Awk cheatsheet

This is a one page quick reference cheat sheet to the GNU awk, which covers commonly used awk expressions and commands.

#Getting Started

#Have a try

$ awk -F: '{print $1, $NF}' /etc/passwd

-	-	-
	`-F:`	Colon as a separator
	`{...}`	Awk program
	`print`	Prints the current record
	`$1`	First field
	`$NF`	Last field
	`/etc/passwd`	Input data file

#Awk program

BEGIN          {<initializations>} 
   <pattern 1> {<program actions>} 
   <pattern 2> {<program actions>} 
   ...
END            {< final actions >}

#Example

awk '
    BEGIN { print "\n>>>Start" }
    !/(login|shutdown)/ { print NR, $0 }
    END { print "<<<END\n" }
' /etc/passwd

#Variables

          $1      $2/$(NF-1)    $3/$NF
           ▼          ▼           ▼ 
        ┌──────┬──────────────┬───────┐
$0/NR ▶ │  ID  │  WEBSITE     │  URI  │
        ├──────┼──────────────┼───────┤
$0/NR ▶ │  1   │  ref.toolkits.cn │  awk  │
        ├──────┼──────────────┼───────┤
$0/NR ▶ │  2   │  google.com  │  25   │
        └──────┴──────────────┴───────┘

# First and last field
awk -F: '{print $1,$NF}' /etc/passwd

# With line number
awk -F: '{print NR, $0}' /etc/passwd

# Second last field
awk -F: '{print $(NF-1)}' /etc/passwd

# Custom string 
awk -F: '{print $1 "=" $6}' /etc/passwd

See: Variables

#Awk program examples

awk 'BEGIN {print "hello world"}'        # Prints "hello world"
awk -F: '{print $1}' /etc/passwd         # -F: Specify field separator

# /pattern/ Execute actions only for matched pattern
awk -F: '/root/ {print $1}' /etc/passwd                     

# BEGIN block is executed once at the start
awk -F: 'BEGIN { print "uid"} { print $1 }' /etc/passwd     

# END block is executed once at the end
awk -F: '{print $1} END { print "-done-"}' /etc/passwd

#Conditions

awk -F: '$3>30 {print $1}' /etc/passwd

See: Conditions

#Generate 1000 spaces

awk 'BEGIN{
    while (a++ < 1000)
        s=s " ";
    print s
}'

See: Loops

#Arrays

awk 'BEGIN {
   fruits["mango"] = "yellow";
   fruits["orange"] = "orange"
   for(fruit in fruits) {
     print "The color of " fruit " is " fruits[fruit]
   }
}'

See: Arrays

#Functions

# => 5
awk 'BEGIN{print length("hello")}'
# => HELLO
awk 'BEGIN{print toupper("hello")}'
# => hel
awk 'BEGIN{print substr("hello", 1, 3)}'

See: Functions

#Awk Variables

#Build-in variables

-	-
`$0`	Whole line
`$1, $2...$NF`	First, second… last field
`NR`	Total `N`umber of `R`ecords
`NF`	`N`number of `F`ields
`OFS`	`O`utput `F`ield `S`eparator (default " ")
`FS`	input `F`ield `S`eparator (default " ")
`ORS`	`O`utput `R`ecord `S`eparator (default "\n")
`RS`	input `R`ecord `S`eparator (default "\n")
`FILENAME`	Name of the file

#Expressions

-	-
`$1 == "root"`	First field equals root
`{print $(NF-1)}`	Second last field
`NR!=1{print $0}`	From 2th record
`NR > 3`	From 4th record
`NR == 1`	First record
`END{print NR}`	Total records
`BEGIN{print OFMT}`	Output format
`{print NR, $0}`	Line number
`{print NR " " $0}`	Line number (tab)
`{$1 = NR; print}`	Replace 1th field with line number
`$NF > 4`	Last field > 4
`NR % 2 == 0`	Even records
`NR==10, NR==20`	Records 10 to 20
`BEGIN{print ARGC}`	Total arguments
`ORS=NR%5?",":"\n"`	Concatenate records

#Examples

Print sum and average

awk -F: '{sum += $3}
     END { print sum, sum/NR }
' /etc/passwd

Printing parameters

awk 'BEGIN {
    for (i = 1; i < ARGC; i++)
        print ARGV[i] }' a b c

Output field separator as a comma

awk 'BEGIN { FS=":";OFS=","}
    {print $1,$2,$3,$4}' /etc/passwd

Position of match

awk 'BEGIN {
    if (match("One Two Three", "Tw"))
        print RSTART }'

Length of match

awk 'BEGIN {
    if (match("One Two Three", "re"))
        print RLENGTH }'

#Environment Variables

-	-
`ARGC`	Number or arguments
`ARGV`	Array of arguments
`FNR`	`F`ile `N`umber of `R`ecords
`OFMT`	Format for numbers (default "%.6g")
`RSTART`	Location in the string
`RLENGTH`	Length of match
`SUBSEP`	Multi-dimensional array separator (default "\034")
`ARGIND`	Argument Index

#GNU awk only

-	-
`ENVIRON`	Environment variables
`IGNORECASE`	Ignore case
`CONVFMT`	Conversion format
`ERRNO`	System errors
`FIELDWIDTHS`	Fixed width fields

#Defining variable

awk -v var1="Hello" -v var2="Wold" '
    END {print var1, var2}
' </dev/null

#Use shell variables

awk -v varName="$PWD" '
    END {print varName}' </dev/null

#Awk Operators

#Operators

-	-
`{print $1}`	First field
`$2 == "foo"`	Equals
`$2 != "foo"`	Not equals
`"foo" in array`	In array

#Regular expression

-	-
`/regex/`	Line matches
`!/regex/`	Line not matches
`$1 ~ /regex/`	Field matches
`$1 !~ /regex/`	Field not matches

#More conditions

-	-
`($2 <= 4 \|\| $3 < 20)`	Or
`($1 == 4 && $3 < 20)`	And

#Operations

#Arithmetic operations

+
-
*
/
%
++
--

#Shorthand assignments

+=
-=
*=
/=
%=

#Comparison operators

==
!=
<
>
<=
>=

#Examples

awk 'BEGIN {
    if ("foo" ~ "^fo+$")
        print "Fooey!";
}'

#Not match

awk 'BEGIN {
    if ("boo" !~ "^fo+$")
        print "Boo!";
}'

#if in array

awk 'BEGIN {
    assoc["foo"] = "bar";
    assoc["bar"] = "baz";
    if ("foo" in assoc)
        print "Fooey!";
}'

#Awk Functions

#Common functions

Function	Description
`index(s,t)`	Position in string s where string t occurs, 0 if not found
`length(s)`	Length of string s (or $0 if no arg)
`rand`	Random number between 0 and 1
`substr(s,index,len)`	Return len-char substring of s that begins at index (counted from 1)
`srand`	Set seed for rand and return previous seed
`int(x)`	Truncate x to integer value
`split(s,a,fs)`	Split string s into array a split by fs, returning length of a
`match(s,r)`	Position in string s where regex r occurs, or 0 if not found
`sub(r,t,s)`	Substitute t for first occurrence of regex r in string s (or $0 if s not given)
`gsub(r,t,s)`	Substitute t for all occurrences of regex r in string s
`system(cmd)`	Execute cmd and return exit status
`tolower(s)`	String s to lowercase
`toupper(s)`	String s to uppercase
`getline`	Set $0 to next input record from current input file.

#User defined function

awk '
    # Returns minimum number
    function find_min(num1, num2){
       if (num1 < num2)
       return num1
       return num2
    }
    # Returns maximum number
    function find_max(num1, num2){
       if (num1 > num2)
       return num1
       return num2
    }
    # Main function
    function main(num1, num2){
       result = find_min(num1, num2)
       print "Minimum =", result
      
       result = find_max(num1, num2)
       print "Maximum =", result
    }
    # Script execution starts here
    BEGIN {
       main(10, 60)
    }
'

#Awk Arrays

#Array with index

awk 'BEGIN {
    arr[0] = "foo";
    arr[1] = "bar";
    print(arr[0]); # => foo
    delete arr[0];
    print(arr[0]); # => ""
}'

#Array with key

awk 'BEGIN {
    assoc["foo"] = "bar";
    assoc["bar"] = "baz";
    print("baz" in assoc); # => 0
    print("foo" in assoc); # => 1
}'

#Array with split

awk 'BEGIN {
    split("foo:bar:baz", arr, ":");
    for (key in arr)
        print arr[key];
}'

#Array with asort

awk 'BEGIN {
    arr[0] = 3
    arr[1] = 2
    arr[2] = 4
    n = asort(arr)
    for (i = 1; i <= n ; i++)
        print(arr[i])
}'

#Multi-dimensional

awk 'BEGIN {
    multidim[0,0] = "foo";
    multidim[0,1] = "bar";
    multidim[1,0] = "baz";
    multidim[1,1] = "boo";
}'

#Multi-dimensional iteration

awk 'BEGIN {
    array[1,2]=3;
    array[2,3]=5;
    for (comb in array) {
        split(comb,sep,SUBSEP);
        print sep[1], sep[2], 
        array[sep[1],sep[2]]
    }
}'

#Awk Conditions

#if-else statement

awk -v count=2 'BEGIN {
    if (count == 1)
        print "Yes";
    else
        print "Huh?";
}'

#Ternary operator

awk -v count=2 'BEGIN {
    print (count==1) ? "Yes" : "Huh?";
}'

#Exists

awk 'BEGIN {
    assoc["foo"] = "bar";
    assoc["bar"] = "baz";
    if ("foo" in assoc)
        print "Fooey!";
}'

#Not exists

awk 'BEGIN {
    assoc["foo"] = "bar";
    assoc["bar"] = "baz";
    if ("Huh" in assoc == 0 )
        print "Huh!";
}'

#switch

awk -F: '{
    switch (NR * 2 + 1) {
        case 3:
        case "11":
            print NR - 1
            break
        
        case /2[[:digit:]]+/:
            print NR
        
        default:
            print NR + 1
        
        case -1:
            print NR * -1
    }
}' /etc/passwd

#Awk Loops

#for...i

awk 'BEGIN {
    for (i = 0; i < 10; i++)
        print "i=" i;
}'

#Powers of two between 1 and 100

awk 'BEGIN {
    for (i = 1; i <= 100; i *= 2)
        print i
}'

#for...in

awk 'BEGIN {
    assoc["key1"] = "val1"
    assoc["key2"] = "val2"
    for (key in assoc)
        print assoc[key];
}'

#Arguments

awk 'BEGIN {
    for (argnum in ARGV)
        print ARGV[argnum];
}' a b c

#Examples

#Reverse records

awk -F: '{ x[NR] = $0 }
    END {
        for (i = NR; i > 0; i--)
        print x[i]
    }
' /etc/passwd

#Reverse fields

awk -F: '{
    for (i = NF; i > 0; i--)
        printf("%s ",$i);
    print ""
}' /etc/passwd

#Sum by record

awk -F: '{
    s=0;
    for (i = 1; i <= NF; i++)
        s += $i;
    print s
}' /etc/passwd

#Sum whole file

awk -F: '
    {for (i = 1; i <= NF; i++)
        s += $i;
    };
    END{print s}
' /etc/passwd

#while

awk 'BEGIN {
    while (a < 10) {
        print "- " " concatenation: " a
        a++;
    }
}'

#do...while

awk '{
    i = 1
    do {
        print $0
        i++
    } while (i <= 5)
}' /etc/passwd

#Break

awk 'BEGIN {
    break_num = 5
    for (i = 0; i < 10; i++) {
        print i
        if (i == break_num)
            break
    }
}'

#Continue

awk 'BEGIN {
    for (x = 0; x <= 10; x++) {
        if (x == 5 || x == 6)
            continue
        printf "%d ", x
    }
    print ""
}'

#Awk Formatted Printing

#Usage

#Right align

awk 'BEGIN{printf "|%10s|\n", "hello"}'

|     hello|

#Left align

awk 'BEGIN{printf "|%-10s|\n", "hello"}'

|hello     |

#Common specifiers

Character	Description
`c`	ASCII character
`d`	Decimal integer
`e`, `E`, `f`	Floating-point format
`o`	Unsigned octal value
`s`	String
`%`	Literal %

#Space

awk -F: '{
    printf "%-10s %s\n", $1, $(NF-1)
}' /etc/passwd | head -n 3

Outputs

root       /root
bin        /bin
daemon     /sbin

awk -F: 'BEGIN {
    printf "%-10s %s\n", "User", "Home"
    printf "%-10s %s\n", "----","----"}
    { printf "%-10s %s\n", $1, $(NF-1) }
' /etc/passwd | head -n 5

Outputs

User       Home
----       ----
root       /root
bin        /bin
daemon     /sbin

#Miscellaneous

#Regex Metacharacters

#Escape Sequences

-	-
`\b`	Backspace
`\f`	Form feed
`\n`	Newline (line feed)
`\r`	Carriage return
`\t`	Horizontal tab
`\v`	Vertical tab

#Run script

$ cat demo.awk
#!/usr/bin/awk -f
BEGIN { x = 23 }
      { x += 2 }
END   { print x }
$ awk -f demo.awk /etc/passwd
69

#Also see

The GNU Awk User's Guide (www-zeuthen.desy.de)
AWK cheatsheet (gist.github.com)

Awk cheatsheet

#Getting Started

#Have a try

#Awk program

#Example

#Variables

#Awk program examples

#Conditions

#Generate 1000 spaces

#Arrays

#Functions

#Awk Variables

#Build-in variables

#Expressions

#Examples

#Environment Variables

#GNU awk only

#Defining variable

#Use shell variables

#Awk Operators

#Operators

#Regular expression

#More conditions

#Operations

#Arithmetic operations

#Shorthand assignments

#Comparison operators

#Examples

#Not match

#if in array

#Awk Functions

#Common functions

#User defined function

#Awk Arrays

#Array with index

#Array with key

#Array with split

#Array with asort

#Multi-dimensional

#Multi-dimensional iteration

#Awk Conditions

#if-else statement

#Ternary operator

#Exists

#Not exists

#switch

#Awk Loops

#for...i

#Powers of two between 1 and 100

#for...in

#Arguments

#Examples

#Reverse records

#Reverse fields

#Sum by record

#Sum whole file

#while

#do...while

#Break

#Continue

#Awk Formatted Printing

#Usage

#Right align

#Left align

#Common specifiers

#Space

#Header

#Miscellaneous

#Regex Metacharacters

#Escape Sequences

#Run script

#Also see