Question

I’ve seen this line in many shell scripts but I don’t understand the effect it has. Could someone explain please?

tempfile=`tempfile 2>/dev/null` || tempfile=/tmp/test$$

Answer 1

It creates a temporary file and puts the path to it in the $tempfile variable.

`tempfile 2>/dev/null`

runs the tempfile command (man tempfile) and discards any error messages. If it succeeds, it returns the name of the newly created temporary file. If it fails, it returns non-zero, in which case the next part of the command runs.

For a command this || that, that only runs if this fails, i.e. returns non-zero.

$$ is a variable in bash that expands to the process ID of the shell. (Compare the results of ps and echo $$.) So tempfile=/tmp/test$$ will expand to something like tempfile=/tmp/test2278.

Presumably, later in the script, something writes to $tempfile.