Scripting Params

script parameters

A bashbash shell script can have parameters. The numbering you see in the script below continues if you have more parameters. You also have special parameters containing the number of parameters, a string of all of them, and also the process id, and the last return code. The man page of bash has a full list.

#!/bin/bash
echo The first argument is $1
echo The second argument is $2
echo The third argument is $3

echo \$ $$  PID of the script
echo \# $#  count arguments
echo \? $?  last return code
echo \* $*  all the arguments

Below is the output of the script above in action.

[paul@RHEL4a scripts]$ ./pars one two three
The first argument is one
The second argument is two
The third argument is three
$ 5610 PID of the script
# 3 count arguments
? 0 last return code
* one two three all the arguments

Once more the same script, but with only two parameters.

[paul@RHEL4a scripts]$ ./pars 1 2
The first argument is 1
The second argument is 2
The third argument is
$ 5612 PID of the script
# 2 count arguments
? 0 last return code
* 1 2 all the arguments
[paul@RHEL4a scripts]$

Here is another example, where we use $0. The $0 parameter contains the name of the script.

paul@debian6~$ cat myname 
echo this script is called $0
paul@debian6~$ ./myname 
this script is called ./myname
paul@debian6~$ mv myname test42
paul@debian6~$ ./test42 
this script is called ./test42

shift through parameters

The shiftshift statement can parse all parameters one by one. This is a sample script.

kahlan@solexp11$ cat shift.ksh 
#!/bin/ksh

if [ "$#" == "0" ] 
 then
  echo You have to give at least one parameter.
  exit 1
fi

while (( $# ))
 do
  echo You gave me $1
  shift
 done

Below is some sample output of the script above.

kahlan@solexp11$ ./shift.ksh one  
You gave me one
kahlan@solexp11$ ./shift.ksh one two three 1201 "33 42"
You gave me one                           
You gave me two
You gave me three
You gave me 1201
You gave me 33 42
kahlan@solexp11$ ./shift.ksh                           
You have to give at least one parameter.

runtime input

You can ask the user for input with the readread command in a script.

#!/bin/bash
echo -n Enter a number:
read number

sourcing a config file

The sourcesource (as seen in the shell chapters) can be used to source a configuration file.

Below a sample configuration file for an application.

[paul@RHEL4a scripts]$ cat myApp.conf 
# The config file of myApp

# Enter the path here
myAppPath=/var/myApp

# Enter the number of quines here
quines=5

And here an application that uses this file.

[paul@RHEL4a scripts]$ cat myApp.bash 
#!/bin/bash
#
# Welcome to the myApp application
#

. ./myApp.conf

echo There are $quines quines

The running application can use the values inside the sourced configuration file.

[paul@RHEL4a scripts]$ ./myApp.bash 
There are 5 quines
[paul@RHEL4a scripts]$

get script options with getopts

The getoptsgetopts function allows you to parse options given to a command. The following script allows for any combination of the options a, f and z.

kahlan@solexp11$ cat options.ksh 
#!/bin/ksh

while getopts ":afz" option;
do
 case $option in
  a)
   echo received -a
   ;;
  f)
   echo received -f
   ;;
  z)
   echo received -z
   ;;
  *)
   echo "invalid option -$OPTARG" 
   ;;
 esac
done

This is sample output from the script above. First we use correct options, then we enter twice an invalid option.

kahlan@solexp11$ ./options.ksh        
kahlan@solexp11$ ./options.ksh -af
received -a
received -f
kahlan@solexp11$ ./options.ksh -zfg
received -z
received -f
invalid option -g
kahlan@solexp11$ ./options.ksh -a -b -z
received -a
invalid option -b
received -z

You can also check for options that need an argument, as this example shows.

kahlan@solexp11$ cat argoptions.ksh 
#!/bin/ksh

while getopts ":af:z" option;
do
 case $option in
  a)
   echo received -a
   ;;
  f)
   echo received -f with $OPTARG
   ;;
  z)
   echo received -z
   ;;
  :)
   echo "option -$OPTARG needs an argument"
   ;;
  *)
   echo "invalid option -$OPTARG" 
   ;;
 esac
done

This is sample output from the script above.

kahlan@solexp11$ ./argoptions.ksh -a -f hello -z
received -a
received -f with hello
received -z
kahlan@solexp11$ ./argoptions.ksh -zaf 42       
received -z
received -a
received -f with 42
kahlan@solexp11$ ./argoptions.ksh -zf   
received -z
option -f needs an argument

get shell options with shopt

You can toggle the values of variables controlling optional shell behaviour with the shoptshopt built-in shell command. The example below first verifies whether the cdspell option is set; it is not. The next shopt command sets the value, and the third shopt command verifies that the option really is set. You can now use minor spelling mistakes in the cd command. The man page of bash has a complete list of options.

paul@laika:~$ shopt -q cdspell ; echo $?
1
paul@laika:~$ shopt -s cdspell
paul@laika:~$ shopt -q cdspell ; echo $?
0
paul@laika:~$ cd /Etc
/etc

practice: parameters and options

1. Write a script that receives four parameters, and outputs them in reverse order.

2. Write a script that receives two parameters (two filenames) and outputs whether those files exist.

3. Write a script that asks for a filename. Verify existence of the file, then verify that you own the file, and whether it is writable. If not, then make it writable.

4. Make a configuration file for the previous script. Put a logging switch in the config file, logging means writing detailed output of everything the script does to a log file in /tmp. solution: parameters and options ================================

1. Write a script that receives four parameters, and outputs them in reverse order.

echo $4 $3 $2 $1

2. Write a script that receives two parameters (two filenames) and outputs whether those files exist.

#!/bin/bash

if [ -f $1 ]
then echo $1 exists!
else echo $1 not found!
fi

if [ -f $2 ]
then echo $2 exists!
else echo $2 not found!
fi

3. Write a script that asks for a filename. Verify existence of the file, then verify that you own the file, and whether it is writable. If not, then make it writable.

4. Make a configuration file for the previous script. Put a logging switch in the config file, logging means writing detailed output of everything the script does to a log file in /tmp.