Shell script Syntax -- if/then/elif/else/fi

Shell script Syntax -- if/then/elif/else/fi is similar to the C language. In Shell, use commands such as if, then, elif, else, And fi to implement branch control. This process control statement is essentially composed of several Shell commands, such as www.2cto.com if [-f ~ /. Bashrc]; then .~ /. Bashrcfi is actually three commands, if [-f ~ /. Bashrc] is the first, then .~ /. Bashrc is the second and fi is the third. If the two commands are written in the same line, they must be separated by a; number. If only one command is written in one line, no; number is required. In addition, there is a line break after then, but this command has not been written, shell will automatically continue the line and process the next line following then as a command. Like [commands, note that commands and parameters must be separated by spaces. The parameters of the if command form a sub-command. if the Exit Status of the sub-command is 0 (true), run the sub-command after then, if Exit Status is not 0 (false), run the subcommand after elif, else, or fi. The sub-commands after if are usually test commands, but can also be other commands. The Shell script does not have {} brackets, so fi is used to indicate the end of the if statement block. See the following example :#! /Bin/sh if [-f/bin/bash] then echo "/bin/bash is a file" else echo "/bin/bash is NOT a file" fiif :; then echo "always true"; fi: A Special Command called an empty command. This command does not do anything, but the Exit Status is always true. In addition, you can run/bin/true or/bin/false to obtain the true or false Exit Status. Let's look at another example :#! /Bin/sh echo "Is it morning? Please answer yes or no. "read YES_OR_NOif [" $ YES_OR_NO "=" yes "]; then echo" Good morning! "Elif [" $ YES_OR_NO "=" no "]; then echo" Good afternoon! "Else echo" Sorry, $ YES_OR_NO not recognized. enter yes or no. "exit 1 fiexit 0 the read command in the above example is used to wait for the user to enter a string and store it in a Shell variable. In addition, Shell also provides the & | syntax, which is similar to the C language and has the Short-circuit feature. Many Shell scripts like to write like this: test "$ (whoami )"! = 'Root' & (echo you are using a non-privileged account; exit 1) & equivalent to "if... then... ", and | equivalent to" if not... then... ". & | Used to connect two commands. The-a and-o mentioned above are only used to connect two test conditions in the test expression. Pay attention to their differences. For example, test "$ VAR"-gt 1-a "$ VAR"-lt 3 is equivalent to test "$ VAR"-gt 1 & test "$ VAR"-lt 3