Quoting

Bash Unix Shell Scripting:

Chapter 5. Quoting

Table of Contents

Quoting means just that, bracketing a string in quotes. This has the effect of protecting in the string from reinterpretation or expansion by the shell or shell script. (A character is "special" if it has an interpretation other than its literal meaning. For example, the represents a wild card character in and ). 

bash$ ls -l [Vv]*
-rw-rw-r--    1 bozo  bozo       324 Apr  2 15:05 VIEWDATA.BAT
 -rw-rw-r--    1 bozo  bozo       507 May  4 14:25 vartrace.sh
 -rw-rw-r--    1 bozo  bozo       539 Apr 14 17:11 viewdata.sh
bash$ ls -l '[Vv]*'
ls: [Vv]*: No such file or directory

In everyday speech or writing, when we "quote" a phrase, we set it apart and give it special meaning. In a Bash script, when we quote a string, we set it apart and protect its literal meaning.

Certain programs and utilities reinterpret or expand special characters in a quoted string. An important use of quoting is protecting a command-line parameter from the shell, but still letting the calling program expand it. 

bash$ grep '[Ff]irst' *.txt
file1.txt:This is the first line of file1.txt.
 file2.txt:This is the First line of file2.txt.

Note that the unquoted grep [Ff]irst *.txt works under the Bash shell.

Quoting can also suppress "appetite" for newlines. 

bash$ echo $(ls -l)
total 8 -rw-rw-r-- 1 bo bo 13 Aug 21 12:57 t.sh -rw-rw-r-- 1 bo bo 78 Aug 21 12:57 u.sh
bash$ echo "$(ls -l)"
total 8
 -rw-rw-r--  1 bo bo  13 Aug 21 12:57 t.sh
 -rw-rw-r--  1 bo bo  78 Aug 21 12:57 u.sh

Notes

Unless there is a file named first in the current working directory. Yet another reason to quote. (Thank you, Harald Koenig, for pointing this out.