Lecture 24: UNIX Shell Programming

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Lecture 24 UNIX Shell Programming

• Homework Read Chapters 15 17 of Sarwar

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Lecture 24 Outline

• Unix Shell programming
• Shell scripts.
• Definition.
• Uses of shell scripts.
• Writing shell scripts.

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Shell scripts

• A shell script is a text file with Unix commands
  in it.
• Shell scripts usually begin with a ! and a shell
  name (complete pathname of shell).
• Pathname of shell be found using the which
  command.
• The shell name is the shell that will execute
  this script.
• E.g !/bin/bash

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Shell scripts (contd.)

• If no shell is specified in the script file, the
  default is chosen to be the currently executing
  shell.

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Shell scripts (contd.)

• Any Unix command can go in a shell script
• Commands are executed in order or in the flow
  determined by control statements.
• Different shells have different control
  structures
• The ! line is very important.
• We will write shell scripts with the Bourne shell
  (bash).

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Shell scripts (contd.)

• A shell script as a standalone is an executable
  program
• Must use chmod to change the permissions of the
  script to be executable also.
• Can run script explicitly also, by specifying the
  shell name.
• E.g bash myscript
• E.g csh myscript

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Shell scripts (contd.)

• Consider the example
• bash myscript
• Invokes the bash shell and then runs the script
  using it.
• Its almost as if we had the line ! /bin/bash in
  the file myscript.
• Similarly with the second example using csh.
• myscript need not be an executable as bash is
  running the script on its behalf.

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Shell scripts (contd.)

• Why write shell scripts ?
• To avoid repetition
• If you do a sequence of steps with standard Unix
  commands over and over, why not do it all with
  just one command?
• Or in other words, store all these commands in a
  file and execute them one by one.

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Shell scripts (contd.)

• Why write shell scripts ?
• To automate difficult tasks
• Many commands have subtle and difficult options
  that you dont want to figure out or remember
  every time .

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Simple Example

• Assume that I need to execute the following
  commands once in a while when I run out of disk
  space

rm -rf HOME/.netscape/cache rm -f
HOME/.netscape/his rm -f HOME/.netscape/cookies
rm -f HOME/.netscape/lock rm -f
HOME/.netscape/.nfs rm -f HOME/.pine-debug rm
-fr HOME/nsmail
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Simple Example (contd.)

• We can put all those commands into a shell
  script, called myscript.

axgopala_at_nitrogen axgopala cat myscript !
/bin/bash rm -rf HOME/.netscape/cache rm -f
HOME/.netscape/his rm -f HOME/.netscape/cookies
rm -f HOME/.netscape/lock rm -f
HOME/.netscape/.nfs rm -f HOME/.pine-debug rm
-fr HOME/nsmail
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Sample Example (contd.)

• To run the script
• Step 1
• chmod ux myscript
• Step 2
• Run the script
• ./myscript
• Each line of the script is processed in order.

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Shell scripts (contd.)

• Shell variables
• Declared by
• varnamevarvalue
• To make them an environment variable, we export
  it.
• export varnamevarvalue

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Shell scripts (contd.)

• Assigning the output of a command to a variable
• Using backquotes, we can assign the output of a
  command to a variable

! /bin/bash filelistls echo filelist
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Shell scripts (contd.)

• Example

axgopala_at_nitrogen public ls a b c
html/ axgopala_at_nitrogen public
filelistls axgopala_at_nitrogen public echo
filelist a b c html/ axgopala_at_nitrogen public
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Shell scripts (contd.)

• The expr command
• Calculates the value of an expression.
• E.g

axgopala_at_nitrogen axgopala valueexpr 1
2 axgopala_at_nitrogen axgopala echo
value 3 axgopala_at_nitrogen axgopala
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Notes on expr

• Why do we need the expr command ???
• E.g

axgopala_at_nitrogen public file12 axgopala_at_nit
rogen public echo file 12 axgopala_at_nitrogen
public
NOTE 12 is copied as it is into val and not the
result of the expression, to get the result, we
need expr.
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Notes on expr

• Variables as arguments

axgopala_at_nitrogen axgopala count5 axgopala_at_ni
trogen axgopala countexpr count
1 axgopala_at_nitrogen axgopala echo
count 6 axgopala_at_nitrogen axgopala
NOTE count is replaced with its value by the
shell!
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Notes on expr

• expr supports the following operators
• arithmetic operators ,-,,/,
• comparison operators lt, lt, , !, gt, gt
• boolean/logical operators ,
• parentheses (, )
• precedence is the same as C, Java

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Control statements

• Without control statements, execution within a
  shell scripts flows from one statement to the
  next in succession.
• Control statements control the flow of execution
  in a programming language.

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Control statements

• The three most common types of control
  statements
• conditionals if/then/else, case, ...
• loop statements while, for, until, do, ...
• branch statements subroutine calls (good
  programming practice), goto (usage not
  recommended).

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for loops

• for loops allow the repetition of a command for a
  specific set of values.
• Syntax
• for var in value1 value2 ...
• do
• command_set
• done
• command_set is executed with each value of var
  (value1, value2, ...) in sequence

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Notes on for

• Example Listing all files in a directory.

! /bin/bash for i in do echo i done
NOTE is a wild card that stands for all files
in the current directory, and for will go through
each value in , which is all the files and i
has the filename.
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Notes on for

• Example output

axgopala_at_nitrogen public chmod ux
listfiles axgopala_at_nitrogen public
./listfiles a b c html listfiles axgopala_at_nitroge
n public
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Conditionals

• Conditionals are used to test something.
• In Java or C, they test whether a Boolean
  variable is true or false.
• In a Bourne shell script, the only thing you can
  test is whether or not a command is successful.

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Conditionals

• Every well behaved command returns back a return
  code.
• 0 if it was successful
• Non-zero if it was unsuccessful (actually 1..255)
• This is different from C.

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The if command

• Simple form
• if decision_command_1
• then
• command_set_1
• fi

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The if command

• Importance of having then on the next line
• Each line of a shell script is treated as one
  command.
• then is a command in itself
• Even though it is part of the if structure, it is
  treated separately.

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Example
if grep unix myfile gt/dev/null then echo "It's
there" fi
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Using else with if

• Example

! /bin/bash if grep "UNIX" myfile
gt/dev/null then echo UNIX occurs in
myfile else echo No! echo UNIX does not
occur in myfile fi
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Using elif with if
! /bin/bash if grep "UNIX" myfile
gt/dev/null then echo UNIX occurs in
myfile elif grep DOS myfile gt /dev/null then
echo DOS appears in myfile not UNIX else echo
nobody is here in myfile fi
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Using colon in shell scripts

• Sometimes, we do not want a statement to do
  anything.
• In that case, use a colon
• if grep UNIX myfile gt /dev/null
• then
• fi
• Does not do anything when UNIX is found in myfile
  .

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The test command

• Use for checking validity.
• Three kinds
• Check on files.
• Check on strings.
• Check on integers

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Notes on test

• Testing on files.
• test f file does file exist and is not a
  directory?
• test -d file does file exist and is a directory?
• test x file does file exist and is executable?
• test s file does file exist and is longer than
  0 bytes?

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Example
!/bin/bash count0 for i in do if test x i
then countexpr count 1 fi done echo
Total of count files executable
NOTE expr count 1 serves the purpose of
count
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Notes on test

• Testing on strings.
• test z string is string of length 0?
• test string1 string2 does string1 equal
  string2?
• test string1 ! string2 not equal?

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Example
! /bin/bash if test -z REMOTEHOST then else
DISPLAY"REMOTEHOST0 export DISPLAY fi
NOTE This example tests to see if the value of
REMOTEHOST is a string of length gt 0 or not,
and then sets the DISPLAY to the appropriate
value.
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Notes on test

• Testing on integers.
• test int1 eq int2 is int1 equal to int2 ?
• test int1 ne int2 is int1 not equal to int2 ?
• test int1 lt int2 is int1 less than to int2 ?
• test int1 gt int2 is int1 greater than to int2
  ?
• test int1 le int2 is int1 less than or equal to
  int2 ?
• test int1 ge int2 is int1 greater than or equal
  to int2 ?

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Example
!/bin/bash smallest10000 for i in 5 8 19 8 7
3 do if test i -lt smallest then
smallesti fi done echo smallest
NOTE This program calculates the smallest among
the numbers 5, 8, 19, 8, 3.
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Notes on test

• The test command has an alias .
• Each bracket must be surrounded by spaces

!/bin/bash smallest10000 for i in 5 8 19 8 7
3 do if i -lt smallest then
smallesti fi done echo smallest
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The while loop

• While loops repeat statements as long as the next
  Unix command is successful.
• Works similar to the while loop in C.

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Example
! /bin/bash i1 sum0 while i -le 100 do
sumexpr sum i iexpr i 1 done echo
The sum is sum.
NOTE The value of i is tested in the while to
see if it is less than or equal to 100.
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The until loop

• Until loops repeat statements until the next
  Unix command is successful.
• Works similar to the do-while loop in C.

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Example
! /bin/bash x1 until x -gt 3 do echo x
x xexpr x 1 done
NOTE The value of x is tested in the until to
see if it is greater than 3.
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Next Lecture

• Unix Shell programming
• Command line arguments.
• Reading input in shell programs.
• Revision
• Storage management.
• Examples.