common way to override a standard utility is to put an executable file of the same name in a directory in your search path ($PATH) which is ahead of the directory where the standard utility is located. In some shells, another way would be to define an alias of the same name.

A umask value of 022 will mask off the write permission bits of any file you create, thus by default not allowing write access to the file by any user other than yourself (the creator of the file). For the ultra paranoid, a umask of 077 might be preferable which masks off all access permission to the file by any other user.

Exercises

This script will use echo (or print in the Korn shell) to display the variables $USER and $SHELL which contain the userís login name and current shell. The date command will also be executed to print out the current date.

A shell can print out $0 to show the name of the command that was used to start it. Output from the ps command will show it running. After executing the next script with exec, the student should notice in the next ps output that the new script name is present but the old one is gone. This is because exec replaces the current program with the new program when it runs it. At each execution of a new script, the old one is replaced and only the new one will show up as currently running. If you execute your first script with exec from your shell, then your shell itself is replaced when the first script runs. Then when the last script in the chain terminates, you may find yourself logged out of the system (or if youíre running in a terminal window, the terminal window will exit)!

The file is unaffected because the umask value only affects the permissions of a file at the time it is created. Since this file was already in existence when the umask value was changed, its permissions were already set. The subsequent date command merely overwrote an existing file.

Such a script will fork off three programs and then execute a wait command. This will cause the shell to sleep until all three programs have terminated. The next command after the wait command could be echo with a simple message.

Project

Shells fundamentally provide the same function, to allow easy access to files and utilities including the ability to execute, rename, delete, or open them. The main issues for this discussion would be whether someone prefers command line access to these functions or a graphical interface.