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