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Execution

The words in a simple command are subjected to a number of substitutions, described later, but eventually becomes simply a list of assignment words, normal words, and I/O words. If all the I/O redirection required by the I/O words is successfully arranged, the shell attempts to execute the command. What is actually done is determined largely by the first of the simple words in the command body.

If this word contains any quoted characters, then only the final option - that of finding an executable file - is attempted, otherwise each of the following is tried in turn.

• If the first word is the name of an alias or function the a new local copy of the ARGV variable is created and assigned all the normal words in the command, the assignment words are used to create local exported variables, and the commands stored for that alias or function are executed. While an alias is being executed, that alias is temporarily disabled thus disabling recursion. Functions are not so inhibited and can be recursive.
• If the first word is the name of a built in command that is not hidden, that command is performed by the shell. Any assignment words are ignored (I think).
• Finally, the shell looks for an executable file with the same name as the first word, which contains a program to be run. If the first word starts with a slash or one or two dots followed by a slash, the word is taken to be a path name and a file with that name must exist. If the first word does not start like this, each directory named in the the variable PATH (which should be a colon separated list of directory names) is searched to see if it contains an executable file with this name. This procedure can be modified by the value assigned to the WHICH variable. If no such file is found the shell gives an error message and aborts the command. Otherwise, the shell attempts to execute the program contained in the file passing it the remainder of the words as arguments. If this fails, the shell assumes that that file contains a shell script and runs the program named in the shell variable SHELL passing it the name of the file and the remainder of the words as arguments.

When a command terminates, it returns an exit-value to the shell which is usually used to indicate success or failure. This value is a positive integer less than 256 where a value of 0 indicates success and any other value indicates failure, the actual number depends on the program and the reason for failure. This number is made available through the $? parameter. Alternately, if a program terminates because it received a signal, the $? parameter is set to 1000 plus the signal number.

This value is also used by the if and while commands as well as the && and || command separators to control the execution of subsequent commands.

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