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What happens to the read variable in Bash?
When the read command sets the variable to the input, that variable is set only in the subshell, not the parent shell. 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.
When to use a variable reference inside another variable?
In older shells, including ksh88 and pdksh, your only recourse when you have a variable containing another variable name and want to use the value of this variable eval, as explained by Bruce Ediger. This solution works in any Bourne/POSIX shell. This is the best method here: it’s simpler and more portable.
How to chop up a variable in Bash?
If you need to do this, you can either use the command substitution above to read the output into a variable and chop up the variable using the bash pattern removal expansion operators or use some variant of the following approach. Beware, however, that this will change the shell’s positional parameters.
When to use dollar sign or variable name in Bash?
When a dollar-sign doesn’tprecede a variable name, or a variable reference is within single-quotes, Bash will interpret the string literally:
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.
Why is my CNT variable never changed in Bash?
You need to use different ways like command substitutions instead of a pipe if you are using bash. This is kind of a ‘common’ mistake. Pipes create SubShells, so the while read is running on a different shell than your script, that makes your CNT variable never changes (only the one inside the pipe subshell).
What happens when the subshell exits in Bash?
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 “correct” reply is given by Dennis. However, I find the process substitution trick extremely unreadable if the loop contains more than a few lines.
How does the time condition loop in Bash work?
In bash and zsh you get integral seconds advancing by the pulse of the system (real-time) clock – i.e., counting behind the scenes does not truly start at 0 (!), but at whatever fraction since the last full time-of-day second the script happened to be started at or the SECONDS variable was reset.
What happens when variable loses value at end of while Read loop?
Iain and Ignacio’s answers have others.) SubShell ends, variables and values are destroyed (they don’t get back to the calling process/script). You echo your original CNT value of 0. Try passing the data in a sub-shell instead, like it’s a file before the while loop.
Which is an example of a while loop in Bash?
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. A subprocess cannot affect its parent’s environment.