Shell Scripting

1.

Shell Scripting
What is Shell
Shell is a consistent buffer between the user and the Unix
system kernel
Shell is a program that reads the commands we type and
converts them into a form readable by the Unix system.
Shell is powerful tool that includes some fundamental
programming constructs that let to make decisions, loops, and
store values in variables

2.

Shell Scripting
Program Execution
$ <program-name> <arguments>
Variable and Filename
Substitution
* ? [...] $ABC
I/O Redirection
< > >>
Pipeline Hookup
$ <prog1> | <prog2>
What is Shell
Environment Control
home, path, prompt,
variables
Interpreted Programming
Language
built-in programming
interpreted language:
analyzing each statement
of one line at a time and
executes it.

3.

Shell Scripting
Basic Commands
Some commands:
date
who
whoami
echo
Some commands for working with files:
ls
cat
more (less)
wc
cp
mv
rm
touch
rename
find

4.

Shell Scripting
Basic Commands
Some commands for working with directories:
pwd
cd
ls
mkdir
cp
ln
rmdir
Current directory
.
Parent directory
..

5.

Shell Scripting
Basic Commands
Substitutions:
*
- any number of any symbol
?
- any symbol
[abc]
- any of “a”, “b” or “c”
[p-u]
- a symbol from range “p, q, r, s, t, u”
[!0-9]
- a symbol not from the range of “0...9”

6.

Shell Scripting
Standard Input and
Standard Output
standard input
standard output
command

7.

Shell Scripting
Standard Input and
Standard Output
Standard input:
$ wc -w
Text to input
<Ctrl-d>
3
$ sort
karen
zaven
aram
<Ctrl-d>
aram
karen
zaven

8.

Shell Scripting
Standard Input and
Standard Output
Standard output:
$ users
armen
karen
gevorg
sergey
$ echo Hello World!
Hello World!
$ pwd
/workspace/shahen/

9.

Shell Scripting
Standard Input and
Standard Output
Redirecting output:
$ echo Hello World! > hello.txt
tee command equal
$ pwd > my_path
$ ls > file_list
$ users >> hello.txt
$ pwd | tee file_name
$ /workspace/shahen
$ cat file_name
$ /workspace/shahen
to 1>&file_name

10.

Shell Scripting
Redirecting input:
$ wc -w < users
4
$ sort < users
armen
gevorg
karen
sergey
Standard Input and
Standard Output

11.

Shell Scripting
Using pipes:
$ ls | wc -w
9
$ users | sort
armen
gevorg
karen
sergey
Pipes and Filters

12.

Shell Scripting
Simple filter:
$ ls | grep user | sort -r
users
all_users
Pipes and Filters

13.

Shell Scripting
Standard Error
$ ls n*
ls: n*: No such file or directory
Redirecting standard error:
$ ls n* > error
ls: n*: No such file or directory
$ ls n* 2> error
0 – standard input
1 – standard output
2 – standard error

14.

Shell Scripting
Multiple commands:
More Than One
Command on a Line
$ date; pwd
Mon May 24 15:43:52 AMST 2005
/workspace/shahen/
$ sort all_users &
[1] 7354
$ command1 && command2
$ command1 || command2
Control operator &&:
command2 is executed only if command1 returns an exit status of zero
Control operator ||:
command2 is executed only if command1 returns a non-zero exit status

15.

Shell Scripting
Processes
List of processes:
$ ps -e
6840
6844
6882
6894
6899
6947
7086
30112
30456
?
?
?
?
?
?
pts/4
pts/6
pts/6
00:00:02
00:00:05
00:00:00
00:00:00
00:00:57
00:01:26
00:00:00
00:00:00
00:00:00
gaim
skype
firefox
run-mozilla.sh
firefox-bin
soffice.bin
bash
bash
ps

16.

Variables
Shell Scripting
Shell variable begins with an alphabetic or underscore
character and is followed by zero or more alphanumeric or
underscore characters
$ ABC=1291
$ _2=”test data”
$ echo $ABC
1291
$ echo $ABC_bugs
$ echo ${ABC}_bugs
1291_bugs
<WRONG!>
_

17.

Shell Scripting
Filename Substitution and Variables
$ X=*
$ echo $X
afile all_users badfiles err file1
file2 filea fileb garbage users
$ echo ”$X”
*
Shell does not perform filename substitution
when assigning values to variables
Variables

18.

Shell Scripting
Indirect expansion
$ X=123
$ Y=X
$ echo $Y
X
$ echo ${!Y}
123
Variables

19.

Variables
Shell Scripting
Built-in Integer Arithmetic
$((expression))
let variable=expression
$ echo $((2+3))
5
$ let A=4+5
$ echo $A
9
$ echo $((A*3))
27
$ result=$(( A >= 0
$ echo $result
1
&&
A <= 100 ))

20.

Shell Scripting
Variables
Command substitution
$ U=users
$ echo `$U` # similar to echo `users`
karen sergey gevorg
$ `which python` --version
Python 2.7.6
$ NOW=$(date)
$ echo $NOW
Mon May 24 16:04:12 AMST 2005
$ echo "we have $(ls | wc -l) files"
we have 10 files
`command`
$(command)

21.

Shell Scripting
Variables
Single quotes and double quotes
$ X=*
$ echo $X
$ echo '$X'
$X
$ echo "$X"
*
$ Y="'Hello', he said"
$ echo $Y
'Hello' he said
$ Z='”Hi”, she answered'
$ echo $Z
”Hi”, she answered
Single quotes:
shell ignores all
characters enclosed
Double quotes:
shell ignores most
characters but:
• Dollar signs
• Back quotes
• Backslashes

22.

Shell Scripting
Backslash
$ echo
syntax
$ echo
>
$ X=*
$ echo
$X
Variables
Removes special meaning
>
error near unexpected token `newline'
\>
\$X
$ LINES=one\
> two
$ echo "$LINES"
onetwo
Used for continuing lines

23.

Shell Scripting
Scripts
Script file ”usr_search”
#!/bin/bash
# let's search users by name
cd data
cat users.list | grep armen
The first line
#!/bin/bash
is not
a comment!
cd ..
$ chmod +x usr_search
Changing mode
(attribute) of the file
to executable

24.

Decisions
Shell Scripting
Exit status
$? variable
$ cp users users.old
$ echo $?
0
If a program completes
execution, it returns
an exit status back to
the system.
$ cp list backup
No such file or directory
$ echo $?
1
This status is a number
that indicates whether
the program successfully
ran.
zero
nonzero
- succeeded
- failed

25.

Shell Scripting
Decisions
”if” statement
if <test_command>
then
<command1>
<command2>
...
else
<command3>
<command4>
...
fi
The command
<test_command>
is executed and its
exit status is tested
If the status is zero
the commands:
<command1>
<command2>
...
executed

26.

Shell Scripting
”if” statement
$ if who | grep sergey > /dev/null
> then
> echo "Hi, Sergey"
> fi
Hi, Sergey
Decisions

27.

Shell Scripting
Decisions
”test” command
test <expression>
or
[ <expression> ]
$
$
$
0
$
$
0
USER=armen
test $USER = armen
echo $?
[ $USER != karen ]
echo $?
( expression )
expression is true
! expression
expression is false
expression1 -a expression2
both
expression1 and expression2
are true
expression1 -o expression2
expression1 or expression2
is true

28.

Shell Scripting
Decisions
”test” command
string1 = string2
strings are equal
integer1 -eq integer2
integer1 is equal to integer2
integer1 -ge integer2
integer1 is
greater than or equal
to integer2
integer1 -gt integer2
integer1 is greater than integer2
string1 != string2
strings are not equal
integer1 -le integer2
integer1 is less than or equal
to integer2
integer1 -lt integer2
integer1 is less than integer2
integer1 -ne integer2
integer1 is not equal to integer2

29.

Shell Scripting
Decisions
”for” statement
for <var> in <word1> <word2> ... <wordN>
do
<command1>
<command2>
...
done
$ for i in 1 2 3
> do
> echo $i
> done
1
2
3

30.

Shell Scripting
$* - all arguments
#!/bin/bash
# my_cmd script
for FILE in $*
do
cat $FILE | grep karen
done
$ my_cmd users all_users
1:karen
22:karena
93:karensh
Decisions

31.

Arguments
Shell Scripting
$# - number of arguments
$1 - first argument, $2 - second argument . . .
#!/bin/bash
# args script
echo total: $#
for A in $*
do
echo $A
done
echo arg1: $1
echo arg2: $2
$ ./args a b c
total: 3
a
b
c
arg1: a
arg2: b
$

32.

Decisions
Shell Scripting
”while” and ”until” statements
while <test_command>
do
<command1>
<command2>
...
done
while command continues
execution as long as
<test_command>
returns a zero exit status
until <test_command>
do
<command1>
<command2>
...
done
until command continues
execution as long as
<test_command>
returns a non-zero exit status

33.

Decisions
Shell Scripting
”case” statement
case <value> in
<pat1>)
<command1>
...
<command2> ;;
<pat2>)
<command3>
...
<command4> ;;
...
esac
case
in
0)
1)
2)
3)
4)
5)
6)
7)
8)
9)
esac
"$1"
echo
echo
echo
echo
echo
echo
echo
echo
echo
echo
zero;;
one;;
two;;
three;;
four;;
five;;
six;;
seven;;
eight;;
nine;;

34.

Shell Scripting
”read” command
read variables
read X Y Z
read TEXT
Input
Shell reads a line from
standard input and assigns
the first word read to the
first variable, the second
word read to the second
variable,
and so on.
If there are more words on
the line than there are
variables listed,
the excess words get
assigned to the last variable

35.

Shell Scripting
Output
”printf” command
printf "<format>" <arg1> <arg2> ... <variables>
$ printf "This is a number: %d\n" 10
This is a number: 10
d Integers
u Unsigned integers
o Octal integers
x Hexadecimal integers, using a-f
c Single characters
s Literal strings

36.

Shell Scripting
Any Character
.
Beginning of Line
^
End of Line
$
Choice of Characters
[...]
Zero or More Characters
*
[A-Za-z][A-Za-z]* [-0-9] [a-z]
Regular Expressions

37.

Shell Scripting
Decisions
”$$” pid parent shell
“$!” pid last process running in background mode
$
$
$
$
$
$
myscript &
17865
echo $!
17865
echo $$
17860

38.

Shell Scripting
Decisions
function declaration
function myFunc ()
{
local a=9
return $(($a + $1))
}
Sum ()
{
b=3
return $(($1 + $2))
}
$ myFunc 10
$ 19
$ Sum 12 56
$ 68
“a” declared as a local
variable in “myFunc”
function
“b” declared as a global
variable in “Sum” function

39.

Shell Scripting
sed
$
$
$
$
and
Decisions
awk commands
sed “s/EXPORT/export/g” file_name > new_file_name
a=“ararat 1973”
echo $a | sed ‘s/\([a-z]*\).*/\1/g’
ararat
$ echo $a | awk ‘{print $2}’
$ 1973
$ echo $a | awk ‘{if ($1 == “ararat”) print $2}’