By Mendel Cooper
This rfile is either an instructional and a reference on shell scripting with Bash. It assumes no prior wisdom of scripting or programming, yet progresses speedily towards an intermediate/advanced point of guide. The routines and heavily-commented examples invite energetic reader participation. nonetheless, it's a paintings in growth. The goal is so as to add a lot supplementary fabric in destiny updates to this rfile, because it evolves right into a entire publication that fits or surpasses any of the shell scripting manuals in print.
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The −n test absolutely requires that the string be quoted within the test brackets. Using an unquoted string with ! −z, or even just the unquoted string alone within test brackets (see Example 7−5) normally works, however, this is an unsafe practice. Always quote a tested string.  Example 7−4. /bin/bash a=4 b=5 # Here "a" and "b" can be treated either as integers or strings. # There is some blurring between the arithmetic and string comparisons, #+ since Bash variables are not strongly typed.
If a string has not been initialized, it has no defined value. # This state is called "null" (not the same as zero). if [ −n $string1 ] # $string1 has not been declared or initialized. " fi # Wrong result. # Shows $string1 as not null, although it was not initialized. echo # Lets try it again. if [ −n "$string1" ] # This time, $string1 is quoted. " fi # Quote strings within test brackets! echo if [ $string1 ] # This time, $string1 stands naked. " fi # This works fine. # The [ ] test operator alone detects whether the string is null.
4. Special Variable Types local variables variables visible only within a code block or function (see also local variables in functions) environmental variables variables that affect the behavior of the shell and user interface In a more general context, each process has an "environment", that is, a group of variables that hold information that the process may reference. In this sense, the shell behaves like any other process. Every time a shell starts, it creates shell variables that correspond to its own environmental variables.
Advanced bash-scripting guide by Mendel Cooper