Bash – Having multi lines in bash command substitution

The most obvious error I can see is that you're not protecting the i loop variable. So when you write $i_trip_point, the shell is looking for a variable named i_trip_point, which is not set so you get the second type of error, like this for example:

cat: /sys/class/thermal/thermal_zone2/trip_point__temp: No such file or directory

Two underscores between point and temp which marks an empty variable being interpolated. You should use the syntax ${i}_trip_point. The braces protect your variable from adjacent characters being interpreted as part of the variable name.

The < isn't directly an aspect of bash command substitution. It is a redirection operator (like a pipe), which some shells allow without a command (POSIX does not specify this behavior).

Perhaps it would be more clear with more spaces:

echo $( < $FILE )

this is effectively* the same as the more POSIX-safe

echo $( cat $FILE )

... which is also effectively*

echo $( cat < $FILE )

Let's start with that last version. This runs cat with no arguments, which means it will read from standard input. $FILE is redirected into standard input due to the <, so cat puts its contents are put into standard output. The $(command) subsitution then pushes cat's output into arguments for echo.

In bash (but not in the POSIX standard), you can use < without a command. bash (and zsh and ksh but not dash) will interpret that as if cat <, though without invoking a new subprocess. As this is native to the shell, it is faster than literally running the external command cat. *This is why I say "effectively the same as."