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diff --git a/awk.html.markdown b/awk.html.markdown new file mode 100644 index 00000000..90f88b1a --- /dev/null +++ b/awk.html.markdown @@ -0,0 +1,359 @@ +--- +language: awk +filename: learnawk.awk +contributors: + - ["Marshall Mason", "http://github.com/marshallmason"] +lang: en +--- + +AWK is a standard tool on every POSIX-compliant UNIX system. It's like a +stripped-down Perl, perfect for text-processing tasks and other scripting +needs. It has a C-like syntax, but without semicolons, manual memory +management, or static typing. It excels at text processing. You can call to it +from a shell script, or you can use it as a stand-alone scripting language. + +Why use AWK instead of Perl? Mostly because AWK is part of UNIX. You can always +count on it, whereas Perl's future is in question. AWK is also easier to read +than Perl. For simple text-processing scripts, particularly ones that read +files line by line and split on delimiters, AWK is probably the right tool for +the job. + +```awk +#!/usr/bin/awk -f + +# Comments are like this + +# AWK programs consist of a collection of patterns and actions. The most +# important pattern is called BEGIN. Actions go into brace blocks. +BEGIN { + + # BEGIN will run at the beginning of the program. It's where you put all + # the preliminary set-up code, before you process any text files. If you + # have no text files, then think of BEGIN as the main entry point. + + # Variables are global. Just set them or use them, no need to declare.. + count = 0 + + # Operators just like in C and friends + a = count + 1 + b = count - 1 + c = count * 1 + d = count / 1 + e = count % 1 # modulus + f = count ^ 1 # exponentiation + + a += 1 + b -= 1 + c *= 1 + d /= 1 + e %= 1 + f ^= 1 + + # Incrementing and decrementing by one + a++ + b-- + + # As a prefix operator, it returns the incremented value + ++a + --b + + # Notice, also, no punctuation such as semicolons to terminate statements + + # Control statements + if (count == 0) + print "Starting with count of 0" + else + print "Huh?" + + # Or you could use the ternary operator + print (count == 0) ? "Starting with count of 0" : "Huh?" + + # Blocks consisting of multiple lines use braces + while (a < 10) { + print "String concatenation is done" " with a series" " of" + " space-separated strings" + print a + + a++ + } + + for (i = 0; i < 10; i++) + print "Good ol' for loop" + + # As for comparisons, they're the standards: + a < b # Less than + a <= b # Less than or equal + a != b # Not equal + a == b # Equal + a > b # Greater than + a >= b # Greater than or equal + + # Logical operators as well + a && b # AND + a || b # OR + + # In addition, there's the super useful regular expression match + if ("foo" ~ "^fo+$") + print "Fooey!" + if ("boo" !~ "^fo+$") + print "Boo!" + + # Arrays + arr[0] = "foo" + arr[1] = "bar" + # Unfortunately, there is no other way to initialize an array. Ya just + # gotta chug through every value line by line like that. + + # You also have associative arrays + assoc["foo"] = "bar" + assoc["bar"] = "baz" + + # And multi-dimensional arrays, with some limitations I won't mention here + multidim[0,0] = "foo" + multidim[0,1] = "bar" + multidim[1,0] = "baz" + multidim[1,1] = "boo" + + # You can test for array membership + if ("foo" in assoc) + print "Fooey!" + + # You can also use the 'in' operator to traverse the keys of an array + for (key in assoc) + print assoc[key] + + # The command line is in a special array called ARGV + for (argnum in ARGV) + print ARGV[argnum] + + # You can remove elements of an array + # This is particularly useful to prevent AWK from assuming the arguments + # are files for it to process + delete ARGV[1] + + # The number of command line arguments is in a variable called ARGC + print ARGC + + # AWK has several built-in functions. They fall into three categories. I'll + # demonstrate each of them in their own functions, defined later. + + return_value = arithmetic_functions(a, b, c) + string_functions() + io_functions() +} + +# Here's how you define a function +function arithmetic_functions(a, b, c, localvar) { + + # Probably the most annoying part of AWK is that there are no local + # variables. Everything is global. For short scripts, this is fine, even + # useful, but for longer scripts, this can be a problem. + + # There is a work-around (ahem, hack). Function arguments are local to the + # function, and AWK allows you to define more function arguments than it + # needs. So just stick local variable in the function declaration, like I + # did above. As a convention, stick in some extra whitespace to distinguish + # between actual function parameters and local variables. In this example, + # a, b, and c are actual parameters, while d is merely a local variable. + + # Now, to demonstrate the arithmetic functions + + # Most AWK implementations have some standard trig functions + localvar = sin(a) + localvar = cos(a) + localvar = atan2(a, b) # arc tangent of b / a + + # And logarithmic stuff + localvar = exp(a) + localvar = log(a) + + # Square root + localvar = sqrt(a) + + # Truncate floating point to integer + localvar = int(5.34) # localvar => 5 + + # Random numbers + srand() # Supply a seed as an argument. By default, it uses the time of day + localvar = rand() # Random number between 0 and 1. + + # Here's how to return a value + return localvar +} + +function string_functions( localvar, arr) { + + # AWK, being a string-processing language, has several string-related + # functions, many of which rely heavily on regular expressions. + + # Search and replace, first instance (sub) or all instances (gsub) + # Both return number of matches replaced + localvar = "fooooobar" + sub("fo+", "Meet me at the ", localvar) # localvar => "Meet me at the bar" + gsub("e+", ".", localvar) # localvar => "m..t m. at th. bar" + + # Search for a string that matches a regular expression + # index() does the same thing, but doesn't allow a regular expression + match(localvar, "t") # => 4, since the 't' is the fourth character + + # Split on a delimiter + split("foo-bar-baz", arr, "-") # a => ["foo", "bar", "baz"] + + # Other useful stuff + sprintf("%s %d %d %d", "Testing", 1, 2, 3) # => "Testing 1 2 3" + substr("foobar", 2, 3) # => "oob" + substr("foobar", 4) # => "bar" + length("foo") # => 3 + tolower("FOO") # => "foo" + toupper("foo") # => "FOO" +} + +function io_functions( localvar) { + + # You've already seen print + print "Hello world" + + # There's also printf + printf("%s %d %d %d\n", "Testing", 1, 2, 3) + + # AWK doesn't have file handles, per se. It will automatically open a file + # handle for you when you use something that needs one. The string you used + # for this can be treated as a file handle, for purposes of I/O. This makes + # it feel sort of like shell scripting: + + print "foobar" >"/tmp/foobar.txt" + + # Now the string "/tmp/foobar.txt" is a file handle. You can close it: + close("/tmp/foobar.txt") + + # Here's how you run something in the shell + system("echo foobar") # => prints foobar + + # Reads a line from standard input and stores in localvar + getline localvar + + # Reads a line from a pipe + "echo foobar" | getline localvar # localvar => "foobar" + close("echo foobar") + + # Reads a line from a file and stores in localvar + getline localvar <"/tmp/foobar.txt" + close("/tmp/foobar.txt") +} + +# As I said at the beginning, AWK programs consist of a collection of patterns +# and actions. You've already seen the all-important BEGIN pattern. Other +# patterns are used only if you're processing lines from files or standard +# input. +# +# When you pass arguments to AWK, they are treated as file names to process. +# It will process them all, in order. Think of it like an implicit for loop, +# iterating over the lines in these files. these patterns and actions are like +# switch statements inside the loop. + +/^fo+bar$/ { + + # This action will execute for every line that matches the regular + # expression, /^fo+bar$/, and will be skipped for any line that fails to + # match it. Let's just print the line: + + print + + # Whoa, no argument! That's because print has a default argument: $0. + # $0 is the name of the current line being processed. It is created + # automatically for you. + + # You can probably guess there are other $ variables. Every line is + # implicitely split before every action is called, much like the shell + # does. And, like the shell, each field can be access with a dollar sign + + # This will print the second and fourth fields in the line + print $2, $4 + + # AWK automatically defines many other variables to help you inspect and + # process each line. The most important one is NF + + # Prints the number of fields on this line + print NF + + # Print the last field on this line + print $NF +} + +# Every pattern is actually a true/false test. The regular expression in the +# last pattern is also a true/false test, but part of it was hidden. If you +# don't give it a string to test, it will assume $0, the line that it's +# currently processing. Thus, the complete version of it is this: + +$0 ~ /^fo+bar$/ { + print "Equivalent to the last pattern" +} + +a > 0 { + # This will execute once for each line, as long as a is positive +} + +# You get the idea. Processing text files, reading in a line at a time, and +# doing something with it, particularly splitting on a delimiter, is so common +# in UNIX that AWK is a scripting language that does all of it for you, without +# you needing to ask. All you have to do is write the patterns and actions +# based on what you expect of the input, and what you want to do with it. + +# Here's a quick example of a simple script, the sort of thing AWK is perfect +# for. It will read a name from standard input and then will print the average +# age of everyone with that first name. Let's say you supply as an argument the +# name of a this data file: +# +# Bob Jones 32 +# Jane Doe 22 +# Steve Stevens 83 +# Bob Smith 29 +# Bob Barker 72 +# +# Here's the script: + +BEGIN { + + # First, ask the user for the name + print "What name would you like the average age for?" + + # Get a line from standard input, not from files on the command line + getline name <"/dev/stdin" +} + +# Now, match every line whose first field is the given name +$1 == name { + + # Inside here, we have access to a number of useful variables, already + # pre-loaded for us: + # $0 is the entire line + # $3 is the third field, the age, which is what we're interested in here + # NF is the number of fields, which should be 3 + # NR is the number of records (lines) seen so far + # FILENAME is the name of the file being processed + # FS is the field separator being used, which is " " here + # ...etc. There are plenty more, documented in the man page. + + # Keep track of a running total and how many lines matched + sum += $3 + nlines++ +} + +# Another special pattern is called END. It will run after processing all the +# text files. Unlike BEGIN, it will only run if you've given it input to +# process. It will run after all the files have been read and processed +# according to the rules and actions you've provided. The purpose of it is +# usually to output some kind of final report, or do something with the +# aggregate of the data you've accumulated over the course of the script. + +END { + if (nlines) + print "The average age for " name " is " sum / nlines +} + +``` +Further Reading: + +* [Awk tutorial](http://www.grymoire.com/Unix/Awk.html) +* [Awk man page](https://linux.die.net/man/1/awk) +* [The GNU Awk User's Guide](https://www.gnu.org/software/gawk/manual/gawk.html) GNU Awk is found on most Linux systems. |