For novice users, this chapter presents basic concepts about the Unix shell. For advanced users, this chapter also summarizes the major similarities and differences between the Bourne, Korn, and C shells. Details on the three shells are provided in Chapter 4 and Chapter 5. Unixshellsshells shells overview of The following topics are presented: Introduction to the shell Purpose of the shell Shell flavors Common features Differing features Let’s suppose that the Unix operating system is a car. When you drive, you issue a variety of “commands”: you turn the steering wheel, press the accelerator, or press the brake. But how does the car translate your commands into the action you want? The car’s drive mechanism, which can be thought of as the car’s user interface, is responsible. Cars can be equipped with front-wheel drive, rear-wheel drive, four-wheel drive, and sometimes combinations of these. The shell is the user interface to Unix, and by the same token, several shells are available in Unix. Most systems provide more than one for you to choose from. Each shell has different features, but all of them affect how commands will be interpreted and provide tools to create your Unix environment. command interpretersshells The shell is simply a program that allows the system to understand your commands. (That’s why the shell is often called a command interpreter.) For many users, the shell works invisibly—“behind the scenes.” Your only concern is that the system does what you tell it to do; you don’t care about the inner workings. In our car analogy, this is comparable to pressing the brake. Most of us don’t care whether the user interface involves disk brakes or drum brakes, as long as the car stops. There are three uses for the shell: Interactive use Customization of your Unix session Programming interactive use of shells When the shell is used interactively, the system waits for you to type a command at the Unix prompt. Your commands can include special symbols that let you abbreviate filenames or redirect input and output. sessionslogin sessions login sessionscustomizing customizing login session A Unix shell defines variables to control the behavior of your Unix session. Setting these variables will tell the system, for example, which directory to use as your home directory, or the file in which to store your mail. Some variables are preset by the system; you can define others in startup files that are read when you log in. Startup files can also contain Unix commands or special shell commands. These are executed every time you log in. scripts, shellshell scripts shell scripts Unix shells provide a set of special (or built-in) commands that let you create programs called shell scripts. In fact, many built-in commands can be used interactively like Unix commands, and Unix commands are frequently used in shell scripts. Scripts are useful for executing a series of individual commands. This is similar to BATCH files in MS-DOS. Scripts can also execute commands repeatedly (in a loop) or conditionally (if-else), as in many high-level programming languages. shellstypes (flavors) of Many different Unix shells are available. This quick reference describes the three most popular shells: sh (Bourne shell) The Bourne (or standard) shell, the most compact shell and also the simplest. ksh (Korn shell) Korn shell (ksh) The Korn shell, a superset of the Bourne shell that lets you edit the command line. There are in fact two commonly available versions of the Korn shell, distinguished by the year they were released, and referred to in this book as ksh88 and ksh93 respectively. csh (C shell) The C shell, which uses C-like syntax and is more convenient for the interactive user than the Bourne shell. Most systems have more than one shell, and people will often use the Bourne shell for writing shell scripts and another shell for interactive use. /etc directory /etc/passwd file The /etc/passwd file determines which shell takes effect during your interactive Unix session. When you log in, the system checks your entry in /etc/passwd. The last field of each entry names a program to run as the default shell. On Solaris or other networked Unix systems, this information may come from NIS or NIS+. Usually, your system administrator will handle this for you; just don’t be surprised if your login name doesn’t appear in /etc/passwd. For example: If the program name is: Your shell is the: /bin/sh Bourne shell /bin/rsh Restricted Bourne shell /bin/jsh Bourne shell, including job control /bin/ksh Korn shell /usr/dt/bin/dtksh The Desktop Korn shell, a version of ksh93 (Solaris only) /bin/rksh Restricted Korn shell /bin/csh C shell You can change to another shell by typing the program name at the command line. For example, to change from the Bourne shell to the Korn shell, type: $ exec ksh rsh (remote shell) remote shellrsh Note that on most systems, rsh is the “remote shell” for executing commands on a remote system across a network. On some systems, though, rsh is indeed the restricted shell, and remsh is the remote shell. Check your local documentation. If you are new to Unix, picking a shell may be a bewildering question. Before ksh was commonly available, the general advice was to use csh for interactive use (because it supported job control and had other features that made it a better interactive shell than the Bourne shell), but to use the Bourne shell for scripting (because it is a more powerful programming language, and more universally available). Today, ksh is widely available; it is upwardly compatible with the Bourne shell as a programming language, and it has all the interactive capabilities of csh, and more. If it is available, it is probably your best choice. sh (Bourne shell) features of csh (C shell) features of ksh (Korn shell) features of The following table displays features that are common to the Bourne, Korn, and C shells. Note that the Korn shell is an enhanced version of the Bourne shell; therefore, the Korn shell includes all features of the Bourne shell, plus some others. The commands bg, fg, jobs, stop, and suspend are available only on systems that support job control. (Essentially all modern Unix systems do.) Symbol/Command Meaning/Action > Redirect output. >> Append to file. < Redirect input. << “Here” document (redirect input). | Pipe output. |& Start a coprocess. Korn shell only. & Run process in background. ; Separate commands on same line. * Match any character(s) in filename. ? Match single character in filename. [ ] Match any characters enclosed. ( ) Execute in subshell. ‘ ‘ Substitute output of enclosed command. " " Partial quote (allows variable and command expansion). ’ ’ Full quote (no expansion). \ Quote following character. $var Use value for variable. $$ Process ID. $0 Command name. $n n th argument (0 ≤ n ≤ 9). $* All arguments as simple words. # Begin comment. bg Background execution. break Break from loop statements. cd Change directory. continue Resume a program loop. echo Display output. eval Evaluate arguments. exec Execute a new shell. fg Foreground execution. jobs Show active jobs. kill Terminate running jobs. shift Shift positional parameters. stop Suspend a background job. suspend Suspend a foreground job (such as a shell created by su). time Time a command. umask Set default file permissions for new files. unset Erase variable or function definitions. wait Wait for a background job to finish. The following table displays features that are different among the three shells. sh ksh csh Meaning/Action $ $ % Prompt. >| >! Force redirection. >>! Force append. > file 2>&1 > file 2>&1 >& file Combine stdout and stderr. { } Expand elements in list. ‘ ‘ ‘ ‘ ‘ ‘ Substitute output of enclosed command. $( ) Substitute output of enclosed command. (Preferred form.) $HOME $HOME $home Home directory. ~ ~ Home directory symbol. var=value var=value set var=value Variable assignment. export var export var=val setenv var val Set environment variable. ${nn} More than nine args can be referenced. "$@" "$@" All args as separate words. $# $# $#argv Number of arguments. $? $? $status Exit status. $! $! Background exit status. $- $- Current options. .  file .  file source file Read commands in file. alias x=y alias x y Name x stands for y. case case switch/case Choose alternatives. cd ~- popd/pushd Switch directories. done done end End a loop statement. esac esac endsw End case or switch. exit [n] exit [n] exit [(expr)] Exit with a status. for / do for / do foreach Loop through variables. print -r glob Ignore echo escapes. hash alias -t hashstat Display hashed commands (tracked aliases). hash cmds alias -t cmds rehash Remember command locations. hash -r PATH=$PATH unhash Forget command locations. history history List previous commands. r !! Redo previous command. r str !str Redo command that starts with str. r x=y [cmd] !cmd:s/x/y/ Edit command, then execute. if [ $i -eq 5 ] if ((i==5)) if ($i==5) Sample if statement. fi fi endif End if statement. ulimit ulimit limit Set resource limits. pwd pwd dirs Print working directory. read read $< Read from standard input. trap 2 trap 2 onintr Ignore interrupts. unalias unalias Remove aliases. until /do until /do Begin until loop. while /do while /do while Begin while loop.