The grep utility searches for text in the contents of text files (not directories on most systems). That’s a good bash tip, but that grep may give you some unexpected output. Note that you don't grep a folder (usually called a directory in UNIX and Linux environments). Install Linux from USB Device or Boot into Live Mode Using Unetbootin and dd Command. Bash uses environment variables to define and record the properties of the environment it creates when it launches. 8 thoughts on “How To Assign Output of a Linux Command to a Variable” Michael Schwager. All this have to be done in a bash. For good and for bad, for all times eternal, Group 2 is assigned to the second capture group from the left of the pattern as you read the regex. It will also match viveki, vivek25, etc. These hold information Bash can readily access, such as your username, locale, the number of commands your history file can hold, your default editor, and lots more. grep , expr , sed and awk are some of them.Bash also have =~ operator which is named as RE-match operator.In this tutorial we will look =~ operator and use cases.More information about regex command cna be found in the following tutorials. Bash substring with regular expression, In a bash script, I´d like to extract a variable string from a given string. Environment Variables. Or, if you grep for username tom, you’ll also get tomcat. i need a string checked for an Regex and the Match should be returned into a variable. This article is for advanced users, who are already familiar with basic regular expressions in Bash. January 30, 2017 at 9:37 am You missed process substitution. I checked that line for an value, if that is true, i need the regexp match as result back, to get stored in a value. So you’ll either want to add a space to the end of your regex like ‘^tom ‘, or instead of a regex use -w for word matching. If you're trying to count the number of files in a directory that have names matching the globbing pattern stored in your shell variable named (confusingly) file_name, you could try something … Capture Groups with Quantifiers In the same vein, if that first capture group on the left gets read multiple times by the regex because of a star or plus quantifier, as in ([A-Z]_)+, it never becomes Group 2. Method 3. I mean, i´d like to extract the string file.txt from the string: This is the file.txt this regex matching on the grep command fails all the time, even if the line contains F08R16 pattern. Use the regular expression match operator =~ if you want to have an extended regular expression on the right-hand side. Execute the shell script, and the variable is successfully converted into array and the strings can be iterated separately # /tmp/ My array: string1 string2 string3 Number of elements in the array: 3 Method 4: Bash split string into array using tr A regular expression can match part of the line; if you want to match the whole line, put ^ at the beginning of the regex and $ at the end. use bash regular expressions to capture the variable name (due to greedy matching, this matches variable names from right to left use variable indirection to get the variable value remove the matched varname from the input string to avoid infinite loop (this destroys the original string, back it … For an introduction to Bash regular expressions, see our Bash regular expressions for beginners with examples article instead. I'm trying to match multiple alphanumeric values (this number could vary) from a string and save them to a bash capture group array. Using the power of regular expressions, one can parse and transform textual based documents and strings. I read line by line through the data, and for that, i have some data i have to extract from that line. Linux bash provides a lot of commands and features for Regular Expressions or regex.