if grep -q "$user2" /etc/passwd; then echo "User does exist!!" Sample code to exercise the function: I tested on MS Windows 7 using bash 4.3.46 (works fine) and bash 3.1.17 (didn't work) The LHS of the =~ should be in quotes. This information is available to you in an interactive bash … Note: not using quotes will cause issues when words contain spaces, etc. does - bash string not contains . You want the -n test ("true if the length of the given string is non-zero"), which is the opposite of the -z test ("true if the length of the given string is zero"). If the string on it somehow represents a regex (like [0-9] for example), there is a higher chance to trigger a "match". The [and [[evaluate conditional expression. When comparing strings in Bash you can use the following operators: string1 = string2 and string1 == string2 - The equality operator returns true if the operands are equal. Always quote in Bash IMO. So as you see now I have used curly braces {} to make sure the separator is not considered part of the variable, now let's check the output from the script: ~]# ./eg_1.sh Hello_World This is the one of the most important thing you should always remember when working with bash string concatenation. The -p test is something completely different ("true if the given file exists and is a named pipe"). Please note that you need at least bash 4 for this use of =~ It doesn't work in bash 3. However, [[is bash’s improvement to the [command. Bash If statement syntax is How to check if a string contains a substring in Bash (14) Compatible answer. Reference: Advanced Bash Script guide. Check if strings are not equal in Bash Instead of checking the quality, let’s do the opposite and check the inequality. You can use (!=) operator to check when both strings are not equal. Here are some examples Bash 4+: Example 1, check for 'yes' in string (case insensitive): if [[ "${str,,}" == *"yes"* ]] ;then Example 2, check for 'yes' in string (case insensitive): Add the following code: #!/bin/bash This is a synonym for the test command/builtin. Bash 4+ examples. validate does not accept substrings (remove the -x option to grep if you want that). Bash check if a string contains a substring . You can quickly test for null or empty variables in a Bash shell script. In my bash script I'm trying to print a line if a certain string does not exist in a file. You can look in the Bash man page in the section called "Parameter Expansion" for information about ${var//string} and ${var#string} and in the section called "Pattern Matching" for [^[:digit:]]` (which is also covered in man 7 regex). Bash also provides the negation operator so that you can easily use “if not equal” condition in shell scripts. Above, PHONE_TYPE="SPACE TEL" would match too. Check If Two Strings are Not Equal (!=) Bash also provides the negation operator to use “if not equal” condition in bash scripts. validate interprets its argument as a fixed string, not a regular expression (remove the -F option to grep if you want that). Now you can use any other special character here to combine both the strings. You need to pass the -z or -n option to the test command or to the if command or use conditional expression.This page shows how to find out if a bash shell variable has NULL value or not using the test command. Let’s create a new test.sh script as shown below: nano test.sh. Example – if -z (to check if string has zero length) Example – if -s (to check if file size is greater than zero) Example – if -n (to check if string length is not zero) Example – if -f (to check if file exists and is a regular file) Syntax of Bash If. 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