Classic Shell Scripting

Evaluation Order and eval

The various expansions and substitutions that we've covered are done in a defined order. The POSIX standard provides the picayune details. Here, we describe things at the level a shell programmer needs to understand things. This explanation is simplified to elide the most petty details: e.g., middles and ends of compound commands, special characters, etc.

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 (|). (Actually, several special symbols separate individual commands: semicolon, ;, pipe, |, ampersand, &, logical AND, &&, and logical OR, ||.) For each pipeline it reads, the shell breaks it up into commands, sets up the I/O for the pipeline, and then does the following for each command, in the order shown:

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.

   It's a subtle point, but variable, command, and arithmetic substitution can be performed while the shell is doing token recognition. This is why the vi ~$user/.profile example presented earlier in Section 7.5.1, actually works as expected.

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 (if and other control-structure openers, {, or (), then the command is actually ...