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How to write shell script



How to write shell script


Following steps are required to write shell script:

(1) Use any editor like vi or mcedit to write shell script.

(2)  After  writing  shell  script  set  execute  permission  for  your  script  as  follows
syntax:
chmod permission your-script-name

Examples:
$ chmod +x your-script-name
$ chmod 755 your-script-name

Note: This will set read write execute(7) permission for owner, for group and other permission is read and execute only(5).

ls -l command earlier presented a long of listing file with a line like the following for each file:

-rw-r--r--    1   root  user  0   2009-04-28 08:26 newfile.txt



Here the first character  in  the first column (-) indicates that  the file is a normal file. The next 9 characters indicate the access permissions for the file. The next set of 9 characters is divided into3 groups of 3 cha- racters. Purpose of these characters is as under:

(-) represents no permission
(r) represents 'read' permission
(w) represents 'write' permission
(x) represents 'execute' permission

Permission
Octal number
Equivalent Symbol
Read
4
r--
Write
2
-w-
Execute
1
--x


The  three  group  represents  user  (owner  of  the  file),  group(to  which  the  owner belongs)  and  others (any  other user of the system) respectively. Three characters in each group are for 'read', 'write' and 'execute' permission respectively.

In our example, the owner has 'read' and 'write' permission for the file and everyone else has only read permission. For a normal file, read, write and execute permissions are obvious.   For a directory,   read and write permissions mean   that   to   read   the contents of  the directory and create new entries in the directory. Execute permission means that one can search in the directory but not read from or write to the directory.

You can use   the chmod command   to change   the access permissions of a  file or a directory. To  specify permissions  for a file  with chmod, any of the following  two methods can be used.

Symbol
Meaning
u
User
g
Group
o
Other
a
All (equals to ugo)
+
Add Permission
-
Remove a permission
r
Read Permission
w
Write permission
x
Execute permission

Syntax: chmod u+x filename

(3) Execute your script as
syntax:
bash your-script-name
sh your-script-name
./your-script-name

Examples:
$ bash bar
$ sh bar
$ ./bar

 

Anurag

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