Shell Scripting - PowerPoint Presentation

Slide1
Slide2

Shell Scripting

Awk

(part1)Slide3

Awk Programming Language

standard

unix

language that is geared for text processing and creating formatted reports

but it is very valuable to seismologists because it uses floating point math, unlike integer only bash, and is designed to work with columnar data

syntax similar to C and bash

one of the most useful

unix

tools at your commandSlide4

considers text files as fields (columns) and records (lines)

performs floating & integer arithmetic and string operations

uses loops and conditionals

define your own functions (subroutines)

execute

unix

commands within the scripts and process the resultsSlide5

versions

awk: original awk

nawk

: new awk, dates to 1987

gawk: GNU awk has more powerful string functionality

the CERI

unix

system has all three. You want to use

nawk

. I suggest adding this line to your .

cshrc

file

alias awk ‘

nawk

’

in OS X, awk is already

nawk

so no changes are necessarySlide6

Command line functionality

you can call awk from the command line two ways:

awk [options] ‘{ commands }’ variables

infile(s

)

awk –f

scriptfile

variables

infile(s

)

or you can create an executable awk script

%

cat << EOF >

test.awk

#!/

usr/bin/nawk

some set of commands

EOF

%

chmod

755

test.awk

%./

test.awkSlide7

How it treats text

awk commands are applied to every record or line of a file

it is designed to separate the data in each line into a field

essentially, each field becomes a member of an array so that the first field is $1, second field $2 and so on.

$

0

refers to the entire recordSlide8

Field Separator

the default

field separator

is one or more white spaces

$1 $2 $3 $4 $5 $6 $7 $8 $9 $10 $11

1 1918 9 22 9 54 49.29 -1.698 98.298 15.0

ehb

Notice that the fields may be integer, floating point (have a decimal point) or strings.

Nawk

is generally smart enough to figure out how to use them.Slide9

Field Separator

the field separator may be modified by resetting the FS built in variable

Look at

passwd

file

%

head -n1 /etc/

passwd

root:x:

0

:1:Super-User:/:/sbin/sh

Separator is “:”, so reset it.

%

awk

–F”:”

‘{ print $1, $3}’ /etc/

passwd

root

0Slide10

print

One of the most common commands used in awk scripts is

print

awk is not sensitive to white space in the commands

%awk –F”:” ‘{ print $1 $3}’ /etc/

passwd

root

0

two solutions to this

%awk –F”:” ‘{ print $1 “

“ $3}’ /etc/

passwd

%awk –F”:” ‘{ print $1, $3}’ /etc/

passwd

root 0Slide11

any string or numeric text can be explicitly output using “”

Assume a starting file like so:

1 1 1918 9 22 9 54 49.29 -1.698 98.298 15.0 0.0 0.0

ehb

FEQ

x

%

awk

'{print

"latitude:"

,$9,"longitude:",$10,"depth:",$11}’ SUMA. loc

latitude: -1.698 longitude: 98.298 depth: 15.0

latitude: 9.599 longitude: 92.802 depth: 30.0

latitude: 4.003 longitude: 94.545 depth: 20.0Slide12

Unlike the shell AWK does not evaluate variables within strings.

The second line, for example, could not be written:

{print "$8\t$3" }

As it

would print ”$8

$3.”

Inside quotes, the dollar sign is not a special character. Outside, it corresponds to a field.Slide13

1 1 1918 9 22 9 54 49.29 -1.698 98.298 15.0 0.0 0.0

ehb

FEQ

x

you can specify a newline in two ways

%

awk '{print "latitude:",$9

;

print

"longitude:",$10}’ SUMA. loc

%

awk '{print "latitude:",$9

”\n”

,”longitude:",$10}’ SUMA. loc

latitude: -1.698

longitude: 98.298Slide14

a trick

If a field is composed of both strings

and numbers,

you can multiple the field by 1 to remove the string.

%

head

test.tmp

1.5 2008/09/09 03:32:10 36.440N 89.560W 9.4

1.8 2008/09/08 23:11:39 36.420N 89.510W 7.1

1.7 2008/09/08 19:44:29 36.360N 89.520W 8.2

%

awk '{print $4,$4*1}'

test.tmp

36.440N 36.44

36.420N 36.42

36.360N 36.36Slide15

Selective execution

awk recognizes regular expressions and conditionals, which can be used to selective execute awk procedures on certain records

%awk –F”:” ‘

/root/

{ print $1, $3}’ /etc/

passwd

#

reg

expr

root

0

or within our example script

#!/

usr/bin/nawk

-f

/root/

{ print $1}Slide16

if

statements are also very useful

%

awk –F”:” ‘ {

if ($1==“root”)

print $1, $3}’ /etc/

passwd

root

0

or within our example script

{

if ($1==“root”)

{ print $1 }

note, this particular if syntax is a bit different from your reading, which suggested

%awk –F”:” ‘

$1==“root”

{print $1, $3}’ /etc/

passwd

the syntax I use is more explicit and more like C or

perl

, so I essentially have to remember less syntaxSlide17

Floating Point Arithmetic

awk does floating point math!!!!!

it stores all variables as strings, but when math operators are applied, it converts the strings to floating point numbers if the string consists of numeric characters

the reading calls this stringy variablesSlide18

Arithmetic Operators

All basic arithmetic is left to right associative

+ : addition

- : subtraction

* : multiplication

/ : division

% : remainder or modulus

^ : exponent

other standard C programming operators Slide19

Assignment Operators

= : set variable equal to value on right

+= : set variable equal to itself plus the value on right

-= : set variable equal to itself minus the value on right

*= : set variable equal to itself times the value on right

/= : set variable equal to itself divided by value on right

%= : set variable equal to the remainder of itself divided by the value on the right

^= : set variable equal to the itself to the exponent following the equal signSlide20

Unary Operations

A unary expression contains one operand and one operator

++ : increment the operand by 1

if ++ occurs after, $

x

++, the original value of the operand is used in the expression and then incremented

if ++ occurs before, ++$

x

, the incremented value of the operand is used in the expression

-- : decrement the operand by 1

+ : unary plus maintains the value of the operand,

x

=+

x

- : unary minus negates the value of the operand,

-1*

x

=-x! : logical negation evaluates if the operand is true (returns 1) or false (returns 0)Slide21

Relational Operators

Returns 1 if true and

0

if false

!!! opposite of bash test command

All relational operators are left to right associative

< : test for less than

<= : test for less than or equal to

> : test for greater than

>= : test for greater than or equal to

== : test for equal to

!= : test for not equal

Slide22

Boolean (Logical) Operators

Boolean operators return

1

for true and

0

for false

&& : logical AND; tests that both expressions are true

left to right associative

|

| :

logical OR ; tests that one or both of the expressions are true

left to right associative

! : logical negation; tests that expression is trueSlide23

Unlike bash, the comparison and relational operators don’t have different syntax for strings and numbers.

ie

: == in awk rather than == or

eq

using

testSlide24

Comparison Operators

~ : pattern match

!~ : pattern does not match

&& : logical AND

|| : logical OR

== : equals (numeric or string)

!= : does not equal (numeric or string)Slide25

Built-In Variables

FS: Field Separator

NR: record number is another useful built-in awk variable

it takes on the current line number, starting from 1

%awk –F”:” ‘ {

if (NR==1)

print $1, $3}’ /etc/

passwd

root

0

this is useful when headers are present in a fileSlide26

RS : record separator specifies when the current record ends and the next begins

default is “\n” or newline

useful option is “”, or a blank line

OFS : output field separator

default is “ “ or a whitespace

ORS : output record separator

default is a “\n” or newlineSlide27

NF : number of fields in the current record

think of this as awk looking ahead to the next RS to count the number of fields in advance

FILENAME : stores the current filename

OFMT : output format for numbers

example OFMT=“%.6f” would make all numbers output as floating pointsSlide28

Accessing shell variables in

nawk

3 methods to access shell variables inside a

nawk

script ...Slide29

1. Assign the shell variables to

awk

variables after the body of the script, but before you specify the input file

VAR1=3

VAR2=“Hi”

awk

'{print v1, v2}' v1=$VAR1 v2=$VAR2

input_file

3 Hi

Note that I am sneaking in the concept of

awk

variables here (v1,v2)Slide30

There

are a couple of constraints with this method

Shell variables assigned using this method are not available in the BEGIN section

If variables are assigned after a filename, they will not be available when processing that filename

awk

'{print v1, v2}' v1=$VAR1 file1 v2=$VAR2 file2

In this case, v2 is not available to

awk

when processing file1.Slide31

Also note:

awk

variables are referred to by just their name (no $ in front)

awk

'{print v1, v2, NF, NR}' v1=$VAR1 file1 v2=$VAR2 file2Slide32

2. Use the -

v

switch to assign the shell variables to

awk

variables.

This works with

nawk

, but not with all

flavours

of

awk

.

nawk

-

v

v1=$VAR1 -

v

v2=$VAR2 '{print v1, v2}'

input_fileSlide33

3. Protect the shell variables from

awk

by enclosing them with "'" (i.e. double quote - single quote - double quote).

awk

'{print "'"$VAR1"'", "'"$VAR2"'"}'

input_file