Learning Linux Commands: awk--reference

http://how-to.linuxcareer.com/learning-linux-commands-awk

1. Introduction

In this case, the title might be a little misleading. And that is because awk is more than a command, it's a programming language in its own right. You can write awk scripts for complex operations or you can use awk from the command line. The name stands for Aho, Weinberger and Kernighan (yes, Brian Kernighan), the authors of the language, which was started in 1977, hence it shares the same Unix spirit as the other classic *nix utilities. If you're getting used to C programming or know it already, you will see some familiar concepts in awk, especially since the 'k' in awk stands for the same person as the 'k' in K&R, the C programming bible. You will need some command-line knowledge in Linux and possibly some scripting basics, but the last part is optional, as we will try to offer something for everybody. Many thanks to Arnold Robbins for all his work involved in awk.

2. What is it that awk does?

awk is a utility/language designed for data extraction. If the word "extraction" rings a bell, it should because awk was one Larry Wall's inspirations when he created Perl. awk is often used with sed to perform useful and practical text manipulation chores, and it depends on the task if you should use awk or Perl, but also on personal preference. Just as sed, awk reads one line at a time, performs some action depending on the condition you give it and outputs the result. One of the most simple and popular uses of awk is selecting a column from a text file or other command's output. One thing I used to do with awk was, if I installed Debian on my second workstation, to get a list of the installed software from my primary box then feed it to aptitude. For that, I did something like that:

  $ dpkg -l | awk ' {print $2} ' > installed

Most package managers today offer this facility, for example rpm's -qa options, but the output is more than I want. I see that the second column of dpkg -l 's output contains the name of the packages installed, so this is why I used $2 with awk: to get me only the 2nd column.

3. Basic concepts

As you have noticed, the action to be performed by awk is enclosed in braces, and the whole command is quoted. But the syntax is awk ' condition { action }'. In our example, we had no condition, but if we wanted to, say, check only for vim-related packages installed (yes, there is grep, but this is an example, plus why use two utilities when you can only use one?), we would have done this:

 $ dpkg -l | awk ' /'vim'/ {print $2} '

This command would print all packages installed that have "vim" in their names. One thing that recommend awk is that it's fast. If you replace "vim" with "lib", on my system that yields 1300 packages. There will be situations where the data you'll have to work with will be much bigger, and that's one part where awk shines. Anyway, let's start with the examples, and we will explain some concepts as we go. But before that, it would be good to know that there are several awk dialects and implementations, and the examples presented here deal with GNU awk, as an implementation and dialect. And because of various quoting issues, we assume you're using bash, ksh or sh, we don't support (t)csh.

4. Examples

Learning Linux awk command with examples
Linux command syntax	Linux command description
awk ' {print $1,$3} '	Print only columns one and three using stdin
awk ' {print $0} '	Print all columns using stdin
awk ' /'pattern'/ {print $2} '	Print only elements from column 2 that match pattern using stdin
awk -f script.awk inputfile	Just like make or sed, awk uses -f to get its' instructions from a file, useful when there is a lot to be done and using the terminal would be impractical
awk ' program ' inputfile	Execute program using data from inputfile
awk "BEGIN { print \"Hello, world!!\" }"	Classic "Hello, world" in awk
awk '{ print }'	Print what's entered on the command line until EOF (^D)
#! /bin/awk -f BEGIN { print "Hello, world!" }	awk script for the classic "Hello, world!" (make it executable with chmod and run it as-is)
# This is a program that prints \ "Hello, world!" # and exits	Comments in awk scripts
awk -F "" 'program' files	Define the FS (field separator) as null, as opposed to white space, the default
awk -F "regex" 'program' files	FS can also be a regular expression
awk 'BEGIN { print "Here is a single \ quote <'\''>" }'	Will print <'>. Here's why we prefer Bourne shells. :)
awk '{ if (length($0) > max) max = \ length($0) } END { print max }' inputfile	Print the length of the longest line
awk 'length($0) > 80' inputfile	Print all lines longer than 80 characters
awk 'NF > 0' data	Print every line that has at least one field (NF stands for Number of Fields)
awk 'BEGIN { for (i = 1; i <= 7; i++) print int(101 * rand()) }'	Print seven random numbers from 0 to 100
ls -l . | awk '{ x += $5 } ; END \ { print "total bytes: " x }' total bytes: 7449362	Print the total number of bytes used by files in the current directory
ls -l . | awk '{ x += $5 } ; END \ { print "total kilobytes: " (x + \ 1023)/1024 }' total kilobytes: 7275.85	Print the total number of kilobytes used by files in the current directory
awk -F: '{ print $1 }' /etc/passwd | sort	Print sorted list of login names
awk 'END { print NR }' inputfile	Print number of lines in a file, as NR stands for Number of Rows
awk 'NR % 2 == 0' data	Print the even-numbered lines in a file. How would you print the odd-numbered lines?
ls -l | awk '$6 == "Nov" { sum += $5 } END { print sum }'	Prints the total number of bytes of files that were last modified in November
awk '$1 ̃/J/' inputfile	Regular expression matching all entries in the first field that start with a capital j
awk '$1 ̃!/J/' inputfile	Regular expression matching all entries in the first field that don't start with a capital j
awk 'BEGIN { print "He said \"hi!\" \ to her." }'	Escaping double quotes in awk
echo aaaabcd | awk '{ sub(/a+/, \ "<A>"); print }'	Prints "<A>bcd"
ls -lh | awk '{ owner = $3 ; $3 = $3 \ " 0wnz"; print $3 }' | uniq	Attribution example; try it :)
awk '{ $2 = $2 - 10; print $0 }' inventory	Modify inventory and print it, with the difference being that the value of the second field will be lessened by 10
awk '{ $6 = ($5 + $4 + $3 + $2); print \ $6' inventory	Even though field six doesn't exist in inventory, you can create it and assign values to it, then display it
echo a b c d | awk '{ OFS = ":"; $2 = "" > print $0; print NF }'	OFS is the Output Field Separator and the command will output "a::c:d" and "4" because although field two is nullified, it still exists so it gets counted
echo a b c d | awk ’{ OFS = ":"; \ $2 = ""; $6 = "new" > print $0; print NF }’	Another example of field creation; as you can see, the field between $4 (existing) and $6 (to be created) gets created as well (as $5 with an empty value), so the output will be "a::c:d::new" "6"
echo a b c d e f | awk ’\ { print "NF =", NF; > NF = 3; print $0 }’	Throwing away three fields (last ones) by changing the number of fields
FS=[ ]	This is a regular expression setting the field separator to space and nothing else (non-greedy pattern matching)
echo ' a b c d ' | awk 'BEGIN { FS = \ "[ \t\n]+" } > { print $2 }'	This will print only "a"
awk -n '/RE/{p;q;}' file.txt	Print only the first match of RE (regular expression)
awk -F\\\\ ’...’ inputfiles ...	Sets FS to \\
BEGIN { RS = "" ; FS = "\n" } { print "Name is:", $1 print "Address is:", $2 print "City and State are:", $3 print "" }	If we have a record like "John Doe 1234 Unknown Ave. Doeville, MA", this script sets the field separator to  newline so it can easily operate on rows
awk ’BEGIN { OFS = ";"; ORS = "\n\n" } > { print $1, $2 }’ inputfile	With a two-field file, the records will be printed like this: "field1:field2 field3;field4 ...;..." Because ORS, the Output Record Separator, is set to two newlines and OFS is ";"
awk ’BEGIN { > OFMT = "%.0f" # print numbers as \ integers (rounds) > print 17.23, 17.54 }’	This will print 17 and 18, because the Output ForMaT is set to round floating point values to the closest integer value
awk ’BEGIN { > msg = "Dont Panic!" > printf "%s\n", msg >} '	You can use printf mainly how you use it in C
awk ’{ printf "%-10s %s\n", $1, \ $2 }’ inputfile	Prints the first field as a 10-character string, left-justified, and $2 normally, next to it
awk ’BEGIN { print "Name Number" print "---- ------" } { printf "%-10s %s\n", $1, \ $2 }’ inputfile	Making things prettier
awk ’{ print $2 > "phone-list" }' \ inputfile	Simple data extraction example, where the second field is written to a file named "phone-list"
awk ’{ print $1 > "names.unsorted" command = "sort -r > names.sorted" print $1 | command }’ inputfile	Write the names contained in $1 to a file, then sort and output the result to another file (you can also append with >>, like you would in a shell)
awk ’BEGIN { printf "%d, %d, %d\n", 011, 11, \ 0x11 }’	Will print 9, 11, 17
if (/foo/ || /bar/) print "Found!"	Simple search for foo or bar
awk ’{ sum = $2 + $3 + $4 ; avg = sum / 3 > print $1, avg }’ grades	Simple arithmetic operations (most operators resemble C a lot)
awk '{ print "The square root of", \ $1, "is", sqrt($1) }' 2 The square root of 2 is 1.41421 7 The square root of 7 is 2.64575	Simple, extensible calculator
awk ’$1 == "start", $1 == "stop"’ inputfile	Prints every record between start and stop
awk ’ > BEGIN { print "Analysis of \"foo\"" } > /foo/ { ++n } > END { print "\"foo\" appears", n,\ "times." }’ inputfile	BEGIN and END rules are executed exactly once, before and after any record processing
echo -n "Enter search pattern: " read pattern awk "/$pattern/ "’{ nmatches++ } END { print nmatches, "found" }’ inputfile	Search using shell
if (x % 2 == 0) print "x is even" else print "x is odd"	Simple conditional. awk, like C, also supports the ?: operators
awk ’{ i = 1 while (i <= 3) { print $i i++ } }’ inputfile	Prints the first three fields of each record, one per line.
awk ’{ for (i = 1; i <= 3; i++) print $i }’	Prints the first three fields of each record, one per line.
BEGIN { if (("date" | getline date_now) <= 0) { print "Can’t get system date" > \ "/dev/stderr" exit 1 } print "current date is", date_now close("date") }	Exiting with an error code different from 0 means something's not quite right. Here's and example
awk ’BEGIN { > for (i = 0; i < ARGC; i++) > print ARGV[i] > }’ file1 file2	Prints awk file1 file2
for (i in frequencies) delete frequencies[i]	Delete elements in an array
foo[4] = "" if (4 in foo) print "This is printed, even though foo[4] \ is empty"	Check for array elements
function ctime(ts, format) { format = "%a %b %d %H:%M:%S %Z %Y" if (ts == 0) ts = systime() # use current time as default return strftime(format, ts) }	An awk variant of ctime() in C. This is how you define your own functions in awk
BEGIN { _cliff_seed = 0.1 } function cliff_rand() { _cliff_seed = (100 * log(_cliff_seed)) % 1 if (_cliff_seed < 0) _cliff_seed = - _cliff_seed return _cliff_seed }	A Cliff random number generator
cat apache-anon-noadmin.log | \ awk 'function ri(n) \ { return int(n*rand()); } \ BEGIN { srand(); } { if (! \ ($1 in randip)) { \ randip[$1] = sprintf("%d.%d.%d.%d", \ ri(255), ri(255)\ , ri(255), ri(255)); } \ $1 = randip[$1]; print $0 }'	Anonymize an Apache log (IPs are randomized)

5. Conclusion

As you can see, with awk you can do lots of text processing and other nifty stuff. We didn't get into more advanced topics, like awk's predefined functions, but we showed you enough (we hope) to start remembering it as a powerful tool.