How is a variable modified inside a while loop not remembered?

Each element of a pipeline, even a builtin or shell function, runs in a separate process, a child of the shell running the pipeline. A subprocess cannot affect its parent’s environment. When the read command sets the variable to the input, that variable is set only in the subshell, not the parent shell.

What happens when you pipe into a while loop in Bash?

When you pipe into a while loop in Bash, it creates a subshell. When the subshell exits, all variables return to their previous values (which may be null or unset). This can be prevented by using process substitution. The solution Dennis provided works, but be aware, that it violates POSIX norm.

Where does a variable go in a loop?

Anything not in a small scope ends up in global. The local scope is inside functions (this includes classes). If you’re doing something in a module (and therefor global) scope, you need to define the variable outside the loop, otherwise it will initialize the variable on every loop.

Is the while loop executed in a subshell?

Here’s a complete test program: The while loop is executed in a subshell. So any changes you do to the variable will not be available once the subshell exits. 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:

How to reduce code in a while loop?

Or use the heredoc method to reduce the amount of code in a subshell. Note the iterative i value can be read outside the while loop.

Which is an example of a while loop?

For example, piping a command’s output into a while loop that repeatedly calls read will result in the same behavior. Each element of a pipeline, even a builtin or shell function, runs in a separate process, a child of the shell running the pipeline.

Why does a while loop run in a subshell?

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. This is because you’re using the loop with a pipe, which automatically causes it to run in a subshell. Here is an alternative solution using a while loop:

How to ” read ” a variable in Bash while loop?

See §3.6.7 “Here Strings” in the Bash Reference Manual. (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.) while read line ; do echo $line done < < ( code

Can you run a while loop in Linux?

) if you don’t mind the while loop executing in a subshell (any variables you modify won’t be visible in the parent after the done ). Script file should be in Linux mode.

What are the troubleshoot variables in parfor-loops?

Troubleshoot Variables in parfor-Loops Ensure That parfor-Loop Variables Are Consecutive Increasing Integers. Loop variables in a parfor-loop must be consecutive increasing integers. For this reason, the following examples return errors:

How to fix a noninteger variable in MATLAB?

You can fix these errors by converting the loop variables into a valid range. For example, you can fix the noninteger example as follows: If MATLAB ® detects that the parfor -loop variable can overflow, it reports an error. The length of the parfor -loop range exceeds the maximum value of the loop variable type.

What happens to a variable when a subshell exits?

When the subshell exits, the value of the variable is lost. Many pipelines that end with read variable can be converted into command substitutions, which will capture the output of a specified command. The output can then be assigned to a variable:

How to alter the main query in WordPress?

This is the easiest and recommended way to alter the main query. This ‘pre_get_posts’ hook is called after the query variable object is created, but before the actual query is run. query_posts () is meant for altering the main loop.

Can a loop contain more than one statement?

The body of a loop can contain more than one statement. If it contains only one statement, then the curly braces are not compulsory. It is a good practice though to use the curly braces even we have a single statement in the body.

How is a while statement evaluated in a loop?

while (condition) { statements; } It is an entry-controlled loop. In while loop, a condition is evaluated before processing a body of the loop. If a condition is true then and only then the body of a loop is executed.

How is a variable modified inside a while loop not remembered?