27 Programming the Bourne Again Shell in .NET Compose EAN/UCC-13 in .NET 27 Programming the Bourne Again Shell

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1000 27 Programming the Bourne Again Shell generate, create ean-13 supplement 2 none in .net projects Microsoft Windows SDK It is a good id EAN/UCC-13 for .NET ea to enclose references to positional parameters between double quotation marks. The quotation marks are particularly important when you are using positional parameters as arguments to commands.

Without double quotation marks, a positional parameter that is not set or that has a null value disappears:. $ cat showargs echo "$0 was called with $# arguments, the first is :$1:." $ ./showargs a b c .

/showargs was called with 3 arguments, the first is :a:. $ echo $xx $ ./showargs $xx a b c .

/showargs was called with 3 arguments, the first is :a:. $ ./showargs "$xx" a b c .

/showargs was called with 4 arguments, the first is ::.. The showargs sc EAN 13 for .NET ript displays the number of arguments ($#) followed by the value of the first argument enclosed between colons. In the preceding example, showargs is initially called with three simple arguments.

Next the echo command demonstrates that the $xx variable, which is not set, has a null value. In the final two calls to showargs, the first argument is $xx. In the first case the command line becomes showargs a b c; the shell passes showargs three arguments.

In the second case the command line becomes showargs "" a b c, which results in calling showargs with four arguments. The difference in the two calls to showargs illustrates a subtle potential problem that you should keep in mind when using positional parameters that may not be set or that may have a null value..

"$*" versus "$@". The $* and $@ p .NET EAN13 arameters work the same way except when they are enclosed within double quotation marks. Using "$*" yields a single argument (with SPACEs or the value of the first character of IFS [page 323] between the positional parameters), whereas using "$@" produces a list wherein each positional parameter is a separate argument.

This difference typically makes "$@" more useful than "$*" in shell scripts. The following scripts help explain the difference between these two special parameters. In the second line of both scripts, the single quotation marks keep the shell from interpreting the enclosed special characters so they are passed to echo and displayed as themselves.

The bb1 script shows that set "$*" assigns multiple arguments to the first command-line parameter:. $ cat bb1 set " $*" echo $# parameters with ""$*"" echo 1: $1 echo 2: $2 echo 3: $3 $ ./bb1 a b c 1 parameters with "$*" 1: a b c 2: 3:. The bb2 script .NET EAN/UCC-13 shows that set "$@" assigns each argument to a different commandline parameter:. Parameters and Variables $ cat bb2 set " Visual Studio .NET European Article Number 13 $@" echo $# parameters with ""$@"" echo 1: $1 echo 2: $2 echo 3: $3 $ ./bb2 a b c 3 parameters with "$@" 1: a 2: b 3: c.

Expanding Null and Unset Variables The expression GS1-13 for .NET ${name} (or just $name if it is not ambiguous) expands to the value of the name variable. If name is null or not set, bash expands ${name} to a null string.

The Bourne Again Shell provides the following alternatives to accepting the expanded null string as the value of the variable: Use a default value for the variable. Use a default value and assign that value to the variable. Display an error.

You can choose one of these alternatives by using a modifier with the variable name. In addition, you can use set o nounset (page 355) to cause bash to display an error and exit from a script whenever an unset variable is referenced..

: Uses a Default Value The : modifier .net framework EAN-13 uses a default value in place of a null or unset variable while allowing a nonnull variable to represent itself: ${name: default} The shell interprets : as If name is null or unset, expand default and use the expanded value in place of name; else use name. The following command lists the contents of the directory named by the LIT variable.

If LIT is null or unset, it lists the contents of /home/max/literature:. $ ls ${LIT:-/ho me/max/literature}. The default can .net vs 2010 EAN13 itself have variable references that are expanded:. $ ls ${LIT:-$HOME/literature}.
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