Working with the Unix OS

Unix Tools
END { print max, maxline }
# Print every line that has at least one field
NF > 0
# Print every line longer than 80 characters
length($0) > 80
# Print the number of fields in every line and the line
AWK – Programming Language
File processing programming language:
- generates reports
- matches patterns
- validates data
- filters data for transmission
{print NF, $0}
# Print the first two fields in opposite order
{print $2, $1}
# Replace first field by the line number
{$1 = NR; print}
# Erase second field
{$2 = ""; print }
! Program Structure
pattern ( action )
pattern ( action )
…
The pattern or action may be omitted, but not both
An awk program has the following structure:
- a BEGIN section – run before input lines are read
- a record section
- an END section – run after all files are processed
! Lexical Units
Awk programs are made up of lexical units called tokens:
- numerical constants – decimal or floating e.g. 12.
- string constants – sequence of characters e.g. "ab"
- keywords
- BEGIN END FILENAME FS NF NR OFS ORS OFMT RS
- break close continue exit exp for getline if in length log next
number print printf split sprintf substr while
- identifiers – variables k arrays
- operators
- assignment += -= *= /= %= ++ --
- arithmetic
- relational
- logical && | !
- regular expression matching
- record and field tokens
- $0 current input record
- $1 first field, $2 second field, etc
- NF number of fields, $NF last field (not defined in BEGIN or END pattern)
- NR number of records (lines so far)
- RS record separator (set to newline)
- FS field separator (set to space)
- RS =" " makes an empty line the RS
- OFS output field separator
- ORS output record separator
- comments
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