Linux环境编程课件Linux环境编程3章节幻灯片.pptVIP

* The simplest type of flow control construct is the conditional, embodied in bashs if statement. You use a conditional when you want to choose whether or not to do something, or to choose among a small number of things to do, according to the truth or falsehood of conditions. Conditions test values of shell variables, characteristics of files, whether or not commands run successfully, and other factors. * The simplest type of flow control construct is the conditional, embodied in bashs if statement. You use a conditional when you want to choose whether or not to do something, or to choose among a small number of things to do, according to the truth or falsehood of conditions. Conditions test values of shell variables, characteristics of files, whether or not commands run successfully, and other factors. The spaces after the opening bracket [ and before the closing bracket ] are required * * * The spaces after the opening bracket [ and before the closing bracket ] are required * conditional expressions inside [ and ] can also be combined using the logical operators and ||. 逻辑测试运算符用于连接测试表达式，逻辑操作符“”和“||”用于连接两个命令。 * It is good practice to surround expressions with quotes, since many characters are treated as special by the shell (e.g., *, #, and parentheses); furthermore, you must quote expressions that include whitespace * 在“[[ ]]”中，允许用括号和逻辑操作符“”和“||”把 test 命令支持的测试组合起来。 * To use which syntax is up to you. it depends on your habits. 如果测试条件是多条命令的话，则以最后一条得到执行的命令的退出状态为准。 * choose in multiple conditions. bashs case construct lets you test strings against patterns that can contain wildcard characters. Like its conventional-language counterparts, case lets you express a series of if-then-else type statements in a concise way. Any of the patterns can actually be several patterns separated by pipe characters (|). If expression matches one of the patterns, its corresponding statements are executed. If there are several patterns separated by pipe characters, the expr