Bash update variable in loop. The other answers provide solutions.
Bash update variable in loop 7 "Here Strings" in the Bash Reference Manual. " May 26, 2015 · This answer assumes that you want your values to overlap -- meaning that a value given as next then becomes curr on the following iteration. 24} does not work in bash v2 (which our servers have here) though it works as a range shortcut on modern bash so that should not cause youproblems. This tip will show you how to correctly get the input from the command and update bash variables correctly. Viewed 148k times 105 . . Mar 18, 2024 · An alternative approach for utilizing multiple variables involves employing IFS with a for loop. Mar 18, 2017 · The for in loop in shell is not numeric, instead it works on a list of strings that is generated from the next token/construct after in and stored in memory. Jan 26, 2012 · If you set a variable in a child process, then the value of the variable in the parent is not affected. Can someone clarify how this works and what steps I should take. Often I have scripts similar to the following, where I cat a file and read it in using a while loop to process variables in the file: #!/bin/bash count=0; cat testfile | while read line do count=$(($count+1)); echo $count; done echo "Total $count"; Oct 5, 2017 · I have a global variable $EMAIL. The other answers provide solutions. 6. These are actually two different variables which just happen to have the same name. echo "Variable \$foo updated to $foo inside if inside while loop" fi. stackexchange. Updating (sequentially adding a value into) a variable Oct 18, 2017 · to update a variable it must be re assing: $(. You will have to update your question with some actual code Apr 11, 2019 · $ bash script. I think the issue I'm ha Jun 25, 2013 · See §3. To separate the variable name from following _, use ${varname} syntax as shown above {0. I have read that the variables inside the loops run on a sub shell. com - In bash, is it possible to use an integer variable in the loop control of a for loop? – blong Commented Jul 24, 2015 at 17:31 Thanks Paul, but I don't understand: the end of the oneliner is awk that searches a specific line in a file and ouputs the last word, false or true, then it always exits with 0. To elaborate, this special shell variable determines how Bash recognizes word boundaries Oct 5, 2017 · Why is a variable global when set with `read variable` inside a `while` loop, but local when set with `while read variable`? 5 Is it while loop or the pipe causing global variable behaving unexpectedly Jan 20, 2021 · Update a bash variable in the while loop test from within the loop? 0. trap 'source /path/to/sessionlength' DEBUG The sourced sessionlength script need only be the content of your loop, without the while and sleep parts around it: The COUNTER is being updated but the update happens in a sub-process. Here is an alternative solution using a while loop: i=$(($i + 1)) echo $i. I want to loop through several files and for each that meets a criteria I will add it to $EMAIL for report generation. Jun 14, 2017 · And then the loop would keep running until the exit code was true. Thus, we use multiple variables within the loop body. Updating (sequentially adding a value into) a variable As you're using bash, you could trap the DEBUG signal, in order to recalculate SESSIONLENGTH before each command:. This can be prevented by using process substitution. The problem is when I redefine $EMAIL inside May 7, 2022 · Having trouble updating my global variable in this shell script. You will have to update your question with some actual code Thanks Paul, but I don't understand: the end of the oneliner is awk that searches a specific line in a file and ouputs the last word, false or true, then it always exits with 0. A B C Q a option A works A B C Q B option B works A B C Q c option c works A B C Q q You entered an invalid option. In practice, the IFS variable is used to tokenize a string into fields based on the specified delimiter. When the subshell exits, all variables return to their previous values (which may be null or unset). (I've also taken the liberty of adding some double-quotes, and adding -r and IFS= to read, to avoid too much mucking around with the contents of your variables. sh #!bin/bash loopControl="z" echo "Choose one of the following options. echo "Value of \$foo in while loop body: $foo" Apr 18, 2015 · Is there a way to assign variables and values and refer to the new variables in a bash for loop? Mar 8, 2023 · If you have the need to update a bash variable inside a loop that reads input from a command, the expected use of a pipe to achieve that fails. FYI, the above small snippet produces an output as Dec 31, 2010 · This is because _ is a valid character in a variable name. ) Dec 19, 2013 · The title is: "Bash incrementing variable in loop" My line does: "incrementing a variable in bash" So I think it does answer a part of the question (at least as much as the example from geekzspot) –. ss=$(date) ss=$(date) SECONDS is a special variable which contains the number of seconds since bash process is running, to have a value changed every 5 seconds, following can be used. There is an error that I often run in to when working with files and while loops in BASH. This is because you're using the loop with a pipe, which automatically causes it to run in a subshell. The second thing to understand is when bash runs a command as a child process. The more conventional way to get a menu in bash is to use select: $ cat script2. Modified 3 years ago. I am trying to assign a variable within a while loop and the script hangs during the read statement. Doesn't seem there is any variables scope limitation in Bash, as long as you don't assign the output "return" of the function to a variable, If you assign the output of the function to a variable the function will be executed in a separate thread which results in isolating the function from the main code variables. sh Choose one of the following options. Here's a rough outline of my code right now, in Bash: Oct 5, 2013 · Update a bash variable in the while loop test from within the loop? 0. echo $((SECONDS/5)) Jun 19, 2013 · How to use variables in a bash for loop [duplicate] Ask Question Asked 11 years, 7 months ago. This question Apr 18, 2015 · NOTE: Be aware that your setup will change/modify/assign variables only in the current process and NOT the parent process if you execute this as a command. To elaborate, this special shell variable determines how Bash recognizes word boundaries @Ida: What would be the advantage of doing that? Doing that would fail: the subshell would execute and return each result on its own line, which would then be smashed into a single string and fed to read at once, which would read until the first embedded newline, loop once and then while, having no more input to process, would terminate. Updating variable in bash script. ) is a command substitution it is expanded to output of specified command. Oct 4, 2008 · Related discusions: bash for loop: a range of numbers and unix. Assuming you encapsulate your code in a function that takes two arguments (current and next) when a next item exists, or one argument when on the last item: Jan 21, 2015 · I'm trying to write a bash script that takes a few variables and then does a find/replace with a given file search using grep to get the list of files that have the string. When the loop is finished (in the subprocess) the original COUNTER variable has not been changed because it was only changed in the sub-process. May 31, 2013 · Instead you can use a here string to re-write the while loop to be in the main shell process; only echo -e $lines will run in a subshell: if [[ "$line" == "second line" ]] then. foo=2. (I plan to implement a loop counter as well for too many failed attempts, but that's a different problem to solve) I think I need a here-string, but I'm not exactly sure how that would look. The indicated [control] variable takes the values from the list, one after another in each iteration; there is no way to manipulate that list. There are two cases relevant to the question: So ideally the expected value for the lineCount would be 2 but since the variable is updated in a sub-shell created by the pipe-line, the update does not reflect once the sub-shell exits. Nov 6, 2017 · In your example the while-loop is executed in a subshell, so changes to the variable inside the while-loop won't affect the external variable. 2. Alternative: Using select. – Jan 8, 2017 · When you pipe into a while loop in Bash, it creates a subshell. klxyipydckoiluatnsnkmcqiovqkvpsauoikgobihcxyhickvof