In Bash (at least as far back as 3.0), if var is a declared/set variable, then declare -p var outputs a declare command that would set variable var to whatever its current type and value are, and returns status code 0 (success). If var is undeclared, then declare -p var outputs ...
22[ FILE1 -nt FILE2 ] 如果 FILE1 has been changedmorerecently than FILE2, or 如果 FILE1 exists and FILE2 does not则为真。23[ FILE1 -ot FILE2 ] 如果 FILE1 比 FILE2 要老, 或者 FILE2 存在且 FILE1 不存在则为真。24[ FILE1 -ef FILE2 ] 如果 FILE1 和 FILE2 指向相同的设备和节...
当我们需要在程序中对一个变量的值进行判断时,if语句可以帮助我们实现这一目的。 在Linux中,if语句可以很方便地判断一个变量是否为空。例如,我们可以使用如下的if语句来判断一个变量是否为空: ```bash if [ -z $variable ]; then echo "Variable if语句...
Why you should not use the || and && operators instead of a Bash If Statement? Detailed Examples & FAQ How to check if a variable exists or is “null”? How to check if a file exists? How to check if a directory exists? How to check if a command succeeds or failed? How to do...
bash 复制代码 if [ 条件 ]; then # 如果条件为真,执行这里的命令 fi 这里的[ 条件 ]是测试条件,可以是比较字符串、数字或文件等。例如,我们可以检查一个文件是否存在: bash 复制代码 if [ -f /path/to/file ]; then echo "File exists." fi ...
bash脚本:if循环 单分支的if语句: if 判断条件; then statement1 statement2 ... fi 例如: 如果用户已存在,则显示已存在 #!/bin/bashNAME = TEST if id $NAME &> /dev/null ; then echo "user exists" fi 双分支的if语句: if 判断条件; then sta ...
Use the if-else statement with the -v option to check if an environment variable is set in bash. The echo command in the if block will be executed if the will
/bin/bash # Date: 2015-04-02 # Author: ArvinLau # Description: # Version: 1.0 UserName=$1 Uid=grep"^$UserName:"/etc/passwd|cut-d: -f3`#此处注意变量不要使用UID,会报./usertype.sh: line 7: UID: readonly variable的错误 if[ $Uid -gt 499 ];then...
bash if [ -v VAR ]; then echo "Variable VAR exists" else echo "Variable VAR does not exist" fi -v 选项用于检查变量 VAR 是否被设置。如果 VAR 存在,则条件为真,执行 then 部分;否则执行 else 部分。 方法二:使用 ${VAR+x} 表达式 bash if [ -z "${VAR+x}" ]; then echo "Variable ...
Check if a File or Directory Exists Using Code Snippets The previous commands work well for a simple two-line command at a command prompt. However, you can also useBashwith multiple commands. When several commands are strung together, they are called ascript. ...