Here command is the name of the command (Cheapest web hosting)
Here command is the name of the command you want to execute. The one problem with this is that if no arguments were specified to your script, some versions of the shell will expand “$@” to “”. If no arguments were specified, you want to execute command, not command “”.To avoid this problem, use the form: command ${@:+”$@”} Here you are using one of the forms of variable substitution discussed in Chapter 8, “Substitution.” In this case you check to see whether the variable $@ has a value. If it does, you substitute the value “$@” for it. If your script was not given any command line arguments, $@ will be null; thus no value will be substituted. How do I use the value of a shell variable in a sed command? The simplest method to use variables in a sed command is to enclose your sed command in double quotes ( “) instead of single quotes ( ‘). Because the shell performs variable substitution on double-quoted strings, the shell will substitute the value of any variables you specify before sed executes. For example, the command sed “/$DEL/d” file1 > file2 deletes all the lines in file1 that contain the value stored in the variable $DEL. How do I check to see whether a variable has a value? There are several methods for determining this. The simplest is the if statement: if [ -z “$VAR” ] ; then list ; fi Here VAR is the name of the variable, and list is the command to execute if VAR does not contain a value. Usually list initializes VAR to some default value. You can initialize variables more succinctly using variable substitution. For example, the previous if statement can be written as : ${VAR:=default} Here default is the default that should be assigned to VAR, if VAR does not have a value. If you need execute a set of commands to obtain a default value, use command substitution with the backquote ( ) operator to obtain the value that should be substituted: : ${VAR:= default } Here default is a list of commands to execute. If VAR does not have a value, the output of these commands will be assigned to it.
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