learnxinyminutes-docs/awk.html.markdown

language	filename	contributors	lang
awk	learnawk.awk	Marshall Mason http://github.com/marshallmason	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.

#!/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
• Awk man page
• The GNU Awk User's Guide GNU Awk is found on most Linux systems.