linux crash course chapter 10 3

Chapter 10: BASH Shell Scripting Fun with fi In this chapter … • Control structures • File descriptors • Variables Control structure tests • Control structures depend on a test that equates either true or false • The test builtin in bash allows logical, relational, and file property-based tests • Syntax: test expression OR [ expression ] test expressions • If expression is true, test returns 0; if false, it returns not (usually 1) • Comparing text strings string1 = string2 string1 != string2 • Comparing numbers num1 –OP num2 – Where OP can be eq, ne, lt, gt, le, ge test expressions con’t • File tests -option filename where option can be: d : file is a directory y : file exists f : file is a regular file Plus many more (check man bash) Other test-commands • Instead of test and [ ] you can use other bash contructs • ((expression)) can be used for integer comparisons • [[expression]] can be used for logical expressions and string comparisons • See pages 505-506 for complete list if … then structure • Syntax: if test-command then commands fi • test-command must evaluate true or false • commands can be zero or more lines if … then … else structure • Syntax: if test-command then commands else commands fi • Same guidelines as if…then if … then … elif structure • Syntax: if test-command then commands elif test-command then commands … else commands fi if … then … elif con’t • You can have one or more elif blocks • Remember, each elif line is following by a then statement • Rather than multiple elif’s, might try a case statement instead select structure con’t • select structure displays a numbered menu allowing user to select an arg • After displaying the menu, select displays the PS3 prompt – by default it’s #? • Set PS3 to customize the prompt to something more intelligible • The user’s selection (what was actually typed) is stored in REPLY File descriptors • Recall 0, 2> … now let’s make more • Syntax: exec n> outfile AND exec m< infile • exec associates streams with files • Then can treat those streams just like the standard ones • To close: exec n>&- AND exec m – the shift builtin rotates through the arguments Null and unset variables • ${varname:-default} : if varname is not set or is null, substitutes for default • ${varname:=default} : if varname is not set or null, substitues for default and sets varname • ${varname:?message} : if varname is not set, displays an error Functions • Syntax: function name () { … } • Note on scope – functions have same scope as calling script/shell … which means you can access (or step on!) existing variables Here document • Allows you to standard input redirection within a script • Denoted by