Shell Programing

Shell Programming
University of Maryland Eastern Shore Dept. of Computer Science Jianhua Yang

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CSDP399 Introduction to Unix and Programming

12/22/12

Outline
Unix Shell programming:
Shell scripts.  Definition.  Uses of shell scripts.  Writing shell scripts.

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CSDP399 Introduction to Unix and Programming

12/22/12

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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CSDP399 Introduction to Unix and Programming

12/22/12

Shell scripts (contd.)
If no shell is specified in the script file, the

default is chosen to be the currently executing shell.

4

CSDP399 Introduction to Unix and Programming

12/22/12

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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CSDP399 Introduction to Unix and Programming

12/22/12

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

6

CSDP399 Introduction to Unix and Programming

12/22/12

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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CSDP399 Introduction to Unix and