E-learning module "Linux Basics"

Introduction to Bash Scripting

  • There are multipe shells with different programming syntax. The most frequently used shell is possibly the bash.
  • A shell script is a plain text file, often with the file extension .sh (to make it easier for the user, not required by the operation system).
  • The text file always has to start with #!/bin/bash  (to set the shell to be used by the script).
  • The script can only be run directly, if the necessery execute permissions are set.
  • $var or ${var} → Usage of a variable.
  • #                           → Comment.
  • var=value           → Set the variable var to value.
  • basename             → Remove the directory information from a path.
  • dirname               → Returns the directory information of a path.
  • read                     → Allows to read values form standard input.
Selected standard variables:
  • $0                 → Name of the script including full path.
  • $#                 → Number of the provided parameters.
  • $1,$2,…,$n → The provided parameters.
  • $*                 → All parameters as a string.
  • $?                 → Contains the exit status after the end of a script.

 
 

Example 1: for-loop in a bash shell script:

#! /bin/bash
dummy=(one two three four)    # Array!

for u in ${dummy[*]}
do
        echo $u
done

Exercise: Try running this with $dummy[*]  instead of ${dummy[*]}. What happens?

 
 

Explanation of Example 1:

dummy=(one two three four)

defines the array dummy containing the four elements (strings) one, two, three and four. The elements are separated by blanks   .

${dummy[0]} denotes the first element of the array, ${dummy[1]} the second one, and so on.

Further reading: "Advanced Bash Scripting", slide 11.

The general structure of a do-loop is:

for arg in list
do
   command
done

In the example above, the command is executed for all elements u of the array dummy.

Further reading: "Advanced Bash Scripting", slide 26.

 
 

Example 2: if-condition:

#! /bin/bash
if [ -e ${HOME}/.bashrc ];
then
        echo ".bashrc exists!"
fi

Explanation of Example 2:

$HOME or ${HOME} is an environment variable and denotes your home directory (and is explained here).

if condition;
then
    command
fi
is the simplest structure of an if-condition.

The condition is expressed by
[  ]
Please note the blanks after [  and before  ] ! They are necessary.

The condition
[ -e filename ]
is true, if the file filename exists, otherwise it is false.

Further reading: "Advanced Bash Scripting", slides 24 and 25.

The content of this e-learning module is licensed under the Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Germany license (CC BY-NC-SA 3.0 DE).



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