One of the good collections that I could able to gather is abt the evaluation of Shell command. Most of this extraction is directly from Classic Shell Scriptin-Oreilly. It's just like a tutorial, pls bear with me, I don't have much time to elaborate it. I think this is good enough. Though it is a fundamental concept, I don't prefer someone who is new to shell env to read this post. Unnecessarily it will create chaos. Ok....let us start.... 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. 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 a compound command. The shell sets things up internally for the compound command, reads the next command, and starts the process again. If the keyword isn't a compound command opener (e.g., is a control-structure middle like then, else, or do, an end like fi or done, or a logical operator), the shell signals a syntax error. 3) Checks the first word of each command against the list of aliases. If a match is found, it substitutes the alias's definition and goes back to step 1; otherwise it goes on to step 4. The return to step 1 allows aliases for keywords to be defined: e.g., alias aslongas=while or alias procedure=function. Note that the shell does not do recursive alias expansion: instead, it recognizes when an alias expands to the same command, and stops the potential recursion. Alias expansion can be inhibited by quoting any part of the word to be protected. 4) Substitutes the user's home directory ($HOME) for the tilde character (~) if it is at the beginning of a word. Substitutes user's home directory for ~user. Tilde substitution (in shells that support it) occurs at the following places:

• As the first unquoted character of a word on the command line
• After the = in a variable assignment and after any : in the value of a variable assignment
• For the word part of variable substitutions of the form ${variable op word}

5) Performs parameter (variable) substitution for any expression that starts with a dollar sign ($).

6) Does command substitution for any expression of the form $(string) or `string`. 7) Evaluates arithmetic expressions of the form $((string)). 8) Takes the parts of the line that resulted from parameter, command, and arithmetic substitution and splits them into words again. This time it uses the characters in $IFS as delimiters instead of the set of metacharacters in step 1. Normally, successive multiple input occurrences of characters in IFS act as a single delimiter, which is what you would expect. This is true only for whitespace characters, such as space and tab. For nonwhitespace characters, this is not true. For example, when reading the colon-separated fields of /etc/passwd, two successive colons delimit an empty field: while IFS=: read name passwd uid gid fullname homedir shell do ... done < /etc/passwd