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Shell Programming and Scripting

^$$ meaning

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# 1  
Old 12-21-2010
^$$ meaning
Hi ,

Can anyone please let me know whta the follwoing piece of code for

Code:
ScriptName=${0##*/}
if pgrep -f "$ScriptName" | grep -v "^$$\$" ; then
  echo `date`": Sctipt $ScritName is already runnig"
  exit
fi

Thnx a lot in advance

Moderator's Comments:
Mod Comment
Please use code tags when posting data and code samples!
Last edited by vgersh99; 12-21-2010 at 09:01 AM.. Reason: code tags, please!
# 2  
Old 12-21-2010
'man ksh' yeilds:
Code:
     $$     The process number of this shell.

# 3  
Old 12-21-2010
Plus grep regex metacharacter ^ is beginning of line; in "..." the shell expands $$ first, else '^$$' is a line with just a '$', as in regex the second '$' makes the first literal, and the second '$' still means end of line.
Last edited by DGPickett; 12-21-2010 at 09:56 AM..
# 4  
Old 12-21-2010
Plus grep regex metacharacter \$ is end of line, so this is trying to match a line that contiains the shell PID only and displays everything else (-v option)
# 5  
Old 12-21-2010
And the \ on the final $ is paranoia, as " . . . $" cannot be shell expanded, as there is no variable after $. I like to use the " less and the ' more, so the shell works less and there are fewer surprises: '^'"$$"'$'
# 6  
Old 12-21-2010
Real paranoia indeed, the shell won't bite Smilie
Code:
grep -v ^$$$

Last edited by Scrutinizer; 12-21-2010 at 12:02 PM..
# 7  
Old 12-21-2010
Not for this, but familiarity breeds contempt and surprises. With '...' there are few surprises! We should teach best practices as we help with problems.
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