The backquotes mean command substitution: the result of expanding
`pidof transmission-gtk` is the output from running the command
pidof transmission-gtk. An alternate way of writing
`pidof transmission-gtk` is
$(pidof transmission-gtk), and that alternate way is recommended (it has no downsides and it avoids issues when the command inside the backquotes itself contains quotes).
The double quotes around that prevent the result of the command substitution from being split into separate words and interpreted as wildcard patterns. If you're just starting with shell scripting, don't worry about this, just remember to always put double quotes around command substitutions. The same goes for variable substitutions, e.g. always write
"$foo", never plain
This snippet as written doesn't make any sense: the test contains a single word, which will be something like
=0 (if there is no running
transmission-gtk process) or
1234=0 (if there is a matching process) or
123 456=0 (a single word in shell terminology, even if it happens to contain a space). A single word in a test is always considered true. To test whether a string is equal to another string, write
[ "string1" = "string2" ] (there must be a space on each side of the brackets, and on each side of the operator). To test for numerical equality, use the
-eq operator, e.g.
[ 1 -eq 01 ] is true.
Here, there are two equivalent ways of testing whether there is a running
Test whether the output is empty:
if [ -z "$(pidof transmission-gtk)" ]; then …
Test whether the return status of the
pidof is nonzero, indicating failure. Since the output of the command is not used, redirect it to
/dev/null (the great void).
if ! pidof transmission-gtk >/dev/null; then