条件语句, if/then/elif 在某些情况下,我们需要做条件判断。比如判断字符串长度是否为0?判断文件foo是否存在?它是一个链接文件还是实际文件?首先,我们需要if命令来执行检查。语法如下: if condition then statement1 statement2 .......... fi 当指定条件不满足时,可以通过else来指定其他执行动作。 i
因为if condition,如果条件为真,就会进入if块区域内执行命令。 如果你是对$?结果很执着的读者,可以去看朱双印的博客原文。 场景一:判断变量是否为空 假如有变量 如上表所示,变量值非空时 condition 为真。使用上述方法判断变量值是否为空时,[ ] 与 [[ ]] 没有区别。 我们可以使用”!”进行取反,使得变量值...
6 elif [ condition2 ] 7 # Same as else if 8 then 9 command4 10 command5 11 else 12 default-command 13 fi使用if test condition-true这种形式和if[condition-true]这种形式是等价的.向我们前边所说的"["是test的标记.并且以"]"结束.在if/test中并不应该这么严厉,但是新版本的Bash需要它.注意:tes...
How to check if a command succeeds or failed? How to do string comparison and check if a string equals to a value? How to check if a string is in an array? How to use the Bash ternary operator? How to negate an if condition in a Bash if statement? (if not command or if not ...
elif [[ condition ]]; then statement else do this by default fi 1. 2. 3. 4. 5. 6. 7. 8. 为了创建有意义的比较,我们也可以使用AND -a和OR -o。 下面的语句翻译成:如果a大于40且b小于6。 if [ $a -gt 40 -a $b -lt 6 ] ...
Bash boolean OR operator takes two operands and returns true if any of the operands is true, else it returns false. OR logical operator combines two or more simple or compound conditions and forms a compound condition. Syntax of OR Operator ...
Another common error is using the wrong operator in the condition. For example, using ‘=’ instead of ‘-eq’ for numerical comparison can lead to unexpected results. a=5b=5# Incorrect syntaxif[$a=$b];thenecho'a is equal to b'fi# Output:# a is equal to b ...
Theifstatement is closed with afi(reverse of if). Pay attention to white space! There must be a space between the opening and closing brackets and the condition you write. Otherwise, the shell will complain of error. There must be space before and after the conditional operator (=, ==, ...
7. Bash 中的 if ## FORMAT if [ condition ]; then cmd elif [ condition ]; then cmd else cmd fi ## EXAMPLE if [ -e file1 ] # file1 exists if [ -f file1 ] # file1 exists and is a regular file if [ -s file1 ] # file1 exists and size > 0 if [ -L file1 ] # fil...
Syntax of NOT Operator </> Copy if ! command; then # commands to run if the command fails fi Here, the!is placed before a command or condition. If the command or condition returns a failure (non-zero exit status), the!negates it and makes theifcondition true. ...