awk

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•   What is awk?
    • Fully functional programming language written for
      processing text and numbers
    • Small, fast, and simple
    • Works field by field
        ‧   As opposed to grep, which works line by line

        ‧   Field = a column of data, separated by a delimiter (e.g., a space,
            a comma, etc.)
        ‧   Record = line of input

    •   Meant for processing column-oriented data, like tables

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•   awk [-F delim][-v var=value] ‘pattern {action; action }’ filename
    •   Basic idea: match a pattern → perform action(s)
        ‧   If no pattern → apply action(s) to all records/lines
        ‧   If no action → default is to print entire record
    •   Options and option arguments:
        ‧   -F = specify field delimiter (default: whitespace)
        ‧   -v = define a variable with its value to be used within awk
              ‧   E.g., -v bird=5
    •   Reads from STDIN and writes to STDOUT
        ‧   Can be used as a filter!

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•   In awk, the nth field is referred to by the variable
    $n
    • E.g.,   $1 → first field, $2 = second field
•   $0 → refers to the whole line

•   Examples:
    • awk     ‘{print $1}’
      ‧   Prints first field of each line
    • awk     ‘{print $0}’
      ‧   Prints each line
•   To use tabs as the delimiter:
    •   awk –F ‘\t’ ‘{print $4}’
•   Another example, this time using colons as delimiter:
    •   awk –F: ‘{print $6}’ /etc/passwd
        ‧   Prints home directory (sixth field) for each user
•   You can print multiple fields as well:
    • awk   –F\t ‘{print $1,$4}
      ‧ Prints first and fourth field separated by a space
      ‧ Comma here → space

    • awk   –F\t ‘{print $1 $4}
      ‧ Prints first and fourth field WITHOUT a space
      ‧ No comma → no space
•   Basically, you can print variables and other text concatenated
    together:
    •   awk –F\t ‘{print “The”, $1, “weighed”,$4,”!”}’
        ‧   Spaces inserted where commas are
        ‧   NOTICE:
              ‧ Single quotes for the outside!
              ‧ Double quotes for the inside!
•   NR = Current record (line) number
•   NF = Number of fields on the current line (columns)
    • $NF   = Last field
•   FS = field separator (defaults to white space)
•   OFS = output field separator
    • Allows   you to change output delimiter
•   Run the following:
•   • awk   ‘{print “Current line:”,NR,”Field Count:”,NF}’
•   Awk is now waiting for text (or EOF, which
    when read will cause awk to stop)

•   Enter some numbers separated by whitespace,
    then hit Enter
•   Awk should then print the current line number
    and field count
•   CTRL+D → send EOF
•   You can also check for a regular expression pattern
    • Must   be enclosed with //
      Matches somewhere in the line (similar to grep)

•   Examples
        awk –F\t ‘/abc/ {print $1}’
       Prints first field if line contains “abc”