Shell programming basics: difference between single quotes and double quotes

 

Both single quotes and double quotes can disable shell processing for special characters. The difference is that double quotes do not have strict single quotes. Single quotes disable all characters with special functions, while double quotes only require shell to ignore the majority. Specifically, it is the dollar sign (②), the quotation mark (③), and The backslash. These three special characters are not ignored. The dollar sign is not ignored, which means shell also replaces variable names in double quotes.

The following describes how to use a simple shell program.

Debian :~ /Learn/shell # cat phonebook

Alice Chebba 973-555-2015

Barbara Swingle 201-555-9257

Liz Stachiw 212-555-2298

Susan Goldberg 201-555-7776

Susan Topple 212-555-4932

Tony Iannino 973-555-1295

Stromboli Pizza 973-555-9478

Debian :~ /Learn/shell #

Debian :~ /Learn/shell # cat lu

# Look someone up in the phone book

Grep "$1" phonebook

Debian :~ /Learn/shell #

This is the correct lu program. The following shows the running result.

Debian :~ /Learn/shell #./lu Susan T

Susan Topple 212-555-4932

Debian :~ /Learn/shell #./lu Tony

Tony Iannino 973-555-1295

Debian :~ /Learn/shell #

If lu is written as ① grep $1 phonebook or ② grep $1 phonebook, the following error occurs (Why ?).

Result 1:

Debian :~ /Learn/shell #./lu Tony // The result is correct.

Tony Iannino 973-555-1295

Debian :~ /Learn/shell #./lu Susan T // The result is incorrect.

Grep: T: No such file or directory

Phonebook: Susan Goldberg 201-555-7776

Phonebook: Susan Topple 212-555-4932

Debian :~ /Learn/shell #

② Results:

Debian :~ /Learn/shell #./lu Tony // The result is incorrect.

Debian :~ /Learn/shell #./lu Susan T // The result is also incorrect.

Debian :~ /Learn/shell #