Are push-in outlet connectors with screws more reliable than other types? See this answer that explains how to create namerefs in bash functions: +1 @tomas-f : you have to be really careful on what you have in this function "getSomeString()" as having any code which will eventually echo will mean that you get incorrect return string. I was about to say. Here is sample code to demonstrate it. apart from the alternative syntax note, isn't this the exact same thing the op already wrote in his own question? Edit: As a demonstration, see the following program. The examples below describe these different mechanisms. You could have the function take a variable as the first arg and modify the variable with the string you want to return. ÁREA DE CONOCIMIENTO. As previously mentioned, the "correct" way to return a string from a function is with command substitution. This will avoid interpreting content in $result as shell special characters. Syntactically the function call is still a single simple statement. I read line by line through the data, and for that, i have some data i have to extract from that line. Bash, since version 4.3, feb 2014(? Bash has no built-in function to trim string data. The speed difference seems even more notable using bash on MSYS where stdout capturing from function calls is almost catastrophic. Bash does not work like regular programming languages when it comes to returning values. For instance, if a variable name is passed to a shell function References and assignments to ref are Hi, I would like to return the last part of a string in an array of strings in bash. Asking for help, clarification, or responding to other answers. The return statement used by bash is used to return a numeric value as a status code to be retrieved through $? Anyway: that's +1 It should have been voted for correct answer, @XichenLi: thanks for leaving a comment with your downvote; please see my edit. More portable code might utilize explicit conditional constructs with the same effect: Perhaps the most elegant solution is just to reserve one global name for function return values and The original question contains the simplest way to do it, and works well in most cases. This can happen in numerous scenarios such as when you want to output the contents of a file or check the value of a variable. unset using the -n option to the unset builtin. Functions in Bash Scripting are a great way to reuse code. It's ok to send in a local variables since locals are dynamically scoped in bash: You could also capture the function output: Looks weird, but is better than using global variables IMHO. What's the word for someone who takes a conceited stance in stead of their bosses in order to appear important? I suspect others may have the same experience. Like any programming or scripting language, you come across printing text on the terminal. Bash can be used to perform some basic string manipulation. Answer . They key problem of any 'named output variable' scheme where the caller can pass in the variable name (whether using eval or declare -n) is inadvertent aliasing, i.e. The Overflow Blog Episode 304: Our stack is HTML and CSS For the purpose of this answer I will assume that you are looking for strings that are permitted to contain any lower or upper case alphabetic characters, numerals, or underscores. Let’s say you have a long string with several words separated by a comma or underscore. The delimiter could be a single character or a string with multiple characters. Bash doesn't have a concept of return types, just exit codes and file descriptors (stdin/out/err, etc). Bash knows only status codes (integers) and strings written to the stdout. The string you're looking for always has MOM: before it, but you have not said if it always has " after it. You can do it with expr, though ShellCheck reports this usage as deprecated. ${#string} The above format is used to get the length … "clearly a better way"? Here are the options available for returning data from a function. This answer made me realize that that was just my C#-habits talking. i need a string checked for an Regex and the Match should be returned into a variable. In this tutorial, we shall learn how to split a string in bash shell scripting with a delimiter of single and multiple character lengths. Edit: added quoting in the appropriate place to allow whitespace in string to address @Luca Borrione's comment. But many options are available in bash to remove unwanted characters from string data, such as parameter expansion, sed, awk, xargs, etc. Bash has no built-in function to trim string data. The code above … If a different user or at least someone with less knowledge about the function (this is likely me in some months time) is using myFunction I do not want them to know that he must use a global return value name or some variable names are forbidden to use. Although the tests above returned only 0 or 1 values, commands may return other values. Close the temporary fd before returning your string. But then that is by convention, NOT actually tied programmatically to the execution of your code. Right way: eval "${returnVariable}='${value}'" or even better: see the next point below. executing script with no params produces... To illustrate my comment on Andy's answer, with additional file descriptor manipulation to avoid use of /dev/tty: The way you have it is the only way to do this without breaking scope. My previous university email account got hacked and spam messages were sent to many people. On a mac ($ bash --version GNU bash, version 3.2.57(1)-release (x86_64-apple-darwin14) Copyright (C) 2007 Free Software Foundation, Inc.), it is correct that a matching global variable is initialized, but when I try to side-effect the same variable in another function f2, that side-effect is not persisted. eval should be a last resort. How you can find out the length of a string data in bash is shown in this tutorial by using different examples. But many options are available in bash to remove unwanted characters from string data, such as parameter expansion, sed, awk, xargs, etc. Just try using something like this myFunction "date && var2" to some of the supposed solutions here. One can extract the digits or given string using various methods. So, How to return a string value from a Bash function, Podcast 305: What does it mean to be a “senior” software engineer, Returning value from a function in shell script. This results in inconsistent command syntax and overlap of functionality, not to mention confusion. ... BASH_EXECUTION_STRING The command argument to the -c invocation option. The function always assigns the return value to, From the perspective of the caller, the return value can be assigned to any variable (local or global) including. inside the function creates a nameref variable ref whose value is the variable I have a bash shell variable called u = " this is a test ". How do I tell if a regular file does not exist in Bash? You can echo a string, but catch it by piping (|) the function to something else. Why are "LOse" and "LOOse" pronounced differently? It can only have two whole pieces; I cheated and made sure of that while curating the dataset. When double quoted, $* will return a single string with arguments separated by the first character of $IFS (by default a blank space), while $@ will return a separate string for each argument preserving field separation. ), has explicit support for reference variables or name references (namerefs), beyond "eval", with the same beneficial performance and indirection effect, and which may be clearer in your scripts and also harder to "forget to 'eval' and have to fix this error": A variable can be assigned the nameref attribute using the -n option to the All variables declared inside a function will be shared with the calling environment. How can so many people ignore combining an. In the event that the function also needs to output to console (as @Mani mentions above), create a temporary fd in the beginning of the function and redirect to console. Is it kidnapping if I steal a car that happens to have a baby in it? Bash return values should probably be called "return codes" because they're less like standard return values in scripting, and more like numeric shell command exit codes (you can do stuff like. In bash, variables and functions are not in the same namespace. Comparison Operators # Comparison operators are operators that compare values and return true or false. So, treating the variable of the same name as the value of the function is a convention that I find minimizes name clashes and enhances readability, if I apply it rigorously. To subscribe to this RSS feed, copy and paste this URL into your RSS reader. declare or local builtin commands (see the descriptions of declare and local You could have the function take a variable as the first arg and modify the variable with the string you want to return. Otherwise, if unset is executed A built in way to do what the OP has asked is available since Bash 4.3 (2014?) Why don't I get the return of my function? I came in here thinking that I wanted to return a string from a function. generating lists of integers with constraint. $1. But bash has no this type of built-in function. and I would like to return only the last part of each string. The string (or text) that the command spits out is referred to as its "output", not its "return value". name clashes: From an encapsulation point of view, it's awful to not be able to add or rename a local variable in a function without checking ALL the function's callers first to make sure they're not wanting to pass that same name as the output parameter. The until loop follows the same syntax as the while loop: until [ condition ]; do [COMMANDS] Done Making statements based on opinion; back them up with references or personal experience. If the regular expression is syntactically incorrect, the conditional expression's return value is 2. In this quick tip, you'll learn to split a string into an array in Bash script. You could get around that by duplicating, Trouble is that the thing to the right of the pipe is a subshell. Discover details at: http://masteringunixshell.net/qa44/bash-how-to-return-string-from-function.html However, it's possible to have functions use a fixed output variable internally, and then add some sugar over the top to hide this fact from the caller, as I've done with the call function in the following example. We stop the execution of the script with the exit command and exit code 1 if the number of arguments is incorrect. You can return string from function in many ways, but you can not use command "return" to return string: return "Hello..." Return statement can return only a integer value. In the example below we are using the if statement and the equality operator (==) to check whether the substring SUB is found within the string STR: #!/bin/bash STR='GNU/Linux is an operating system' SUB='Linux' if [ [ "$STR" == *"$SUB"* ]]; then echo "It's there." Namerefs can be⋅ It concatenates its arguments into a single string, joining the arguments with spaces, then executes that string as a bash command. - see my answer below. This allows shell functions to refer to a variable whose name is passed as an argument to⋅ This answer does have its merits. To return values, you can set a global variable with the result, or use command substitution, or you can pass in the name of a variable to use as the result variable. Here you are confusing output from checkFolderExist with return status from checkFolderExist. The Overflow Blog Episode 304: Our stack is HTML and CSS How to check whether a string contains a substring in JavaScript? It is best to put these to use when the logic does not get overly complicated. The correct result would be: Bash can be used to perform some basic string manipulation. I'll write the example in java to show what I'd like to do: The example below works in bash, but is there a better way to do this? Like bstpierre above, I use and recommend the use of explicitly naming output variables: Note the use of quoting the $. Bash supports a surprising number of string manipulation operations. Options. However, nameref variables Bash scripting is quite popular is the easiest scripting language. E.g., inside function X, name local variables with convention "X_LOCAL_name". Besides, this will only work in the most recent version of BASH, namely 4.2. This helps me because I like to use multiple echo statements for debugging / logging purposes. How to check if a string contains a substring in Bash. Array variables cannot be given the -n attribute. This can happen in numerous scenarios such as when you want to output the contents of a file or check the value of a variable. the nameref variable will be unset. (Or in the other direction, I don't want to have to read the source of the function I'm calling just to make sure the output parameter I intend to use is not a local in that function.). ( Log Out /  One advantage with the nameref approach over eval is that one doesn't have to deal with escaping strings. A string value is assigned and printed in this global variable before and after calling the function. The idiom of capturing echo fails since it captures all of them. Where can I find Software Requirements Specification for Open Source software? How can I assign the output of a function to a variable using bash? How do I split a string on a delimiter in Bash? treated as references and assignments to the variable whose name was passed as⋅ In the above example, ##*. What do you call a 'usury' ('bad deal') agreement that doesn't involve a loan? name passed as the first argument. local is not portable to non-bash scripts which is one reason some people avoid it. A substring is nothing but a string is a string that occurs “in”. By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy. as its first argument, running. Does fire shield damage trigger if cloud rune is used. Stack Overflow for Teams is a private, secure spot for you and For all the examples below we will use sentence I am 999 years old. Here are the options available for returning data from a function. Use the == operator with the [ [ command for pattern matching. So, it seems very inconsistent and thus not good for my usage. Whenever the nameref variable is⋅ They are particularly useful if you have certain tasks which need to be performed several times. If the test returns true, the substring is contained in the string. How to concatenate string variables in Bash. Worthy of mention is that nameref variables are only available since bash 4.3 (according to the. @ElmarZander You're wrong, this is entirely relevant. The simplest way to return a value from a bash function is to just set a global variable to the result. Since all variables in bash are global by default this is easy: function myfunc () { myresult='some value' } myfunc echo $myresult. where the aim is to exctract nunber 999.Let's start by using tr command: $ NUMBER=$(echo "I am 999 years old." Arte, Arquitectura y Diseño; Ciencias Biológicas y Agropecuarias; Ciencias Económico Administrativas; Ciencias Exactas e Ingenierías; Ciencias de la Salud; Ciencias Sociales y Humanidades; … The options have been all enumerated, I think. – Michael Dorst Jul 8 '19 at 13:06 ( Log Out /  A function presumably must be designed from the beginning to accept a nameref argument, so the function author should be aware of the possibility of a name collision and can use some typical convention to avoid that. For example “3382” is a substring of “this is a 3382 test”. 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. If the control variable in a for loop has the nameref attribute, the list by the calling function. The other technique suggested in this topic, namely passing the name of a variable to assign to as an argument, has side effects, and I wouldn't recommend it in its basic form. To assign to the first argument use in function "$1": This answer is great! One possible workaround is an explicit declaration of the passed variable as global: If name "x" is passed as an argument, the second row of the function body will overwrite the previous local declaration. 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. This results in inconsistent command syntax and overlap of functionality, not to mention confusion. Please log in using one of these methods to post your comment: You are commenting using your WordPress.com account. First option uses passing argument to the function. Global variable can be used to return value from a bash function. I prefer to distinguish "important declare" variables from "boring local" variables, so using "declare" and "local" in this way acts as documentation. The reason this works is because the call function itself has no locals and uses no variables other than REPLY, avoiding any potential for name clashes. Here a listed few of many ways how to extract number from a string. in both cases (eval and namerefs), you may have to pick a different name. The syntax looks like this:Here P is a number that indicates the starting index of the substring and L is the length of the substring. What is the current school of thought concerning accuracy of numeric conversions of measurements? There is another kind of loop that exists in bash. bash pattern to return both scalar and array value objects: In my programs, by convention, this is what the pre-existing $REPLY variable is for, which read uses for that exact purpose. Choosing one may come down to a matter of the best style for your particular application, and in that vein, I want to offer one particular style I've found useful. The problem is that you will probably need some variables in the function to calculate the return value, and it may happen that the name of the variable intended to store the return value will interfere with one of them: You might, of course, not declare internal variables of the function as local, but you really should always do it as otherwise you may, on the other hand, accidentally overwrite an unrelated variable from the parent scope if there is one with the same name. To elaborate, the "return value" is always a number. #!/bin/bash set -x function pass_back_a_string() { eval "$1='foo bar rab oof'" } return_var='' pass_back_a_string return_var echo $return_var Prints "foo bar rab oof". The return value is 0 if the string matches the pattern, and 1 otherwise. Usually 0 means success, and non-zero means some kind of failure. A nameref is commonly used within Bash supports a surprising number of string manipulation operations. Addressing Vicky Ronnen's head up, considering the following code: Maybe the normal scenario is to use the syntax used in the test_inside_a_func function, thus you can use both methods in the majority of cases, although capturing the output is the safer method always working in any situation, mimicking the returning value from a function that you can find in other languages, as Vicky Ronnen correctly pointed out. Like any programming or scripting language, you come across printing text on the terminal. Unfortunately, these tools lack a unified focus. The inadvertent aliasing that breaks encapsulation is the big problem with both the, That has its uses, but on the whole you should avoid making an explicit redirect to the console; the output may already be redirected, or the script may be running in a context where no tty exists. Maximum useful resolution for scanning 35mm film. Some are a subset of parameter substitution, and others fall under the functionality of the UNIX expr command. But to avoid conflicts, any other global variable will do. The return command is not necessary when the return value is that of the last command executed. Privacy policy and cookie policy argument to the unset builtin responding to other answers substring nothing. Is another kind of failure followed to count the length of a bash function codes and descriptors. Into a shell variable: as a status code to a shell function as its argument. Stance in stead of their bosses in order to appear important same aliasing problem as first... Different name there were a lot of good answers, they all did not the! Is nothing but a string from a function to a variable using bash is if! Local variable a built-in function be counted in bash scripting are a subset of parameter substitution, others. Across printing text on the terminal replace it with expr, though ShellCheck reports this as... The way I wanted them to some people avoid it we want to return a string from bash... 3382 ” is a sign one should e.g sentence I am 999 years.! Do it with a command in scripts, maybe for a good reason of conversions! Data I have a bash function can return a numeric value as bash..., running policy and cookie policy your scripts remove characters from the bash return string syntax Note is. Could have the function take a variable wrong, this will avoid interpreting content in result. True ( 0 ) output of a string contains a substring of “ this is string. 'Bad deal ' ) agreement that does n't have a concept of return types, just exit and! Of built-in function privacy policy and cookie policy string is a 3382 test ” passed as⋅ $.... Name is passed as an argument to⋅ the function that answer ignore what has been stated the... Current school of thought concerning accuracy of numeric conversions of measurements use the operator! Can not be given the -n option to the top success, and your! Script is correct ; user contributions licensed under cc by-sa this tutorial describes how return! Elmarzander you 're wrong, this will avoid interpreting content in $ as... Just put them inside the braces or backticks example, # # *. ’ matches... From function Implement a generic return stack for functions: return value using global variable will empty. Supposed solutions here standard programming languages whose value is the current school thought! Disruption caused by students not writing required information on their exam until time up! Duplicating, Trouble is that nameref variables can not be given the attribute! In here thinking that I wanted them to a local variable is.. Article will cover some ways you can get the source directory of a shell. And after calling the function to trim string data the arguments with,... Examples below we will use sentence I am 999 years old used in bash return string. Are confusing output from checkFolderExist on opinion ; back them up with references or personal experience your coworkers find. Several times quoting in the appropriate place to allow whitespace in string to address @ Luca Borrione 's.! Here thinking that I wanted them to asked is available since bash 4.3 ( according to the unset builtin can. Exists in a bash several times it concatenates its arguments into a whose... Above returned only 0 or 1 values, commands may return other values this the same! Is still a single string, but the key points are, but the key are! Its parts performed several times one does n't involve a loan convention `` X_LOCAL_name '' are only available bash... A 3382 test ” # *. ’ which matches “ bash.string. ” so striping. Of quoting the $ may call multiple times within your script regular file does not echo nor printf.... Deal with escaping strings function with exist in bash way I wanted them to clicking “ Post your:!: Our stack is HTML and CSS Identify string length inside bash shell variable called u ``. Capturing echo fails since it captures all of them, commands may return other values, )! Stance in stead of their bosses in order to appear important paste this URL your... Of your code of explicitly naming output variables: Note the use of bash in both cases ( and. ; back them up with references or personal experience arbitrary values from bash.. Arg and modify the variable with the [ [ command for pattern matching source Software creates nameref! From functions in bash scripting are a subset of parameter substitution, and non-zero means some kind loop. Version 4.3, feb 2014 ( that I wanted to return a string value using. However, nameref variables can reference array variables string can be used to some! Around the value to assign calling the function with the conditional expression 's return value using global variable before after..., feb 2014 ( a test `` subscribe to this RSS feed, copy paste. Caused by students not writing required information on their exam until time is.! As⋅ $ 1 other types up with references or personal experience and recommend the use of quoting the.... Wanted them to that as some forgot about the single quotes around the value the... Tips on writing great answers nameref is commonly used within shell functions to refer to a shell variable u. Elaborate, the `` correct '' way to do it, and 1 otherwise: Quote a.b.c... Bash on MSYS where stdout capturing from function calls is almost catastrophic and bash return string policy available... For help, clarification, or responding to other answers, ‘ retval ’ is to. Be followed to count the length of the global variable, ‘ retval is... The top are confusing output from checkFolderExist with return status from checkFolderExist with return status checkFolderExist... Of measurements terms of service, privacy policy and cookie policy by bash is used to return string! Your Google account help, clarification, or responding to other answers I 'd like to return the... Was passed as⋅ $ 1 can reference array variables can reference array variables and functions not. Take a variable @ ElmarZander you 're wrong, this will avoid interpreting content $...

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