Awk is a flexible pattern matching, text processing and line-oriented language, Linux provides us with an implementation version of it, gawk.

We can use the awk or gawk command to demonstrate. Awk is a symbolic link to gawk:

                    
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                          ll /bin/awk

OPTIONS

Here are some common options for gawk command:

Option	Meaning
-f program-file or ‑‑file program-file	Reads the gawk execution script from the file.
-F fs or ‑‑field-separator fs	Specifies the value of the predefined variable FS. Use this value as the delimiter for the column.
-v var=val or ‑‑assign var=val	Assign the value val to the variable var.

VARIABLES, RECORDS AND FIELDS

Here are some common built-in variables for gawk command:

Option	Meaning
ARGC	The number of command-line arguments.
ARGV	Command line argument array.
FNR	The number of records from the current input file.
FS	The field separator for input content; the default is a space.
NF	The number of fields from the input.
NR	The number of records from the input.
OFS	The field separator for output content; the default is a space.
ORS	The record separator for output content; the default is a newline.
RS	The record separator for input content; the default is a newline.

PATTERNS AND ACTIONS

Here are some common patterns and actions for gawk command:

PATTERNS

Option	Meaning
BEGIN	Print once at the beginning.
END	Print once at the end.
/regular expression/	Executes a regular expression for each line of content.
relational expression	Performs operator calculations on each line of content.
pattern && pattern or pattern \|\| pattern or ! pattern	Performs logical operations on each line of content.
pattern ? pattern : pattern	Performs a ternary operation on each line of content.
pattern1, pattern2	Start with the pattern1-matched row until the pattern2-matched row.

ACTIONS

Option	Meaning
{}	Everything in parentheses are executable statements.

Operators

Option	Meaning
$N	Refers to the Nth field.
++ –	Auto-increment and auto-decrement.
+ - ~	Plus, minus, and negation.
* / %	Multiplication, division, and modulus.
space	String concatenation.
< > <= >= == !=	Relational operators.
&& \|\|	Logical AND and OR.
?:	Ternary operator. expr1 ? expr2 : expr3.
= += -= *= /= %= ^=	Assignment.

I/O Statements

Option	Meaning
print	Outputs the current record, ending with the value defined by the built-in variable ORS.
printf	Output formatted content.

The printf Statement

Option	Meaning
%a, %A	A hexadecimal floating point number.
%c	A single character.
%d, %i	The integer part of a decimal number.
%e, %E	A floating point number.
%o	An unsigned octal number.
%u	An unsigned decimal number.
%s	A character string.
%x, %X	An unsigned hexadecimal number.

Examples

Run Program Only

Output Argument Count

With the ARGC variable, we can get the number of command-line arguments:

                    
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                          awk 'BEGIN{print ARGC}'    # Print the command line argument count

Output Argument Value

With the ARGV variable, we can get the value of the command-line argument:

                    
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                          awk 'BEGIN{print ARGV[0]}'    # Print the command line first argument value

Output Integer Variable

                    
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                          awk 'BEGIN{var=123;print var}'    # Print the self-defined integer variable

Output Integer Variables Summation

We can obtain the summation of two integer variables before the output:

                    
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                          awk 'BEGIN{var=123;var2=321;print var+var2}'    # Print the summation of these two integer variable

Output String Variable

                    
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                          awk 'BEGIN{var="abc";print var}'    # Print the self-defined string variable

Use the Input Stream as Content

Awk can also handle content from the input stream:

                    
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                          ll /
ll / | awk '{print $1}'

Handle File Content

As well as dealing with the content from the file.

Let’s prepare some test content first:

                    
                      
                        
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                          printf '%s\n' \
'user1:10020:10010:Dong Chen:/bin/bash:active' \
'user2:10021:10010:Dong Chen1:/bin/bash:inactive' \
'user3:10022:10010:Dong Chen2:/bin/bash:active' \
'user4:10023:10010:Dong Chen3:/bin/bash:inactive' \
'user5:10024:10010:Dong Chen4:/bin/bash:active' \
'user6:10025:10010:Dong Chen5:/bin/bash:inactive' > awk_file
cat awk_file
                        
                      

                  

Output Header and Footer

The BEGIN and END patterns will only print once:

                    
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                          awk -F: 'BEGIN{print "HEADER"}END{print "FOOTER"}{print $1,$2,$3,$4,$5}' awk_file

Output With A Customized Field Separator

Through -v argument, we can customize the output field separator’s variable OFS:

                    
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                          awk -v OFS=', ' -F: '{print $1, $4, $2, $3, $5}' awk_file

Output Formatted Content

Through printf command and its formats, we can print formatted content:

                    
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                          awk -F: '{printf "User Account: %s, Username: %s, User ID: %d, Group ID: %d, Login Shell: %s\n", $1, $4, $2, $3, $5}' awk_file

If Else Ternary Operation

We can use a ternary operator to do conditional statement operations:

                    
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                          awk -F: '($6=="active" ? status="Working hard" : status="Lazy"){printf "%s is %s.\n", $1, status }' awk_file

References The GNU Awk User’s Guide, GAWK(1)

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59. Advanced Text Processing - Awk's Basic Usage

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