str - the String to check, may be null Returns: true if the String is not empty and not null 判断是否null、空串或空白串 是null或空串或空白串:方法一 if (s == null || s.length() == 0 || s.trim().length() == 0) { // do something } 是null或空串或空白串:方法二 if (s ...
The statements[ -z "$X" ]andecho "The environment variable X is not set."andusageare used to test if the string is null (-z) and if so, then perform the echo statement saying that it is not set and invoke the "usage" function discussed below. If your script does not use flags,...
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 string comparison and check if a string equals to a value? How to check if a string is in an array?
-n is one of the supported bash string comparison operators used for checking null strings in a bash script. When -n operator is used, it returns true for every case, but that’s if the string contains characters. On the other hand, if the string is empty, it won’t return true. ...
To check for non-null/non-zero string variable, i.e. if set, use if [ -n "$1" ] It's the opposite of -z. I find myself using -n more than -z. You would use it like: if [ -n "$1" ]; then echo "You supplied the first parameter!" else echo "First parameter not ...
I need to catch an error when lifting a service. The response can benull, a string error message like error services-migration/foobar: Not found: services-migration/foobar or a valid JSON when everything is fine. I was wondering if there is a way withjqto simply check if the provided ...
Here, we list some basic bash syntax with a brief explanation. It is a good starting point if you are a beginner.
In this example, the substitution command is used with $() to capture the output of grep, and then the -z option of the test command is used to check if the resulting string is empty or not. Here, the grep command searches for the pattern pattern in the file dummy.txt. The output...
Here, -q is used to suppress the output to only return the exit status.If the given string is found the exit status code 0 (zero) will be returned and the if block will be executed. On the other hand, if the given string is not found non-zero exit status will be returned and ...
本篇开始,介绍shell脚本编程,更确切的说是bash脚本编程(版本:4.2.46(1)-release)。我们从变量开始。...非交互式shell启动时(如脚本中),会继承派生出此shell的父shell的环境变量并执行环境变量BASH_ENV的值中所指代的文件。...作用环境变量的作用主要是影响shell的行为