Classic Shell Scripting

Chapter 7. Input and Output, Files, and Command Evaluation

This chapter completes the presentation of the shell language. We first look at files, both for I/O and for generating filenames in different ways. Next is command substitution, which lets you use the output of a command as arguments on a command line, and then we continue to focus on the command line by discussing the various kinds of quoting that the shell provides. Finally, we examine evaluation order and discuss those commands that are built into the shell.

Standard Input, Output, and Error

Standard I/O is perhaps the most fundamental concept in the Software Tools philosophy. The idea is that programs should have a data source, a data sink (where data goes), and a place to report problems. These are referred to by the names standard input, standard output, and standard error, respectively. A program should neither know, nor care, what kind of device lies behind its input and outputs: disk files, terminals, tape drives, network connections, or even another running program! A program can expect these standard places to be already open and ready to use when it starts up.

Many, if not most, Unix programs follow this design. By default, they read standard input, write standard output, and send error messages to standard error. As we saw in Chapter 5, such programs are called filters because they "filter" streams of data, each one performing some operation on the data stream and passing it down the pipeline to the next ...