Learning the bash Shell, Second Edition

Command-Line Processing

We’ve seen how the shell uses read to process input lines: it deals with single quotes (''), double quotes (“”), and backslashes (\); it separates lines into words, according to delimiters in the environment variable IFS; and it assigns the words to shell variables. We can think of this process as a subset of the things the shell does when processing command lines.

We’ve touched upon command-line processing throughout this book; now is a good time to make the whole thing explicit. Each line that the shell reads from the standard input or a script is called a pipeline; it contains one or more commands separated by zero or more pipe characters (|). For each pipeline it reads, the shell breaks it up into commands, sets up the I/O for the pipeline, then does the following for each command (Figure 7.1):

Figure 7-1. Steps in command-line processing

1. Splits the command into tokens that are separated by the fixed set of metacharacters: SPACE, TAB, NEWLINE, ;, (, ), <, >, |, and &. Types of tokens include words, keywords, I/O redirectors, and semicolons.

2. Checks the first token of each command to see if it is a keyword with no quotes or backslashes. If it’s an opening keyword, such as if and other control-structure openers, function, {, or (, then the command is actually a compound command. The shell sets things up internally for the compound command, reads the next command, ...