## Shells 

In DragonFly, a lot of everyday work is done in a command line interface called a shell. A shell's main job is to take commands from the input channel and execute them. A lot of shells also have built in functions to help everyday tasks such as file management, file globbing, command line editing, command macros, and environment variables. DragonFly comes with a set of shells, such as `sh`, the Bourne Shell, and `tcsh`, the improved C-shell. Many other shells are available from pkgsrc®, such as `zsh` and `bash`.

Which shell do you use? It is really a matter of taste. If you are a C programmer you might feel more comfortable with a C-like shell such as `tcsh`. If you have come from Linux or are new to a UNIX® command line interface you might try `bash`. The point is that each shell has unique properties that may or may not work with your preferred working environment, and that you have a choice of what shell to use.

One common feature in a shell is filename completion. Given the typing of the first few letters of a command or filename, you can usually have the shell automatically complete the rest of the command or filename by hitting the  **Tab**  key on the keyboard. Here is an example. Suppose you have two files called `foobar` and `foo.bar`. You want to delete `foo.bar`. So what you would type on the keyboard is: `rm fo[ **Tab** ].[ **Tab** ]`.

The shell would print out `rm foo[BEEP].bar`.

The [BEEP] is the console bell, which is the shell telling me it was unable to totally complete the filename because there is more than one match. Both `foobar` and `foo.bar` start with `fo`, but it was able to complete to `foo`. If you type in `.`, then hit  **Tab**  again, the shell would be able to fill in the rest of the filename for you.

Another feature of the shell is the use of environment variables. Environment variables are a variable key pair stored in the shell's environment space. This space 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 what they mean:

[[!table  data="""
|<tablestyle="width:100%"> Variable | Description 
<tablestyle="width:100%"> `USER` | Current logged in user's name. 
 `PATH` | Colon separated list of directories to search for binaries. 
 `DISPLAY` | Network name of the X11 display to connect to, if available. 
 `SHELL` | The current shell. 
 `TERM` | The name of the user's 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. e.g., DragonFly. 
 `MACHTYPE` | The CPU architecture that the system is running on. 
 `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. |
"""]]
Setting an environment variable differs somewhat from shell to shell. For example, in the C-Style shells such as `tcsh` and `csh`, you would use `setenv` to set environment variables. Under Bourne shells such as `sh` and `bash`, you would use `export` to set your current environment variables. For example, to set or modify the `EDITOR` environment variable, under `csh` or `tcsh` a command like this would set `EDITOR` to `/usr/pkg/bin/emacs`:

    
    % setenv EDITOR /usr/pkg/bin/emacs


Under Bourne shells:

    
    % export EDITOR="/usr/pkg/bin/emacs"


You can also make most shells expand the environment variable by placing a `$` character in front of it on the command line. For example, `echo $TERM` would print out whatever `$TERM` is set to, because the shell expands `$TERM` and passes it on to `echo`.

Shells treat a lot of special characters, called meta-characters as special representations of data. The most common one is the `*` character, which represents any number of characters in a filename. These special meta-characters can be used to do filename globbing. For example, typing in `echo *` is almost the same as typing in `ls` because the shell takes all the files that match `*` and puts them on the command line for `echo` to see.

To prevent the shell from interpreting these special characters, they can be escaped from the shell by putting a backslash (`\`) character in front of them. `echo $TERM` prints whatever your terminal is set to. `echo \$TERM` prints `$TERM` as is.

### Changing Your Shell 

The easiest way to change your shell is to use the `chsh` command. Running `chsh` will place you into the editor that is in your `EDITOR` environment variable; if it is not set, you will be placed in `vi`. Change the ***Shell:*** line accordingly.

You can also give `chsh` the `-s` option; this will set your shell for you, without requiring you to enter an editor. For example, if you wanted to change your shell to `bash`, the following should do the trick:

    
    % chsh -s /usr/pkg/bin/bash


 **Note:** The shell that you wish to use ***must*** be present in the `/etc/shells` file. If you have installed a shell from the [ pkgsrc tree ](pkgsrc.html), then this should have been done for you already. If you installed the shell by hand, you must do this.

For example, if you installed `bash` by hand and placed it into `/usr/local/bin`, you would want to:

    
    # echo "/usr/local/bin/bash" >> /etc/shells


Then rerun `chsh`.