Programming - Linux Scripting

Front 1

$man bash

Back 1

displays the linux bash shell manual

Front 2

$echo $?

Back 2

displays exit status (0 = ok, other=error)

Front 3

Wild Cards

*
?
[..]

Back 3

* matches any number of characters
? matches exactly one character
$ ls /bin/[a-c]* - will find any files in the bin directory that begins with an a, b or c and is followed by anything else.
! or ^ - does not match expression that follows

Front 4

adding more then one command on a line

Back 4

separate commends with a semicolon

date;who

Front 5

read

Back 5

accepts input from user and stores it in variable.

read fname:

Example: read user_name

Front 6

$#

Back 6

holds number of arguments specified on command line

Front 7

$* or $@

Back 7

refer to all arguments passed to script. Or $1..$9 refers to the 1st thru the 9th parameters passed.

Front 8

$0

Back 8

Contains the name of the script.

Front 9

Comparing String Variable to Literal

Back 9

userName=""
targetName="Jack"
echo -n "Enter your name: "
read userName
if [ $userName == "Jack" ]
then
echo "Hello $userName"
else
echo "Go away $userName"
fi

Front 10

chmod a+rx filename

or

chmod 755 filename

Back 10

makes a file executable, by setting the executable bit, and readable by setting the read bit.

Front 11

#!/bin/sh

Back 11

tells Unix that the file is to be executed by /bin/sh. This is the standard location of the Bourne shell on just about every Unix system. If you're using GNU/Linux, /bin/sh is normally a symbolic link to bash.

Front 12

mkdir rc{0,1,2,3,4,5,6,S}.d

or

for runlevel in 0 1 2 3 4 5 6 S
do
mkdir rc${runlevel}.d
done

Back 12

creates multiple directories rc0..rcS

Front 13

while loop

Back 13

while [ condition ]
do
...
executable code
...
done

Example:

while [ "$user_name != "Quit" ] (note space)
do
read user_name
echo $user_name
done

Front 14

for loop

Back 14

for i in <set of values>
do
...
executable code
...
done

Hint 14

Examples:

i=1
for day in "Mon Tue Wed Thu Fri"
do
echo "Weekday $((i++)) : $day"
done


i=1
weekdays="Mon Tue Wed Thu Fri"
for day in $weekdays
do
echo "Weekday $((i++)) : $day"
done


i=1
for day
do
echo "Weekday $((i++)) : $day"
done

i=1
for username in `awk -F: '{print $1}' /etc/passwd`
do
echo "Username $((i++)) : $username"
done


i=1
cd ~
for item in *
do
echo "Item $((i++)) : $item"
done


i=1
for file in /etc/[abcd]*.conf
do
echo "File $((i++)) : $file"
done

Front 15

if statement

Back 15

if [ ... ]
then
# if-code
else
# else-code
fi

Front 16

if else elsif statement

Back 16

if [ something ]; then
echo "Something"
elif [ something_else ]; then
echo "Something else"
else
echo "None of the above"
fi

Front 17

awk '{ print $0 }' /etc/passwd

Back 17

Prints the entire line of the file /etc/passwd. In awk, when a print command appears by itself, the full contents of the current line are printed. So awk '{ print $0 }' /etc/passwd and awk '{ print } do exactly the same thing.

Front 18

$0 (in awk)

Back 18

Represents the entire current line. So awk '{ print $0 }' /etc/passwd and awk '{ print }' do exactly the same thing.

Front 19

AWK Variables

Back 19

awk variables are initialized to either zero or the empty string the first time they are used.
Variables
Variable declaration is not required
May contain any type of data, their data type may change over the life of the program
Must begin with a letter and continuing with letters, digits and underscores
Are case sensitive
Some of the commonly used built-in variables are:

NR -- The current line's sequential number
NF -- The number of fields in the current line
FS -- The input field separator; defaults to whitespace and is reset by the -F command line parameter
FS - Input Field Seperator
RS - Input Field Seperator
OFS - Output Field Separator
ORS - Output Record Separator

Front 20

write shell script to read a text file with multiple columns and then create a new file with the columns in reverse order.

Back 20

awk -F, '{ print $3","$2","$1 }' names.txt > names2.txt

Hint 20

first_name,last_name,age

to

age,last_name,first_name

Front 21

awk OFS

Back 21

Output Field Seperator

Hint 21

OFS=\;

sets the output field separator to a semicolon ;

Front 22

create a output file called dataout.txt using input from file data.txt. creating new data field layout separated by semicolons. new data fields are created fom substrings of original datafields as follows:

Column1 contains first six characters of first number in text file
Column2 contains last five characters of first number in text file
Column3 contains complete semicolon separated number of text file
Column4 contains first six characters of last number in text file
Column4 contains last four characters of last number in text file

Back 22

awk '{ print substr($1,1,6), substr($1,7,11),$2,substr($3,1,6)
, substr($3,7,11)}' OFS=\; data.txt > dataout.txt

Front 23

awk \t

Back 23

awk tab

Hint 23

Example:

print $1"\t"$3"\t"$4"\t"$6;

Front 24

awk FILENAME

Back 24

gives the file name of the file awk is processing.

Hint 24

awk BEGIN {