🖥Scripting

The basics of shell script programming

Script output (such as "Hello World" below) is displayed at the start of the line.

$ echo '#!/bin/sh' > my-script.sh          # Creat and write in file
$ echo 'echo Hello World' >> my-script.sh  # update file contcont 
$ chmod 755 my-script.sh                   # Set file Permission 
$ ./my-script.sh
Hello World
$

# File permission 

# 4 stands for "read",
# 2 stands for "write",
# 1 stands for "execute", and
# 0 stands for "no permission."

# 7 = 4 + 2 + 1 --> rwx
# 5 = 4 + 1     --> r-x

# owner-group-others

# 10 character permission

# first character is object type
# a hyphen represents a plain file
# d -> directory file
# l -> symlink file
# s -> socket file
# p -> pipe (FIFO, Read and Write)


# 2-4 character is owner perminssion
# 5-7 character is group permission
# 8-10 character is other permission

# permission are
# r->Read, w->write, x-> excute, s->Stiky bit)

Chmod Terminology

chmod [options] mode[,mode] file1 [file2 ...]


-R Recursive, i.e. include objects in subdirectories.
-v verbose, show objects changed (unchanged objects are not shown).


$ ls -l findPhoneNumbers.sh
-rwxr-xr--  1 dgerman  staff  823 Dec 16 15:03 findPhoneNumbers.sh

$ stat -c %a findPhoneNumbers.sh
754

The r, w, and x specify the read, write, and execute access. 


#	Permission	rwx	Binary
7	read, write and execute	rwx	111
6	read and write	rw-	110
5	read and execute	r-x	101
4	read only	r--	100
3	write and execute	-wx	011
2	write only	-w-	010
1	execute only	--x	001
0	none	---	000

Permission	rwx	Binary
7	read, write and execute rwx	111
6	read and write rw-	110
5	read and execute r-x	101
4	read only r--	100
3	write and execute -wx	011
2	write only -w-	010
1	execute only --x	001
0	none ---	000

Reference	Class	Description
u	user	file owner
g	group	members of the file's group
o	others	users who are neither the file's owner nor members of the file's group
a	all	all three of the above, same as ugo

Operator	Description
+	adds the specified modes to the specified classes
-	removes the specified modes from the specified classes
=	the modes specified are to be made the exact modes for the specified classes

#!/bin/sh
# This is a comment!
echo Hello World	# This is a comment, too!

Note that to make a file executable, you must set the eXecutable bit, and for a shell script, the Readable bit must also be set:

$ chmod a+rx my-script.sh
$ ./my-script.sh

Note:

One weakness in many shell scripts is lines such as:

cat /home/username/somefile | grep "string_to_find"

which would run much faster as:

grep "string_to_find" /home/username/somefile