BASH script quick reference

1. Special character

• ; --Command separator
• ;; --double terminator in case ....esac
• # -- comment
• . -- equivalent to source command. e.g. Note: dot has other meanings as well.

  . data-file  # source data-file

• "STRING" --partial quoting preserve most of the special characters within STRING
  
• 'STRING' -- full quoting preserves all special characters within STRING
• , -- comma separator ,links together a series of arithmetic operations. All are evaluated, but only the last one is returned.
• \-- escape
  
• ` -- command substitutions. `command` makes available the output of command for assignment to a variable
• : -- null command=NOP , :=true
• ! -- invert/negate/reverse the sense of a test or exit status[bang]
• () -- command group,(a1=1,b=2), or array initialization
  
• {a,b,c}--brace expansion,eg. cat {file1,file2,file3}>combined_file,eg. cp file22.{txt,backup}
• {a..z} -- extended brace expansion. echo {a..z} #a,b,c,d....z
• [] -- test expression between []
• [[]]-- test.
  
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2.