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Shell Programming

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Shell Programming
The basics
Variables
Quoting
Pre-defined variables
'If' statements
Conditionals
Case statement
Loops

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Reading: The Unix Programming Environment, Chapter 5

The basics

• A shell script is a collection of Unix commands, with a little bit of programming syntax thrown in to make it all work
• A comment begins with '#' and can start anywhere on the line
• The first line of the script tells what shell to use to interpret the script, in the form '#!/bin/sh'
• A really simple example [/bin/true from OpenBSD]:

 
 #! /bin/sh
 # $OpenBSD: true.sh,v 1.2 1996/06/26 05:41:53 deraadt Exp $
 
 exit 0

Variables

• Variables are assigned with the syntax var=something

• Note the lack of spaces
• If you do have a space, e.g. var = something, the shell will think you're trying to run the command var, with arguments = and something.

• Variables are referenced by using a "$" in front when referencing the value.
• If there is any confusion about what the variable name is, you can enclose it in braces like ${var}.
• Example:

 
 var="This is a variable"
 var1="This is var1"
 echo $var
 echo $var1
 echo ${var}1

• Produces

 
 This is a variable
 This is var1
 This is a variable1

Quoting

• There are three main types of quotes
• Single quote (' -- next to the semicolon on most keyboards)

• Anything in single quotes is taken literally, not interpreted, and gathered into one argument

• Double quote (" -- next to the semicolon, use the shift key)

• Anything in double quotes is interpreted, and gathered into one argument

• Backquotes (` -- next to 1 on most keyboards)

• Anything in backquotes is taken as a shell command, and the output of the command is placed on the command line in the place of the backquoted text

• Example

 today=`date`
 echo 'Running on `uname`: The value of today is $today.'
 echo "Running on `uname`: The value of today is $today."

• Example output

 Running on `uname`: The value of today is $today.
 Running on Darwin: The value of today is Mon Apr 7 18:08:38 PDT 2003.

Pre-defined variables

• There are some special variables that are pre-defined for you whenever a shell script is running.
• $0

• $0 is the name of the currently running script, as specified to the system

• If you run the script with "./script", then $0 will be "./script"
• If you run the script with "/home/joeblow/bin/script", then $0 will be "/home/joeblow/bin/script"

• $#

• The number of arguments that were given to the script

• $*

• All of the arguments together, but be careful, because you lose quoting (see below)

• $1, $2, $3, ... $9

• $n is the nth argument, and is undefined if there was no nth argument
• The shift command moves every argument "down a notch"

• $2 becomes $1
• $3 becomes $2
• $4 becomes $3
• etc.
• (The value of the previous $1 is discarded)

• $@

• Similar to $*, but preserves spaces in arguments in certain situations

• $$

• The PID of the script
• Often used to assign unique temporary filenames

• $?

• The exit value of the last command run from the script

• Example

 echo "\$$ is '$$'"
 echo "\$0 is '$0'"
 echo "\$# is '$#'"
 echo "\$* is '$*'"
 echo "\$1 is '$1'"
 echo "\$2 is '$2'"
 echo "\$3 is '$3'"
 shift
 echo "after shift...."
 echo " \$* is '$*'"
 echo " \$# is '$#'"
 echo " \$1 is '$1'"
 echo " \$2 is '$2'"
 echo " \$3 is '$3'"

• Example output

With the command: ./vartest one "two three" four 'five six'
 $$ is '11300'
 $0 is './vartest'
 $# is '4'
 $* is 'one two three four five six'
 $1 is 'one'
 $2 is 'two three'
 $3 is 'four'
 after shift....
 $* is 'two three four five six'
 $# is '3'
 $1 is 'two three'
 $2 is 'four'
 $3 is 'five six'

• Illustrating the difference between $* and $@

• The script:

  echo "\$* is $*"
 echo " Breaking it down...."
 for var in $*
 do
 echo " $var"
 done
 echo
 echo "\$@ is $@"
 echo " Breaking it down...."
 echo " (without the quotes)"
 for var in $@
 do
 echo " $var"
 done
 echo " (with the quotes)"
 for var in "$@"
 do
 echo " $var"
 done

• Produces:

 When run as ./argtest one "two three" 'four five'
 $* is one two three four five
 Breaking it down....
 one
 two
 three
 four
 five
 
 $@ is one two three four five
 Breaking it down....
 (without the quotes)
 one
 two
 three
 four
 five
 (with the quotes)
 one
 two three
 four five

'If' statements

• if

• Syntax

 
 if <<condition>>
 then
 <<one or more statements>>
 fi

• Example

 
 if [ $var -eq 0 ]
 then
 echo "Don't try to divide by \$var!"
 fi

• More on what goes in "<<condition>>" in a minute

• if-then-else

• Syntax

 
 if <<condition>>
 then
 <<one or more statements>>
 else
 <<one or more statements>>
 fi

• Example

 
 if [ $var -eq 0 ]
 then
 echo "Don't try to divide by \$var!"
 else
 echo "It's ok to divide by \$var."
 fi

• if-then-elif-fi

• Syntax

 
 if <<condition>>
 then
 <<one or more statements>>
 elif <<condition>>
 <<one or more statements>>
 else
 <<one or more statements>>
 fi

• Example

 
 if [ $var -eq 0 ]
 then
 echo "Don't try to divide by \$var!"
 elif [ $var -eq 1 ]
 echo "There's no point in dividing by \$var."
 else
 echo "It's ok to divide by \$var."
 fi

Conditionals

• Any Unix command can be used as a conditional
• If the command returns 0, it's "true"
• If the command returns anything else, it's "false"
• One often-used command is the "test" command (which is also aliased as "[" to make your program look more like other programming languages)

• All sorts of tests on file existence, permissions, etc are possible
• String and numeric comparisons
• The test command has a huge number of options, it's best to study the man page to learn them all. Just about anything you want to test can be tested by the test command

Case statement

• Functionally equivalent to if-then-elif-elif-elif-else-fi
• But cleaner-looking
• Matching of cases is done as strings using shell pattern matching (what you'd use with the ls command, for example).
• Syntax

 
 case $var in
 <<pattern1>>)
 <<things to do for pattern 1>>
 ;;
 <<pattern2>>)
 <<things to do for pattern 1>>
 ;;
 *) # The default condition
 <<things to do in the default case>>
 ;;
 esac

• Example (/etc/rc.d/init.d/misc on Geoff Allen’s old laptop)

 
 #!/bin/sh
 
 . /etc/rc.d/init.d/functions
 
 # See how we were called.
 case "$1" in
 start)
 echo -n "Starting misc daemons: "
 
 daemon /usr/local/sbin/wwwoffled
 
 echo
 ;;
 stop)
 echo -n "Stopping misc daemons: "
 
 killproc wwwoffled
 
 echo
 ;;
 status)
 # this is way overkill, but at least we have some status output...
 echo "Status of misc daemons"
 
 status wwwoffled
 ;;
 restart|reload)
 # do not do anything; this is unreasonable
 $0 stop; $0 start
 ;;
 *)
 # do not advertise unreasonable commands that there is no reason
 # to use with this device
 echo "Usage: misc {start|stop|status|restart|reload}"
 exit 1
 esac
 
 exit 0

Loops

• for

• Syntax

 
 for var in <<item1>> <<item2>> <<item3>>
 do
 <<statements -- some using $var>>
 done

• Example ( /etc/rc.d/init.d/killall)

 
 #!/bin/sh
 
 # Bring down all unneeded services that are still running (there shouldn't
 # be any, so this is just a sanity check)
 
 for i in /var/lock/subsys/*; do
 # Check if the script is there.
 [ ! -f $i ] && continue
 
 # Get the subsystem name.
 subsys=${i#/var/lock/subsys/}
 
 # Bring the subsystem down.
 if [ -f /etc/rc.d/init.d/$subsys.init ]; then
 /etc/rc.d/init.d/$subsys.init stop
 else
 /etc/rc.d/init.d/$subsys stop
 fi
 done

• while

• Syntax

 
 while <<condition>>
 do
 <<statements>>
 done

• until

 
 until <<condition>>
 do
  <<statements>>
 done