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Top Forums Shell Programming and Scripting How to get the value of a variable which is having another value in environmental script? Post 302439759 by Franklin52 on Friday 23rd of July 2010 01:40:29 PM
Old 07-23-2010
Quote:
Originally Posted by TFord
What does the \$$filename do? I don't think I have ever seen this before.
eval is used here to obtain the value of a variable whose name is derived from the value of another variable.

Suppose you have an environment variable FILE1 with the content as in the scenario above:
Code:
FILE1="/clocal/data/user/userdata.txt"

If you assign the name of the environment variable to the variable like:
Code:
filename="FILE1"

filename doesn't have the value "/clocal/data/user/userdata.txt" but just "FILE1".

With the command:
Code:
eval "file=\$$filename"

eval expands $filename to FILE1, then the shell expands $FILE1 (hence the escaped $) and assigns the content to the variable $file.

Now the variable $file contains "/clocal/data/user/userdata.txt".
This User Gave Thanks to Franklin52 For This Post:
 

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GETFLAGS(8)						      System Manager's Manual						       GETFLAGS(8)

NAME
getflags, usage - command-line parsing for shell scripts SYNOPSIS
getflags $* usage [ progname ] DESCRIPTION
Getflags parses the options in its command-line arguments according to the environment variable $flagfmt. This variable should be a list of comma-separated options. Each option can be a single letter, indicating that it does not take arguments, or a letter followed by the space-separated names of its arguments. Getflags prints an rc(1) script on standard output which initializes the environment variable $flagx for every option mentioned in $flagfmt. If the option is not present on the command-line, the script sets that option's flag vari- able to an empty list. Otherwise, the script sets that option's flag variable with a list containing the option's arguments or, if the option takes no arguments, with the string 1. The script also sets the variable $* to the list of arguments following the options. The final line in the script sets the $status variable, to the empty string on success and to the string usage when there is an error parsing the command line. Usage prints a usage message to standard error. It creates the message using $flagfmt, as described above, $args, which should contain the string to be printed explaining non-option arguments, and $0, the program name (see rc(1)). If run under sh(1), which does not set $0, the program name must be given explicitly on the command line. EXAMPLE
Parse the arguments for leak(1): flagfmt='b,s,f binary,r res,x width' args='name | pid list' if(! ifs=() eval `{getflags $*} || ~ $#* 0){ usage exit usage } SOURCE
/src/cmd/getflags.c /src/cmd/usage.c SEE ALSO
arg(3) GETFLAGS(8)
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