branch
if/then/elif/else/fi
Similar to the C language, the commands of if , then , elif , else , and fi are used to implement branch control in Shell . This flow control statement is also essentially composed of several shell commands, such as the previously mentioned
if [ -f ~/.bashrc ]; then . ~/.bashrc be
其实是三条命令，if [ -f ∼/.bashrc ]是第一条，then . ∼/.bashrc是第二条，fi是第三条。如果两条命令写在同一行则需要用;号隔开，一行只写一条命令就不需要写;号了，另外，then后面有换行，但这条命令没写完，Shell会自动续行，把下一行接在then后面当作一条命令处理。和[命令一样，要注意命令和各参数之间必须用空格隔开。if命令的参数组成一条子命令，如果该子命令的Exit Status为0（表示真），则执行then后面的子命令，如果Exit Status非0（表示假），则执行elif、else或者fi后面的子命令。if后面的子命令通常是测试命令，但也可以是其它命令。Shell脚本没有{}括号，所以用fi表示if语句块的结束。见下例：
#! /bin/sh if [ -f /bin/bash ] then echo "/bin/bash is a file" else echo "/bin/bash is NOT a file" be if :; then echo "always true"; fi
" : " is a special command called empty command which does nothing but Exit Status is always true. In addition, you can also execute /bin/true or /bin/false to get a 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_NO if [ "$YES_OR_NO" = "yes" ]; then echo "Good morning!" elif [ "$YES_OR_NO" = "no" ]; then echo "Good afternoon!" else echo "Sorry, $YES_OR_NO not recognized. Enter yes or no." exit 1 be exit 0
The function of the read command in the above example is to wait for the user to input a line of string and store the string in a Shell variable.
In addition, Shell also provides && and || syntax, similar to C language, with Short-circuit feature, many Shell scripts like to write like this:
test "$(whoami)" != 'root' && (echo you are using a non-privileged account; exit 1)
&& is equivalent to " if ... then ..." and || is equivalent to " if not ... then ...". && and || are used to connect two commands, while -a and -o mentioned above are only used to connect two test conditions in a test expression, pay attention to their differences, for example:
test "$VAR" -gt 1 -a "$VAR" -lt 3
is equivalent to the following
test "$VAR" -gt 1 && test "$VAR" -lt 3