You can see system variables by giving command like $ set
http://freeos.com/guides/l
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Question
what does $$ mean in a shell script
Hi,
In one of scripts I am looking at there is an output being directed to a file
the notation is like this
grep -v xyz > file1.$$
and then later on they are deleting file1.$$ (that's something else)
I was wondering what does $$ mean, any idea.
Thanks in advance.
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Answers
Or rather, parameter expansion is indicated by $
The `$' character introduces parameter expansion, command substitution,
or arithmetic expansion. The parameter name or symbol to be expanded
may be enclosed in braces, which are optional but serve to protect the
variable to be expanded from characters immediately following it which
could be interpreted as part of the name.
${parameter}
The value of parameter is substituted. The braces are required
when parameter is a positional parameter with more than one
digit, or when parameter is followed by a character which is not
to be interpreted as part of its name.
in this case, the name of the parameter is also $
$$ :Process ID of the script itself. The $$ variable often finds use in scripts to construct "unique" temp file names
http://tldp.org/LDP/abs/ht
When you need to create a temp file to capture some output, then you need to have a unique name somehow. When you run a script then a process is created and that process has a unique id for its life time. So, you can always attach the process id to a file name to make it unique at that time. Each process has a set of env variables and its process id is referred to by $. env variables values are accessed via evaluation operator $. so to get the process id you use $$.
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by: ozoPosted on 2009-03-18 at 11:27:42ID: 23921935
man sh
$ Expands to the process ID of the shell. In a () subshell, it
expands to the process ID of the current shell, not the sub-
shell.