Parameter Substitution

As we saw in the chapter on variables, you can put braces around a variable name to set it off from its surroundings:

$ curr=myfile
$ rm ${curr}.jpeg

There are also some nifty tricks you can perform inside the braces, such as changing parts of the string. Suppose you have a file named mypicture.jpeg instead of myfile.jpeg. You could alter the $curr variable when you insert it into a command:

$ rm ${curr/file/picture}.jpeg

Playing Safe With Variables That Don't Exist

Sometimes you might be using variables that have been removed (which you can do with the unset command) or were never initialized in the first place.  Since by default the shell uses an empty string for a nonexistent or undefined variable, as in the case of the rm command we showed earlier, it's useful to be able to substitute a default value for a variable.

$ cat "${VARIABLE_FILE_NAME:-/home/user/file}"

The ':-' operator asks the shell to check whether the variable is set. to see if it exists and is set to some string.  If it was never defined, or has no value, the shell substitutes the text after ':-'.

$ cat "${VARIABLE_FILE_NAME:=/home/user/file}"

The ':=' operator will do much the same, but instead of just substituting /home/user/file in the current cat command, if VARIABLE_FILE_NAME doesn't exist, the shell will also set the variable to the alternative text.

Cutting Corners With Variable Expansion

Variable expansion is by no means limited to filenames. It is also a handy way to pass complex, frequently used options to commands.

$ export ALT_LS='--color=always -b -h --filetype'
$ ls $ALT_LS

Since the alternative options are stored and expanded in variable form, you can use whatever defaults you like for ninety-percent of your work but quickly use an alternative form in special cases.

Parameter expansion is an excellent way to abstractly deal with multiple files or tediously long series of options.  Once you understand it, it's bound to expand your abilities on the command line.