As Paul pointed out, don't use it in a loop. Bash scripting entails quite a lot of things: writing conditional statements, understanding the various concepts of bash, built-in commands, and how to indicate if a script executed successfully or not. Find And Delete Files That "does not" contain specific text in their names in Linux. The -z operator functions similarly like -n operator. To check if two strings are not equal in bash scripting, use bash if statement and not equal to!= operator. ==), not the regex operator =~. Bash 4+ examples. Piping out to 100000 greps is predictably painful! Example – if -z (to check if string has zero length) (yuck!) The old rule about using external utilities without need holds true. This is another example to read a file line by line and check if a string contains a specific substring and do some operation on it. Note that the double hash is obligatory with some shells (ash). Instinctively you think that this "language" requires you to follow an if with a [or a [[.Both of those are just commands that return an exit status indicating success or failure (just like every other command). A string can be any sequence of characters. Instinctively you think that this "language" requires you to follow an if with a [ or a [[. Above will evaluate [ stringvalue ] when the substring is not found. Where ?? Here, we have provided every detail on how to check for empty variables in a bash script, as well as introduce a handy freeware for managing Linux partitions on Windows PC. If you’d just like a possible solution: As our string StdName has now a value Aqsa Yasin, and the -z string operator knows that the string is not null, so it executes the else part. Many non-Linux nixes have only sh (which may or may not be some variant of ksh on solaris, others, I dunno). This Stack Overflow answer was the only one to trap space and dash characters: My .bash_profile file and how I used grep: If the PATH environment variable includes my two bin directories, don't append them, Original source: http://unstableme.blogspot.com/2008/06/bash-search-letter-in-string-awk.html, Use sed to remove instance of substring from string, If new string differs from old string, substring exists. Only sh is installed. If statement can accept options to perform a specific task. Also, you need spaces between [[, the elements of the comparison, and ]].Spaces are critical delimiters in the shell; do not leave them out. $ bash CheckStrings.sh. One of these days, you may experience it. The original string will look like: some textmore textzero-or-more-chars The last tag and zero or more chars part should be deleted ONLY if the zero-or-more-chars does NOT contain any of the following: a-z A-Z 0-9,.: fi` is useful when using BusyBox's shell, the inequality of difference. So the simple substitution option predictably wins whether in an extended test or a case. The output of the following line of code will be forward 2. https://stackoverflow.com/questions/229551/how-to-check-if-a-string-contains-a-substring-in-bash/229585#229585, This is probably the best solution since it is portable to posix shells. Leave a space between the brackets and the contents. How do you tell if a string contains another string in POSIX sh? The if statement allows you to specify courses of action to be taken in a shell script, depending on the success or failure of some command. It returns no error. The easiest method to perform string concatenation in bash is to write variables side by side. If there are no spaces, bash will complain about that. The -- argument is pure POSIX paranoia, used to protected against input strings similar to options, such as --abc or -a. -n is one of the supported bash string comparison operators used for checking null strings in a bash script. In my last article I shared some examples to get script execution time from within the script.I will continue with articles on shell scripts. You should remember that shell scripting is less of a language and more of a collection of commands. You don't need to quote variables inside [[ ]]. Hi, if empty strings are false, why do you consider it clumsy? https://stackoverflow.com/questions/229551/how-to-check-if-a-string-contains-a-substring-in-bash/229606#229606. It is written in Bash. For that reason I'd use grep, and not the [ command. The valid variable (string) must contain only letters. This book contains many real life examples derived from the author's experience as a Linux system and network administrator, trainer and consultant. If you don't have, https://stackoverflow.com/questions/229551/how-to-check-if-a-string-contains-a-substring-in-bash/8090175#8090175, @AloisMahdal I never down-voted, I'm just postulating on why downvotes were given. 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