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I wonder the general rule to use regular expression in if clause in bash?
Here is an example
$ gg=svm-grid-ch $ if [[ $gg == *grid* ]] ; then echo $gg; fi svm-grid-ch $ if [[ $gg == ^....grid* ]] ; then echo $gg; fi $ if [[ $gg == ....grid* ]] ; then echo $gg; fi $ if [[ $gg == s...grid* ]] ; then echo $gg; fi $ Why the last three fails to match?
Hope you could give as many general rules as possible, not just for this example.
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If-Then-Else Conditionals in Regular Expressions. A special construct (? ifthen|else) allows you to create conditional regular expressions. If the if part evaluates to true, then the regex engine will attempt to match the then part.
Regex is a very powerful tool that is available at our disposal & the best thing about using regex is that they can be used in almost every computer language. So if you are Bash Scripting or creating a Python program, we can use regex or we can also write a single line search query.
You can use the test construct, [[ ]] , along with the regular expression match operator, =~ , to check if a string matches a regex pattern (documentation). where commands after && are executed if the test is successful, and commands after || are executed if the test is unsuccessful.
A null string in Bash can be declared by equalizing a variable to “”. Then we have an “if” statement followed by the “-n” flag, which returns true if a string is not null. We have used this flag to test our string “name,” which is null.
When using a glob pattern, a question mark represents a single character and an asterisk represents a sequence of zero or more characters:
if [[ $gg == ????grid* ]] ; then echo $gg; fi When using a regular expression, a dot represents a single character and an asterisk represents zero or more of the preceding character. So ".*" represents zero or more of any character, "a*" represents zero or more "a", "[0-9]*" represents zero or more digits. Another useful one (among many) is the plus sign which represents one or more of the preceding character. So "[a-z]+" represents one or more lowercase alpha character (in the C locale - and some others).
if [[ $gg =~ ^....grid.*$ ]] ; then echo $gg; fi Use =~
for regular expression check Regular Expressions Tutorial Table of Contents
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