FreeBSD provides a command line interface called a shell. A shell receives commands from the input channel and executes them. Many shells provide built in functions to help with everyday tasks such as file management, file globbing, command line editing, command macros, and environment variables. FreeBSD comes with several shells, including sh, the Bourne Shell, and tcsh, the improved C-shell. Other shells are available from the FreeBSD Ports Collection, such as zsh and bash.
The shell that is used is really a matter of taste. A C programmer might feel more comfortable with a C-like shell such as tcsh. A Linux user might prefer bash. Each shell has unique properties that may or may not work with a user's preferred working environment, which is why there is a choice of which shell to use.
One common shell feature is filename completion. After a user types the first few letters of a command or filename and presses Tab, the shell will automatically complete the rest of the command or filename. Consider two files called foobar and foo.bar. To delete foo.bar, type rm fo[Tab].[Tab].
The shell should print out rm foo[BEEP].bar.
The [BEEP] is the console bell, which the shell used to indicate it was unable to complete the filename because there is more than one match. Both foobar and foo.bar start with fo. By typing ., then pressing Tab again, the shell would be able to fill in the rest of the filename.
Another feature of the shell is the use of environment variables. Environment variables are a variable/key pair stored in the shell's environment. This environment can be read by any program invoked by the shell, and thus contains a lot of program configuration. Here is a list of common environment variables and their meanings:
| Variable | Description |
|---|---|
| USER | Current logged in user's name. |
| PATH | Colon-separated list of directories to search for binaries. |
| DISPLAY | Network name of the Xorg display to connect to, if available. |
| SHELL | The current shell. |
| TERM | The name of the user's type of terminal. Used to determine the capabilities of the terminal. |
| TERMCAP | Database entry of the terminal escape codes to perform various terminal functions. |
| OSTYPE | Type of operating system. |
| MACHTYPE | The system's CPU architecture. |
| EDITOR | The user's preferred text editor. |
| PAGER | The user's preferred text pager. |
| MANPATH | Colon-separated list of directories to search for manual pages. |
How to set an environment variable differs between shells. In tcsh and csh, use setenv to set environment variables. In sh and bash, use export to set the current environment variables. This example sets the default EDITOR to /usr/local/bin/emacs for the tcsh shell:
% setenv EDITOR /usr/local/bin/emacs
The equivalent command for bash would be:
% export EDITOR="/usr/local/bin/emacs"
To expand an environment variable in order to see its current setting, type a $ character in front of its name on the command line. For example, echo $TERM displays the current $TERM setting.
Shells treat special characters, known as meta-characters, as special representations of data. The most common meta-character is *, which represents any number of characters in a filename. Meta-characters can be used to perform filename globbing. For example, echo * is equivalent to ls because the shell takes all the files that match * and echo lists them on the command line.
To prevent the shell from interpreting a special character, escape it from the shell by starting it with a backslash (\). For example, echo $TERM prints the terminal setting whereas echo \$TERM literally prints the string $TERM.
The easiest way to permanently change the default shell is to use chsh. Running this command will open the editor that is configured in the EDITOR environment variable, which by default is set to vi. Change the “Shell:” line to the full path of the new shell.
Alternately, use chsh -s which will set the specified shell without opening an editor. For example, to change the shell to bash:
% chsh -s /usr/local/bin/bash
Note: The new shell must be present in /etc/shells. If the shell was installed from the FreeBSD Ports Collection, it should be automatically added to this file. If it is missing, add it using this command, replacing the path with the path of the shell:
# echo /usr/local/bin/bash >> /etc/shellsThen rerun chsh.