CIS 218 Advanced UNIX 1
CIS 218 – Advanced UNIX
(g)awk
CIS 218 Advanced UNIX 2
Overview
• awk is a programming language
• Awk uses syntax based on grep and sed for
handling numbers and text
• awk provides field level addressability.
And within a field (word) using substring
commands
• awk works field by field
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awk command syntax
• There are two ways to execute an awk
program/script:
– awk [-F field-separator] ‘program’ target-file
– awk [-F field-separator] -f program.file target
• From our discussion of sed, and
Refrigerator Rule No. 5, I would hope you
are firmly committed to the second form!
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awk Variables
• There are a number of awk variables that
are very useful
– FS (The field separator, defaults to white space)
– OFS (Output field separator, can be critical)
– NR (Number of records, a sequential counter)
– NF (Number of fields in the current record)
– FILENAME (Name of the current target file)
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awk Variables (cont.)
– $0 (The entire line as read from the target file)
– $n (Where n is the nth field in the record. This is how we get field level addressability in awk)
• nawk, gawk, etc give us more variables, the most significant two are:
– ARGC (the count of the command line arguments)
– ARGV (an array of the command line arguments)
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Parts of a program
• All programs are composed of one or more of the following three constructs:
– sequence (a series of instructions, one following the next, executed sequentially)
– selection (the ability of the code to decide which instructions to execute, conditional execution)
– iteration (adding looping so that selected code will be repeated over an over)
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awk Program Format
• Awk programs are composed of pattern {action} pairs (actions must be enclosed in French braces {} )
– a pattern without a corresponding action takes the default action, print $0
– an action without a corresponding pattern is applied to every line
– each input line is submitted to every pattern/action pair
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awk Program Format (cont.)
• Placement of the open French brace is critical
– pattern { both patterns are
action 1 executed for lines
action 2 matching the pattern }
– pattern lines matching the pattern
{action 1 are printed, and both
action 2 actions are performed on
} every line!
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Patterns
• In an awk program, the pattern is the
selection tool that decides what actions are
applied to which lines.
• Patterns can be:
– relational expressions
– regular expressions
– magic patterns
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Relational Expression patterns
Symbol Meaning Symbol Meaning
< Less than == equal to
<= Less than or
equal to
~ contains the RE
> Greater than !~ doesn't contain
RE
>= Greater than or
equal to
&& logical and
!= not equal to || logical or
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Regular Expression patterns
• Must be enclosed in slashes /RE/
• Anchors apply to the entire line if they are used as the only pattern
• Remember, you can use regular expressions in relational patterns with ~ and !~ to apply them to fields
• Both true regular expressions and fixed patterns can be used as REs in awk
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Pre/Post Processing
• There are two in awk:
– BEGIN {the action associated is performed before the
target file is opened}
– END {the action associated is performed after the target
file is successfully closed}
• Both are coded in UPPER CASE
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# comments
• Like most scripting languages # indicates a
comment
• awk scripts should be well documented
• Comments should explain what you are
doing and why.
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• The print command is the simplistic output
tool for awk. Basically and “echo”/
• You can direct print to send its data to a file
with the > operator
• Generally print is used for simple output or
debugging output
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printf
• Similar in concept to the “C” language command.
The format of a printf command is:
printf (“formatting string”,variables)
• The formatting characters correspond to the
variables one for one in both lists.
• Each formatting character is prefixed by %
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printf (cont.)
• The formatting specifiers contain then
following characters:
– - indicates that the data should be left justifed
– n indicates the minimum width of the field
– .n indicates the maximum width of the field
“%-5s”
indicates a string field, left justified, of
width 5 bytes
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printf formatting characters
Format Meaning Format Meaning
%c single ASCII
character
%G shortest of %E or
%f
%d decimal integer %i decimal integer
%e scientific notation %o octal number
%E SCIENTIFIC
NOTATION
%s string
%f floating point %x hexadecimal (lc)
%g shortest of %f or
%e
%X HEXADECIMAL
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printf spacing characters
• There are two characters available to
change the spacing of your text:
– \n inserts a newline character. You must use
this if you want your output to occur on
successive lines.
– \t inserts a tab character
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getline
• getline is used to read from the keyboard
• It can also capture the results of a command but this form is seldom used
• Read from the keyboard using getline variable < “/dev/tty”
• If you don’t supply a variable, awk will use $0, so in most cases you want to use a variable.
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rand() srand()
• The rand() function generates pseudo-random numbers in the range 0 - 1.
• Given the same seed, it will always generate the same series of numbers.
• srand() is used to supply a new seed to rand().
• If you don’t supply srand() a value, it uses the current time as the seed.
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system()
• The system() function allows you to execute
system commands within an awk script.
• You must enclose the system command in
quotation marks.
• You cannot capture the output from the
system() function within the script but you
can capture the return code.
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length()
• The length([argument]) function returns the
length of the argument in bytes.
• If you give length() a number, it will return
the number of digits in the number.
• If you don’t give length() an argument, it
will use $0 by default.
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index()
• The index(string,target) function returns the
position of the first occurrence of the target
within the string.
• The index() function is often used to set the
boundary for the substr() function.
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substr()
• The substr(string,start[,length]) function
will return the part of the string beginning
with start and continuing for length bytes.
• If you don’t give it a length, it will return all
the bytes between the start and the end of
the string.
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split()
• You will use split(string, array[, separator])
to divide a string into parts using separator
to parse them, storing the resultant parts in
the array.
• If you don’t code a separator, the function
will use the field separator to parse the
string.
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if • Besides using patterns, if gives us another
way to perform selection
• The format of an if statement is if (condition) {verb(s)} [else { verb(s)}]
• If you have more than one verb, they must be enclosed in French braces.
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if conditions
A < B A is less than B
A <= B A is less than or equal to B
A == B A equals B (note 2 =)
A > B A is greater than B
A >= B A is greater than or equal to B
A != B A is not equal to B
A ~ /RE/ A contains the regular
expression RE
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if
• A sample if
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exit
• The input file is closed
• Control is transferred to the action
associated with the END magic pattern if
there is one
• Generally used as a bailout in case of
catastrophic errors
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for loop • This is a counted loop
• executes until the counter reaches the target
value
• Increment (count up) or decrement (count
down)
• also works with the elements of an array
• multiple verbs must be enclosed in { }
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for loop example
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while loop
• The while loop is an example of conditional
execution
• The loop cycles as long as the condition
specified is true
• A while loop always checks to see if it
should execute
• multiple verbs must be enclosed in { }
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while loop example
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do/while
• Even though it has a while in it, this is an
example of until logic.
• Until logic is shunned by conscientious
coders.
• ‘nuff said
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break
• Used to exit from a loop
• Control is passed to the line following the
end of the loop
• Causes an exit from the loop but NOT the
awk script. If you want to bail out of the
whole script, use the exit command.
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break example
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continue
• Causes awk to skip the rest of the body of
the loop for the current value
• In a for loop the counter is incremented, and
the next cycle of the loop is started
• In a while loop, the next iteration of the
loop starts
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continue example
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next
• Causes the script to start over
• takes the next element from standard input
or the target file
• Like exit, this command effects the whole
script
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next example