Wednesday, May 30, 2012

Understanding UNIX Shell Scripts

Special characters that can be used to manipulate commands in the command line include

1.    Backslash (\)
2.    Greater than (>)
3.    Less than (<)
4.    Pipe (|)
5.    Ampersand (&)

Character prevents the shell from treating another character as a special character through a process called backslash escaping.
This allows us to split a command statement across multiple lines. When we place the backslash at the end of a line and then press Enter, we can continue the statement on the next line. The backslash prevents the shell from treating the Enter keystroke – or new line character – as a special character.

For Example:
$echo Long pieces of test may not always fit onto to a single \
Line of the command line interface, so it becomes \
Necessary to split them across multiple lines using \

Greater than (>):
Allows us to direct the standard output of a command to a file or a device such as a printer instead of to the terminal screen.

For Eample:
$ ls –l /usr/home > userdirs

Less than (<):
Allows us to send the contents of a file to a command as its standard output.

For Eg:
$ sort –d < list

Pipe line (|):
Direct the output of one command to the input of another command.

For Example:
$ cat good | grep ‘India’

Character at the end of a command statement allows us to run commands in the background.

For Eg:
$ find / -name che &
[1]  48748
$ che

If we want to use special character in command line text without the shell recognizing them as special characters, we have to enclose them with a backslash (\).

For Example:
$echo Tours \& Accommodation
Tours & Accommodation

This eg shows an ‘echo’ command in which the ‘echo’ text contains an ampersand. There’s backslash
In front of the ampersand, which prevents the shell from treating it as a special character.

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