What do you mean by substitution in Bash?
Think of command substitution as running a command as usual and dumping the output at the point where you are running the command. The output of commands can be used as arguments to another command, to set a variable, and even for generating the argument list in a for 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.
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.
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 $lines will run in a subshell:
What does the loss of newlines in command substitution mean?
The loss of newlines in command substitution Earlier, I quoted from the Bash documentation on command expansion. Here’s an emphasized version of the excerpt: Command substitution allows the output of a command to replace the command itself.
How to change the name of a variable in Bash?
Whenever Bash encounters a dollar-sign, immediately followed by a word, within a command or in a double-quoted string, it will attempt to replace that token with the value of the named variable. This is sometimes referred to as expanding the variable, or parameter substitution: user@host:~$ var_a=”Hello World”user@host:~$ another_var=42
How does the expansion in Bash take place?
Bash performs the expansion by executing command and replacing the command substitution with the standard output of the command, with any trailing newlines deleted. Embedded newlines are not deleted, but they may be removed during word splitting.