Advanced UNIX BASH

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Advanced UNIX BASH

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explain how to write Bourne and Bash Shell scripts. Overview. 1. Making a File Executable ... echo Definitely Bourne Shell Script. continued ... – PowerPoint PPT presentation

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Title: Advanced UNIX BASH

1
Advanced UNIX (BASH)

Objectives
explain how to write Bourne and Bash Shell scripts

2
Overview

1. Making a File Executable
2. Combining Commands
3. Redirecting Output
4. Executing Scripts
5. Variables
6. Control Flow

continued
3

7. Functions
8. Other Commands
9. Here Documents
10. Debugging
11. More Information

4
1. Making a File Executable

cat whosondateecho Users currently logged
onwho
Wrong
whosonwhoson Permission denied

5
Right

ls -lg whoson-rw-r--r-- 1 ad pubs 42 Jun 17
1055 whoson chmod ux whoson ls -lg
whoson-rwxr--r-- 1 ad pubs 42 Jun 17 1055
whoson
whosonTue Nov 7 132134 ICT 2000Users
currently logged inad consol Nov 7
0826jenny tty02 Nov 7 1004

6
Possible PATH Problem

whosonwhoson Command not found
Due to PATH shell variable (see later)
Quick fixes ./whoson or sh whoson

7
2. Combining Commands

Sequencing
a b c
same as
a b c

8
Process Creation ()

a b c14271 (PID for a)14272 (PID for
b)
a b c 142901429114292
a b c 14302 (PID for piped commands)

9
Processes in Action

cat aecho -n aaaaaaaaaaaaaaaaaaaaaecho -n
aaaaaaaaaaaaaaaaaaaaasleep 2echo -n
aaaaaaaaaaaaaaaaaaaaaecho -n
aaaaaaaaaaaaaaaaaaaaa
Similarly for b and c
Try the following a few times
a b c

On the lab machines there isn't much variation
unless the machine is loaded.
10
3. Redirecting Output

1gt redirect standard output (stdout)2gt redirect
standard error (stderr)
cat a b 1gt out 2gt err

Files
stdout
out
cat a b
err
stderr
11
gt

redirect one stream into another
2gt1 redirect stderr into stdout
cat a b 1gt theLot 2gt1

theLot
stdout
cat a b
stderr
12
4. Executing Scripts

Make sure that a script is executed by the Bourne
Shell
sh whoson (no need for chmod)
or
cat boss!/bin/shecho Definitely Bourne
Shell Script

continued
13

On Linux machines (e.g. calvin), the Bourne shell
has been replaced by Bash
sh means the Bash shell

14
5. Variables

5.1. User-defined Variables
5.2. Environment Variables
5.3. Readonly Shell Variables

15
5.1. User-defined Variables
No spaces around the

personalex echo personperson echo
personalex
var returns the value stored in var
called substitution

16
5.1.1. Switch off Substitution

Swich off substitution with 'var' or \var
echo 'person'person echo \personperson

17
5.1.2. Switch off Special Chars (")

"" switches off the special meaning of characters
used in filename generation (e.g. , ?)
ls // directory contentsad.reportad.summary
memoad echo "memo"ad echo
memoad.report ad.summary

means only
means any number of characters
18
5.1.3. Exporting Variables

Normally a variable is local to the running
script (the process).
It is sometimes useful if running scripts
(processes) can access another scripts
variables.
e.g.

we want to use cheese in subtest
calls
extest
subtest
cheeseenglish
19
No Exporting extest1 subtest
Using " is a good habit, see later

cat extest1cheeseenglishecho "extest1 1
cheese"subtestecho "extest1 2 cheese"cat
subtestecho "subtest 1 cheese"cheeseswissech
o "subtest 2 cheese"

continued
20
subtest does not see extest1's cheese value

extest1extest1 1 englishsubtest 1 subtest
2 swissextest1 2 english

extest1 is not affected by subtest's setting
of cheese
21
Exporting extest2 subtest

cat extest2export cheesecheeseenglishecho
extest2 1 cheesesubtestecho extest2 2
cheese
extest2extest2 1 englishsubtest 1
englishsubtest 2 swissextest2 2 english

cheese value passed in
change not exported from subtest
22
5.1.4. Reading
read inputs everything up to the newline

cat readlnecho -n Type read lnecho You
entered ln
readlnType The Wizard of OzYou entered The
Wizard of Oz

23
No Quotes

cat readlnnqecho -n Type read lnecho You
entered ln
lsad.report summary1 readlnnqType You
entered ad.report summary1

directory contents
24
5.1.5. Executing Commands
A very simple shell

cat proc_cmdecho -n Enter a command read
commandcommandecho Thanks
proc_cmdEnter a command echo Display
thisDisplay thisThanks

25
5.1.6. Splitting Input
Text is split based on white space.

cat split3echo -n Enter something read
word1 word2 word3echo Word 1 is word1echo
Word 2 is word2echo Word 3 is word3
split3Enter something this is somethingWord1
is thisWord2 is isWord3 is something

split3Enter something this is something else,
xWord1 is thisWord2 is isWord3 is something
else, x

The last variable gets everything that is left in
the input.
27
5.1.7. Command Subsitution
Must use

cat mydirthis_dirpwdecho Using the
this_dir directory.this_date(date)echo
"Today's date this_date"
mydirUsing /home/ad/teach/adv-unix/bourne
directoryToday's date Tue Nov 7 135246 ICT
2000

A Bash addition
28
5.2. Environment Variables

Most environment variables get their values from
the shell at login.
The values of some of the variables can be set by
editing the .profile file in your home directory
Bash uses .bash_profile and .bashrc

29
5.2.1. Examples

HOME pathname of your home directory
pwd/home/ad/planning echo HOME/home/ad
cd pwd/home/ad

cd uses HOME to return to your home directory
continued
30

PATH
directories where executable can be found
represented as a string of pathnames separated by
s
echo PATH/usr/local/bin/usr/bin/bin
PATHSPATH"/home/ad/bin." echo
PATH/usr/local/bin/usr/bin/bin/home/ad/bin.

Extend the default PATH
31
Note for SysAdmins

If you are the system administrator (superuser,
root) for your machine, do not extend your path
with "."
it opens you to potential attack by hackers
e.g. 'fake' UNIX utilities placed in the current
directory

32
5.2.2. Typical .profile

cat .profileTERMvt100PATHPATH"/home/ad/bin
."PS1ad CDPATHHOMEexport TERM PATH PS1
CDPATHstty kill u
. .profile

export needed in the Bourne shell
33
5.2.3. Typical .bash_profile

On calvin, .bash_profile simply invokes your
.bashrc (if it exists)
umask 002if -f /.bashrc -a PS1"\ "
then . /.bashrcfi

These shell commands will be explained later
continued
34
Typical .bashrc

PS1"\u_at_\h " PS1"\w\ "PATHPATH"."ali
as ls'/bin/ls -F'alias dir'ls -ba'alias
cls"clear" psgrep() ps aux grep 1
grep -v grep

No export needed
These features will be explained later.
35
5.2.4. set

The current settings for the environment
variables can be listed with set
set moreBASH/bin/bash PATH/usr/local/b
in/usr/bin/bin. PS1'\u_at_\h '

36
5.3. Readonly Shell Variables

These are environment variables that cannot have
their values changed.
Most of them relate to the arguments supplied to
a script when it is called.

37
5.3.1. Script Name (0)

cat abcecho The name of this script is 0
abcThe name of this script is abc

38
5.3.2. Script Arguments (1, 2,..., 9)

cat display_5argsecho The first five command
lineecho arguments are 1 2 3 4 5
display_5args jenny alex helenThe first five
command linearguments are jenny alex helen

If the variable has no value, then nothing is
printed.
39
5.3.3. All Arguments ()

cat display_allecho
display_all a b c de fg hi jk mno pqr stu
w x y za b c de fg hi jk mno pqr stu w x y z
_at_ is like but puts ... around each printed
argument

40
5.3.4. Number of Arguments ()

cat num_argsecho This script has
arguments.
num_args helen alex jennyThis script has 3
arguments

41
5.3.5. The shift Commnd

shift moves argument values on the command line
one ltnumbergt to the left.
Overcomes limit of 9 argument variables(1, 2,
..., 9)

continued
42

cat demo_shiftecho arg1 1 arg2 2 arg3
3shiftecho arg1 1 arg2 2 arg3
3shiftecho arg1 1 arg2 2 arg3
3shift
demo_shift alice helen jenny junearg1 alice
arg2 helen arg3 jennyarg1 helen arg2 jenny
arg3 june arg1 jenny arg2 june arg3

jenny "moves" to the left.
43
5.3.6. The set Command (Again)

set cmd (must use )
evaluates cmd and assigns its values to the
script command line arguments (1, 2, ..., 9,
)
the values in cmd output are separated by
whitespace

continued
44

dateFri Jun 17 230409 GMT7 1996 cat
datesetset dateecho echoecho Argument 1
1echo Argument 2 2echo Argument 3
3echo 2 3, 6

The date values are assigned to 1, 2, etc.
continued
45

datasetFri Jun 17 230413 GMT7
1996Argument 1 FriArgument 2 JunArgument 3
17Jun 17, 1996

46
6. Command Flow

6.1. Branching
6.2. Test Forms
6.3. Looping
6.4. break, continue,
6.5. trap

47
6.1. Branching

cat same_wordecho -n word 1 read
word1echo -n word 2 read word2if test
"word1" "word2"then echo Matchfiecho End
of Program

Use " to stop filename expansion
continued
48

same_wordword1 peachword2 peachMatchEnd
of Program

49
6.1.1. Second Format
Redirect stdout to stderr

cat chkargsif 0 then echo Usage
chkargs argument... 1gt2 exit 1fiecho Program
runningexit 0
chkargsUsage chkargs argument... chkargs
abcProgram running

50
6.1.2. if-then-else
C programmers prefer this format

cat showif 0 then echo Usage show
-v filenames 1gt2 exit 1fiif 1 ! -v
then cat _at_else shift more _at_fi

Use show -v f1.txt f2.txt
51
6.1.3. if-then-elif
For multiway branches

cat same3echo -n word 1 read word1echo -n
word 2 read word2echo -n word 3 read
word3if word1 word2 -a \ word2
word3 then echo Match words 1, 2, and 3

-a means "and" \ means "continued on next line"
continued
52

elif word1 word2 then echo Match
words 1 and 2elif word1 word3 then
echo Match words 1 and 3elif word2
word3 then echo Match words 2 and
3else echo No matchfi

53
6.1.4. case
Better style specify the shell, and comment the
code.

cat command_menu!/bin/sh menu interface to
simple commandsecho \n COMMAND MENU\necho
a. Current date and timeecho b. Users
currently logged inecho c. Name of working
directoryecho d. Contents of working
directoryecho -n Enter a, b, c, or d read
answerecho

continued
54

case answer in a) date b) who
c) pwd d) ls -C
) echo answer not legal
esacecho

is the default always include it at the end
55

command_menu COMMAND MENU a.
Current date and time b. Users currently logged
in c. Name of working directory d. Contents
of working directoryEnter a, b, c, or d aFri
Jun 17 141157 GMT 1996

56
Other case Patterns

? matches a single character
... any character in the brackets. Use - to
specify a range (e.g. a-z)
or (e.g. aA)
it can be used to match "any number of
characters"

cat timeDay!/bin/shecho Is it morning? Answer
yes or noread timeofdaycase "timeofday" in
"yes" "y" "Yes" "YES" ) echo "Good
Morning" nN ) echo "Good
Afternoon" ) echo "Uhh??" esac

58
6.2. test Forms
Used in the conditions of if's and loops.

Format
test expr
expr
e.g.
test word1 word2
1 ! -v
word2 word3
word1 word2 -a \ word2
word3

59
6.2.1. String Expressions

string1 string2
string1 ! string2
-n string (true if string is not )
-z string (true if string is )

60
6.2.2. Numerical Expressions

number1 -eq number2 (equality)
number1 -ne number2 (inequality)
number1 -lt number2 (lt)
number1 -le number2 (lt)
number1 -gt number2 (gt)
number1 -ge number2 (gt)

61
6.2.4. File Test Expressions

-f file (file exists)
-d file (file exists but is a directory)
-r file (file is readable)
-w file (file is writable)
-x file (file is executable)

62
6.2.5. Combining Expressions

! expr (not expr)
expr1 -a expr2 (and)
Bash allows as well
expr1 -o expr2 (or)
Bash allows as well
( expr )

63
6.3. Looping (for-in)

Format
for loop-index in argument-listdo
commandsdone

cat fruitfor fruit in apples oranges pearsdo
echo fruitdoneecho Task completed
fruitapplesorangespearsTask completed

65
Looking for for in files

First version of script
cat file-fors1for f in lsdo echo fdone

cat file-fors2for f in lsdo grep -i for
f gt /dev/null if ? 0 then echo
f fidone

Ignore output
67
6.3.1. for

cat whos Give user details from
/etc/passwdif 0 then echo Usage whos
id... 1gt2 exit 1fifor i read as for i in
_at_do awk -F print 1, 5 /etc/passwd
grep -i idone

awk variables with
68
6.3.2. while

cat countnumber0while number -lt 10 do
echo -n "number" numberexpr number
1doneecho
count0123456789

69
Watching for Mary to log in

while sleep 60do who grep marydone
Disadvantages
if Mary is already logged in, then we must wait
60 secs to find out
we keep being told that Mary is logged in

70
6.3.3. until

Format
until commanddo loop body until the
command evaluates to truedone
Watching (v.2)
until who grep marydo sleep 60done

loop until grep returns true (0)
71
6.3.4. Extending Watching

Generalise so can watch for anyone
watchfor ad
Watch for everyone logging in/out
watchwho

72
watchfor

!/bin/sh watchfor watch for person supplied
as argumentif 0 then echo Usage
watchfor person exit 1fiuntil who grep
1do sleep 60done

73
watchwho

Once a minute, run who and compare its output to
that from a minute ago. Report any differences.
Keep the who output in files in /tmp
Give the files unique names by adding the shell
variable .
is the PID of the users shell

!/bin/sh watchwho watch who logs in and
outnew/tmp/wwho1.old/tmp/wwho2.gt old
create an empty filewhile truedo who gt
new diff old new mv new old sleep
60done awk /gt/ 1 in print /lt/
1 out print

An advanced Shell programming technique
75
Use
initial users

watchwhoin root tty1 Nov 6 0932in ad pts/3
Nov 8 0849 (myrrh.coe.psu.ac.th)in s4010441
pts/5 Nov 8 1011 (192.168.0.134)in ad pts/4
Nov 8 1012 (myrrh.coe.psu.ac.th)in s4010143
pts/17 Nov 7 2357 (192.168.0.204)out ad pts/4
Nov 8 1012 (myrrh.coe.psu.ac.th)in ad pts/4
Nov 8 1016 (myrrh.coe.psu.ac.th)out ad pts/4
Nov 8 1018 (myrrh.coe.psu.ac.th)

76
Notes

diff uses lt and gt to distinguish data from
old and new
diff old newlt adgt mary

old
new
johnad
johnmary
continued
77
while loop
awk

The output from the while loop is piped into awk
only one call to awk is required
the "pipe" programming style
A calvin problem
awk had to be called with the -W interactive
option so that its output was not buffered
otherwise nothing would appear

78
6.3.5. Checking mail

Have a script watch your mailbox periodically,
and report whenever the mailbox changes by
printing You have mail.
Usage
checkmail checks every 60 secs
checkmail 120 checks every 120 secs

79
checkmail

!/bin/sh checkmail watch mailbox for
growthtime1-60oldls"ls -l MAIL"while
truedo newls"ls -l MAIL" echo oldls
newls oldls"newls" sleep timedone awk
5 lt 14 print You have mail

uses the pipe technique
80
Notes

MAIL is a builtin shell variable, with the value
/var/spool/mail/USER
t1-60 sets t to 1 or, if no argument is
provided, to 60
General form var-thingreturns value of var
if defined otherwise thing

continued
81

Use awk to print a message only when the mailbox
gets bigger
awk 5 14 compares the size fields of the two ls
-l calls output at the end of each iteration
e.g. ls -l foo-rw-r--r-- 1 ad ad 34512 Nov
13 1996 foo ls -l foo-rw-r--r-- 1 ad ad 34512
Nov 13 1996 foo

5th value
14th value
82
6.4. break, continue,

break and continue are used as in C
break escapes from a loop
for file in freddo if -d "file" then
deleted? break finish loop fi
do something done

continued
83

continue goes to the top of a loop
for file in freddo if -d "file" then
deleted? continue go back to loop top
fi do something done

continued
84

The '' command is the same as true, but runs a
tiny bit faster
often used to simplify control logic
if -f fred then is fred a file?
do nothingelse do something fi

85
6.5. trap

Capture user interrupts or system call failures.
Format (with )
trap commands signal-numbers-or-name
Some signal numbers (names)
2 (INT) press delete or control-C
3 (QUIT) press control- or control-\
15 (TERM) kill command signal

continued
86

A complete list of available signals is given by
typing trap -l at the command line.
To ignore a signal, set the trap command to be
empty ()
To reset signal processing, set the command to -

continued
87

cat intertrap echo PROGRAM INTERRUPTED exit
1 INTwhile truedo echo Program running.
sleep 2done

88
7. Functions

Bash allows shell scripts to use functions
function_name() statements
Functions must be defined textually before they
are used.

continued
89

Functions can return integer values
Function parameters are passed by modifying ,
_at_, , 1 -- 9 while the function is
executing.
Local variables are defined with the local
keyword
the other variables in a script are global by
default

90
my_name

!/bin/shyes_or_no() a function echo
"Is your name ?" while true do echo -n
"Enter yes or no " read x case "x" in
y yes ) return 0 n no ) return
1 ) echo "Answer yes or no"
esac done end of yes_or_no()

continued
91