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What is the meaning of this style in bash?
${PUBLIC_INTERFACE:-eth0} What is the purpose of :-?
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Bash and sh both use colons (":") in more than one context. You'll see it used as a separator ($PATH, for example), as a modifier (${n:="foo"}) and as a null operator ("while :").
-d is a operator to test if the given directory exists or not. For example, I am having a only directory called /home/sureshkumar/test/. The directory variable contains the "/home/sureshkumar/test/" if [ -d $directory ]
Then we have an “if” statement followed by the “-n” flag, which returns true if a string is not null. We have used this flag to test our string “name,” which is null. It means that the “if” condition will not be executed since the value of the “-n” flag will be false in this case.
It's nothing, these colons are part of the command names apparently. You can verify yourself by creating and running a command with : in the name. The shell by default will autoescape them and its all perfectly legal. Follow this answer to receive notifications.
If $PUBLIC_INTERFACE exists and isn't null, return its value, otherwise return "eth0".
There are actually a few of these documented in the bash man page:
${parameter:-word} Use Default Values. If parameter is unset or null, the expansion of word is substituted. Otherwise, the value of parameter is substituted.
${parameter:=word} Assign Default Values. If parameter is unset or null, the expansion of word is assigned to parameter. The value of parameter is then substituted. Positional parameters and special parameters may not be assigned to in this way.
${parameter:?word} Display Error if Null or Unset. If parameter is null or unset, the expansion of word (or a message to that effect if word is not present) is written to the standard error and the shell, if it is not interactive, exits. Otherwise, the value of parameter is substituted.
${parameter:+word} Use Alternate Value. If parameter is null or unset, nothing is substituted, otherwise the expansion of word is substituted.
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