Content Comparison

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1. single quoting (e.g. 'some text') – serves two purposes
   • it groups together all text inside the quotes into a single token
   • it tells the shell not to "look inside" the quotes to perform any evaluation
     
     • all metacharacters inside the single quotes are ignored
     • in particular, any environment variables in single-quoted text are not evaluated
   • single quoting preserves any whitespace present in the text (spaces or newlines)
2. double quoting (e.g. "some text") – also serves two purposes
   • it groups together all text inside the quotes into a single token
   • it allows environment variable evaluation, but inhibits some metacharcters
     • e.g. asterisk ( * ) pathname globbing and some other metacharacters
       
   • double quoting also preserves any special characters present whitespace in the text
     • e.g. spaces, newlines (\n) or , and Tabs (\t)
3. backtick quoting (e.g. `date`)
   
   • evaluates the expression inside the backtick marks ( ` )
   • the standard output of the expression replaces the text inside the backtick marks ( ` )
   • note that the syntax $(<expression>) is equivalent to `<expression>`

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Single vs double quotes examplesExamples:

echo "My Unix group is $MY_GROUP"   # The text "$MY_GROUP" is evaluated 
                                    #   and its value substituted  
echo 'My Unix group is $MY_GROUP'   # The text "$MY_GROUP" is left as-is

foo="My USER name is $USER"; echo $foo         # The text "$USER" is evaluated and
its value                                        #   and its value substituted
foo='My USER name is $USER'; echo $foo         # The text "$USER" is left as-is

FOO="Hello world!"
echo "The value of variable 'FOO' is \"$FOO\""  # Escape the double quotes inside double quotes

                                                #   inside double quotes

# Same output - the shell removes whitespace
echo Hello world!; echo Hello   world!

# Echo a multi-line text variable without quotes, and the
# shell removes whitespace (here the newline)
foo='aa
bb'; echo $foo

# But enclose the multi-line variable in double quotes and whitespace
# is preserved
echo "$foo"

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date          # Calling the date command just displays just displays 
              #   date/time information
echo date     # Here "date" is treated as a literal word, and 
              #   written to output
echo `date`   # The date command is evaluated and its output replaces
   the command  today=$( date );      #   replaces echothe $todaycommand

# environmentAssign variablea "today"string isincluding assigned today's date to variable "today"
today="Today is: `date`"; echo $today  # "today" is assigned a string including today's date

More at: Intro Unix: Writing text: Quoting in the shell

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ls haiku.txt xxx.txt                # displays both output and error text 
                                   #   on the Terminal
ls haiku.txt xxx.txt 2>/dev/null    # displays only output text on the the 
                                   #   Terminal
ls haiku.txt xxx.txt 1>/dev/null    # displays only error text on the Terminal 
                    # And this syntax (2>&1) sends standard output to outerr.log and standard error to the #   sameTerminal
place
as# standardAnd out.this So data fromsyntax (2>&1) sends both standard output and standard error 
# to willthe same beplace writtenas tostandard outerrout.log 
ls haiku.txt xxx.txt 1>outerr.log 2>&1 

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• Understanding the tree-like structure of directories and files in the file system hierarchy
  • Absolute paths start with a slash ( / ), the root of the file system hierarchy
    
  • More at: Intro Unix: Files and File Systems: The file system hierarchy
    
• Knowing how to navigate the file system using the cd (change directory) command, Tab key completion, and relative path syntax:
  • use the dot ( . ) metacharacter for the current directory
  • use the dot-dot ( .. ) metacharacters for the parent directory
  • More at:
    • Intro Unix: Files and File Systems: Navigating the file system 
    • Intro Unix: Files and File Systems: Relative pathname syntax
• Selecting multiple files using pathname wildcards (a.k.a. "globbing")
  
  • asterisk ( * ) to match any length of characters
  • brackets ( [ ] ) match any character between the brackets, including hyphen ( - ) delimited character ranges such as [A-G]
  • braces ( {  } ) enclose a list of comma-separated strings to match (e.g. {dog,pony})
  • More at: Intro Unix: Files and File Systems: Pathname wildcards 
• A basic understanding of file attributes such as
  • file type (file, directory)
  • owner and group
    
  • permissions (read, write, execute) for the owner, group and everyone
  • More at: Intro Unix: Files and File Systems: File attributes
• Familiarly with basic file manipulation commands (mkdir, cp, mv, rm)
  • More at: Intro Unix: Files and File Systems: Basic file manipulation commands