Shell Scripting

1
Shell Scripting

• WeeSan Lee ltweesan_at_cs.ucr.edugt
• http//www.cs.ucr.edu/weesan/cs183/

2
Roadmap

• Introduction
• Prompt Alias
• How to Create a Shell Script?
• Sharp-Bang Comments
• Always Use Absolute Pathname!
• Command Separator , ,
• Backquote/Backtick
• Variables
• Positional Parameters
• Misc.
• References

3
Introduction

• Shell
• A command interpreter
• For example
• sh, bash, csh, tcsh, kosh, zsh,
• Shell Script
• Whole bunch of Unix commands saved into a text
  file with flow control
• Your shell scripting skills depends on
• Unix commands you know
• how well you put them together to do the right
  thing

4
Prompt Alias

• PS1"\u_at_\h\w \
• \u the username of the current user
• \h the hostname
• \w the current working directory
• \ if the effective UID is 0, a , otherwise a
• alias v'ls -l --colorauto
• /.bashrc
• After editing
• source /.bashrc

5
How to Create a Shell Script?

• Edit hello.sh
• !/bin/sh
• echo "Hello World!"
• Make hello.sh an executable
• chmod x hello.sh
• Run hello.sh
• ./hello.sh
• sh hello.sh
• Debug hello.sh
• sh -x hello.sh

6
Sharp-Bang (!) Comments

• Which shell to run the script?
• ! Sharp-Bang, a two-byte magic number
• cat hello.sh
• !/bin/sh
• My first shell script
• echo "Hello World!

7
Always Use Absolute Pathname!

• cat cal.sh
• !/bin/sh
• cal 2008
• cat cal
• !/bin/sh
• /usr/bin/bash
• cat cal2.sh
• !/bin/sh
• CAL/usr/bin/cal
• CAL 2008

8
Command Separator, ,

• cd /var/log tail message
• alias todo'echo cat -n /TODO echo
• cd /var/log
• echo "Cannot change to /var/log"
• exit 1
• cat or.sh
• !/bin/sh
• -e 1 exit 1
• /usr/bin/less 1
• lpr file.tmp rm file.tmp

9
Backquote/Backtick

• echo "The date is date"
• wget cat z.txt
• for i in cat kilo.txt do
• echo i has ssh i who wc -l users
• done

10
Exit Status

• Each Unix command returns a exit status (?)
• Unlike C, 0 means true, false otherwise
• /bin/true
• echo ?
• 0
• cat abc
• echo ?
• 1
• touch abc
• cat abc
• echo ?
• 0

11
Exit Status

• Return a exit status using exit built-in
  command
• cat exit2.sh
• !/bin/sh
• exit 2
• exit2.sh
• echo ?
• 2
• exit-1.sh
• !/bin/sh
• exit -1
• echo ?
• 255

12
Exit Status

• The script returns the status of the last
  executable
• cat exit3.sh
• !/bin/sh
• /usr/bin/who /bin/false
• exit3.sh
• echo ?
• 1

13
Variables

• a3
• Define a variable a
• echo a
• echo a
• echo "a a"
• a is a reference to its value
• a 3
• run a with 3 as parameter
• a 3
• run 3 with empty environment a
• a"a b c"
• echo a
• echo "a"
• Double quoting a variable preserves whitespaces
• echo 'a'
• Single quoting a variable disables var.
  referencing

• a
• Set a to a null value
• unset a
• adate
• echo a
• als -l
• Can assign anything to a variable
• echo a
• echo "a"
• Double quotes preserves formatting
• echo "'a' is a"
• echo "\"a\" is a"
• echo "\a is a"

14
Variables

• cat user.sh
• /bin/sh
• NUM"who cut -d' ' -f1 sort uniq wc -l"
• echo "hostname has NUM users."

15
Positional Parameters

• cat param.sh
• !/bin/sh
• echo "Program 0"
• echo " of parameters "
• echo "\1 1"
• echo "\2 2"
• echo "\3 3"
• echo "\10 10"
• echo "\_at_ _at_"
• echo "\ "

• param.sh seq 1 10
• Program ./param.sh
• of parameters 10
• 1 1
• 2 2
• 3 3
• 10 10
• _at_ 1 2 3 4 5 6 7 8 9 10
• 1 2 3 4 5 6 7 8 9 10
• echo
• Current process ID

16
Positional Parameters Soft-link Trick

• ln -s param.sh newcmd.sh
• newcmd.sh seq 1 10
• Program ./newcmd.sh
• of parameters 10
• 1 1
• 2 2
• 3 3
• 10 10
• _at_ 1 2 3 4 5 6 7 8 9 10
• 1 2 3 4 5 6 7 8 9 10

17
Branching

• Syntax
• if ltconditiongt then
• ltstmtsgt
• elif ltconditiongt then
• ltstmtsgt
• else
• ltstmtsgt
• fi

18
How to check if a given path is a file or a
directory?

• test.sh /etc/foo
• /etc/foo does not exist.
• test.sh /etc/passwd
• /etc/passwd is a file.
• test.sh /etc
• /etc is a directory.

19
How to check if a given path is a file or a
directory?
If -ne 1 then echo "Usage 0 file"
exit 1 fi

• !/bin/sh
• if test -e 1 then
• if -f 1 then
• echo "1 is a file."
• elif -d 1 then
• echo "1 is a directory."
• else
• echo "1 has an unknown type."
• fi
• else
• echo "1 does not exist."
• fi

-eq 1 echo "Usage 0 file"
exit 1
20
File Test Operators

• -e file exists
• -f regular file
• -s file is not zero size
• -d directory
• -b block device
• -c char. device
• -L symbolic link
• -r file has read permission
• -w write
• -x execute

21
Comparison Operators

• !/bin/sh
• a1
• b2
• if "a" -ne "b" then Numerical
  comparision 1 ! 2
• echo "a and b are not equal"
• else
• echo "a and b are equal"
• fi
• if "a" ! "b" then String comparision
  "1" ! "2
• echo "a and b are not equal"
• else
• echo "a and b are equal"
• fi

22
Comparison Operators

• Integer Comparison Operators
• -eq is equal to
• -ne is not equal to
• -gt is greater than
• -ge is greater than or equal to
• -lt is less than
• -le is less than or equal to
• String Comparison Operators
• is equal to
• same as
• ! is not equal to
• -n string is not null
• -z string is null

23
-n example

• !/bin/sh
• Wrong, should do 1
• if -n 1 then
• echo "\1 is non-null"
• else
• echo "\1 is null"
• fi
• Correct
• if -n "1" then
• echo "\1 is non-null"
• else
• echo "\1 is null"
• fi

24
Case Statement

• Syntax
• case "var" in
• value1)
• ltstmtsgt
• value2)
• ltstmtsgt
• )
• ltstmtsgt
• esac

25
Case Statement - an example

• !/bin/sh
• echo -n "Hit a key, then hit Enter "
• read key
• case "key" in
• abcdefghijklmnopqrstuvw
  xyz)
• echo "key is a lower case."
• ABCDEFGHIJKLMNOPQRSTUVW
  XYZ)
• echo "key is an upper case."
• 0123456789)
• echo "key is a number."
• )
• echo "key is a punctuation."
• esac

26
Case Statement - a better example

• !/bin/sh
• read -n1 -p "Hit a key " key
• echo
• case "key" in
• a-z)
• echo "key is a lower case."
• A-Z)
• echo "key is an upper case."
• 0-9)
• echo "key is a number."
• )
• echo "key is a punctuation."
• esac

27
for loop

• Syntax
• for ltvargt in ltlistgt do
• ltstmtsgt
• done
• For example
• for i in 1 2 3 4 5 6 7 8 9 10 do
• echo i
• done

for i in seq 1 10 do
28
for loop
/usr//man gets expanded first!

• for i in /usr//man do echo i done
• /usr/bin/man
• /usr/csshare/man
• /usr/kerberos/man
• /usr/local/man
• /usr/share/man

29
while until loop

• while loop syntax
• while ltconditiongt do
• ltstmtsgt
• done
• until loop syntax
• until ltconditiongt do
• ltstmtsgt
• done

30
How to keep track if a user has been logged out?

• Null command always returns true
• echo ?
• 0
• Create a zero-sized file
• gt out.txt
• cat /dev/null gt out.txt
• touch out.txt

• !/bin/sh
• usermanager
• while do
• who grep user gt /dev/null
• if ? -ne 0 then
• break
• else
• sleep 30
• continue
• fi
• done
• echo "user has logged out at date"

Do we care about the output?
31
How to keep track if a user has been logged out?

• !/bin/sh
• usermanager
• while who grep user gt /dev/null do
• sleep 30
• done
• echo "user has logged out at date"

32
How to keep track if a user has been logged in?

• !/bin/sh
• userweesan
• until who grep user gt /dev/null do
• sleep 30
• done
• echo "user has logged in at date"

33
read

• !/bin/sh
• while read line do
• echo "line"
• done lt /etc/passwd
• !/bin/sh
• OIFSIFS IFS
• while read name passwd uid gid fullname ignore
  do
• echo "name (fullname)"
• done lt /etc/passwd
• IFSOIFS

34
How to find the UID of weesan?

• Parse /etc/passwd line-by-line
• while IFS read name passwd uid gid fullname
  ignore do
• if "name "weesan" then
• echo uid
• fi
• done lt /etc/passwd
• Or, use grep cut
• grep weesan /etc/passwd cut -d -f3
• Or, simply
• id -u weesan

35
How to remove weesan from /etc/passwd?

• Parse /etc/passwd line-by-line
• while IFS read name ignore do
• if "name" ! "weesan" then
• echo "uidignore
• fi
• done lt /etc/passwd
• Or use grep
• grep -v weesan /etc/passwd

36
How to find lines starting or ending with '1'?

• cat data.txt
• 1a
• 2
• 3
• 411
• grep 1 data.txt
• 1a
• 411
• grep 1 data.txt
• 1a
• grep '1' data.txt
• 411

37
Command Line Options

• argv.sh -a -b -c -d 1 -e -f foo.txt
• Option a
• Option b
• Option c
• Option d with 1
• Option e
• Unknown option -f
• The rest is foo.txt

38
Command Line Options

• cat argv.sh
• !/bin/sh
• for arg do
• case "1" in
• -a) echo "Option a"
• -b) echo "Option b"
• -c) echo "Option c"
• -d) shift
• echo "Option d with 1"
• -e) echo "Option e"
• -) echo "Unknown option 1"
• ) FILE"FILE 1"
• esac
• shift
• done
• echo "The rest is FILE"

• cat getopts.sh
• !/bin/sh
• while getopts "abcde" arg do
• case "arg" in
• a) echo "Option a"
• b) echo "Option b"
• c) echo "Option c"
• d) echo "Option d with OPTARG"
• e) echo "Option e"
• esac
• done
• shift ((OPTIND - 1))
• echo "The rest is 1"

39
Command Line Options

• getopts.sh -abc -d1 -ef foo.txt
• Option a
• Option b
• Option c
• Option d with 1
• Option e
• getopts.sh illegal option -- f
• The rest is foo.txt

40
Here Document

• !/bin/sh
• cat ltltEOF
• This is a here document.
• Usually it has multiple lines
• and long.
• EOF

41
Basename, dirname

• dirname /boot/System.map
• /boot
• basename /boot/System.map
• System.map
• basename /boot/System.map .map
• System
• Good for renaming
• a"a.txt"
• tmp"basename a .txt.tmp
• grep -v weesan a gt tmp mv tmp a

42
eval

• !/bin/sh
• cmd'/usr/bin/grep string file'
• read -p "File " file
• read -p "String " string
• eval cmd

43
Parameter Substitution

• !/bin/sh
• echo "\user is not set"
• echo user-whoami
• echo "\user is set"
• user"foo"
• echo user-whoami
• echo "\user is set and non-empty"
• user"foo"
• echo user-whoami
• echo user-whoami
• echo "\user is set but empty"
• user
• echo user-whoami
• echo user-whoami

• user is not set
• weesan
• user is set
• foo
• user is set and non-empty
• foo
• foo
• user is set but empty
• weesan

44
Function Call

• !/bin/sh
• start()
• echo "Start"
• stop()
• echo "Stop"
• restart()
• echo "Restart"
• case "1" in
• Sstart) start
• Sstop) stop
• Rrestart) restart
• ) echo "Usage 0 startstoprestart"
• esac

45
Function Call - parameters

• !/bin/sh
• foo()
• echo " of para "
• echo "\0 0"
• echo "\1 1"
• echo "\2 2"
• echo "\3 3"
• foo 1 2 3

• of para 3
• 0 ./func2.sh
• 1 1
• 2 2
• 3 3

46
find

• find -nouser -xdev
• find /home -print0 xargs -0 ls -l
• find /bin /usr/bin /sbin -perm -4000 -or -perm
  -2000
• find /bin /usr/bin /sbin -perm -4000 -or -perm
  -2000 xargs ls -l
• find /bin /usr/bin /sbin -perm -4000 -or -perm
  -2000 -exec ls -l '' \

47
References

• Advance Bash-Script Guide
• http//www.tldp.org/LDP/abs/abs-guide.pdf
• Unix Power Tools, 3rd Edition
• Shelley Powers, Jerry Peek, Tim O'Reilly, Mike
  Loukides