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Tuesday, January 03, 2012

Special shell variables : Unix/Linux

Special shell variables  There are some variables which are set internally by the shell and which are available to the user:

$1 - $9
these variables are the positional parameters.
the name of the command currently being executed.
the number of positional arguments given to this invocation of the shell.
the exit status of the last command executed is given as a decimal string.  When a command impletes successfully, it returns the exit status of 0 (zero), otherwise it returns a non-zero exit status
$$             the process number of this shell - useful for including in filenames, to make them unique.
the process id of the last command run in the background.
the current options supplied to this invocation of the shell.
a string containing all the arguments to the shell, starting at $1.
same as above, except when quoted.

Tips ::
$* and $@ when unquoted are identical and expand into the arguments.
"$*" is a single word, comprising all the arguments to the shell, joined together with spaces. For example '1 2' 3 becomes "1 2 3".
"$@" is identical to the arguments received by the shell, the resulting list of words completely match what was given to the shell. For example '1 2' 3 becomes "1 2" "3"..

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