Question

export SRVHOME=`readlink -f \`dirname $0\`/../`

I know it exports a directory variable . but what means by the symbol “\`” or “`” surround the command?

Answer 1

Whatever you put between two ` characters is a command run on a fork of your shell.
In this case, the command itself contains another pair of ` characters – hence, they must be escaped, which is done here by prepending a \ character.

An other example:

$ set -x
$ date -d @`echo \`date +%s\``
+++ date +%s
++ echo 1357377825
+ date -d @1357377825
Sat Jan  5 11:23:45 EET 2013

Note: set -x displays all steps of a command. Helpful for debugging.

In bash you can run several encapsulated commands with $(command) instead:

$ date -d @$(echo $(date +%s))
+++ date +%s
++ echo 1357378055
+ date -d @1357378055
Sat Jan  5 11:27:35 EET 2013