The Linux Command Line (2012)

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Tilde Expansion

As you may recall from our introduction to the cd command, the tilde character (˜) has a special meaning. When used at the beginning of a word, it expands into the name of the home directory of the named user or, if no user is named, the home directory of the current user:

[me@linuxbox ˜]$ echo ˜
/home/me

If user foo has an account, then

[me@linuxbox ˜]$ echo ˜foo
/home/foo

Arithmetic Expansion

The shell allows arithmetic to be performed by expansion. This allows us to use the shell prompt as a calculator:

[me@linuxbox ˜]$ echo $((2 + 2))
4

Arithmetic expansion uses the following form:

$(( expression))

where expression is an arithmetic expression consisting of values and arithmetic operators.

Arithmetic expansion supports only integers (whole numbers, no decimals) but can perform quite a number of different operations. Table 7-1 lists a few of the supported operators.

Table 7-1. Arithmetic Operators

Operator	Description
+	Addition
-	Subtraction
*	Multiplication
/	Division (But remember, because expansion supports only integer arithmetic, results are integers.)
%	Modulo, which simply means remainder
**	Exponentiation

Spaces are not significant in arithmetic expressions, and expressions may be nested. For example, multiply 5² by 3:

[me@linuxbox ˜]$ echo $(($((5**2)) * 3))
75

Single parentheses may be used to group multiple subexpressions. With this technique, we can rewrite the example above and get the same result using a single expansion instead of two:

[me@linuxbox ˜]$ echo $(((5**2) * 3))
75

Here is an example using the division and remainder operators. Notice the effect of integer division:

[me@linuxbox ˜]$ echo Five divided by two equals $((5/2))
Five divided by two equals 2
[me@linuxbox ˜]$ echo with $((5%2)) left over.
with 1 left over.

Arithmetic expansion is covered in greater detail in Chapter 34.

Brace Expansion

Perhaps the strangest expansion is called brace expansion. With it, you can create multiple text strings from a pattern containing braces. Here’s an example:

[me@linuxbox ˜]$ echo Front-{A,B,C}-Back
Front-A-Back Front-B-Back Front-C-Back

Patterns to be brace expanded may contain a leading portion called a preamble and a trailing portion called a postscript. The brace expression itself may contain either a comma-separated list of strings or a range of integers or single characters. The pattern may not contain embedded whitespace. Here is an example using a range of integers:

[me@linuxbox ˜]$ echo Number_{1..5}
Number_1 Number_2 Number_3 Number_4 Number_5

Here we get a range of letters in reverse order:

[me@linuxbox ˜]$ echo {Z..A}
Z Y X W V U T S R Q P O N M L K J I H G F E D C B A

Brace expansions may be nested:

[me@linuxbox ˜]$ echo a{A{1,2},B{3,4}}b
aA1b aA2b aB3b aB4b

So what is this good for? The most common application is to make lists of files or directories to be created. For example, if we were photographers and had a large collection of images that we wanted to organize by years and months, the first thing we might do is create a series of directories named in numeric year-month format. This way, the directory names will sort in chronological order. We could type out a complete list of directories, but that’s a lot of work and it’s error prone too. Instead, we could do this:

[me@linuxbox ˜]$ mkdir Pics
[me@linuxbox ˜]$ cd Pics
[me@linuxbox Pics]$ mkdir {2009..2011}-0{1..9} {2009..2011}-{10..12}
[me@linuxbox Pics]$ ls
2009-01 2009-07 2010-01 2010-07 2011-01 2011-07
2009-02 2009-08 2010-02 2010-08 2011-02 2011-08
2009-03 2009-09 2010-03 2010-09 2011-03 2011-09
2009-04 2009-10 2010-04 2010-10 2011-04 2011-10
2009-05 2009-11 2010-05 2010-11 2011-05 2011-11
2009-06 2009-12 2010-06 2010-12 2011-06 2011-12

Pretty slick!

Parameter Expansion

We’re only going to touch briefly on parameter expansion in this chapter, but we’ll be covering it extensively later. It’s a feature that is more useful in shell scripts than directly on the command line. Many of its capabilities have to do with the system’s ability to store small chunks of data and to give each chunk a name. Many such chunks, more properly called variables, are available for your examination. For example, the variable named USER contains your username. To invoke parameter expansion and reveal the contents of USER, you would do this:

[me@linuxbox ˜]$ echo $USER
me

To see a list of available variables, try this:

[me@linuxbox ˜]$ printenv | less

You may have noticed that with other types of expansion, if you mistype a pattern, the expansion will not take place and the echo command will simply display the mistyped pattern. With parameter expansion, if you misspell the name of a variable, the expansion will still take place but will result in an empty string:

[me@linuxbox ˜]$ echo $SUER

[me@linuxbox ˜]$

Command Substitution

Command substitution allows us to use the output of a command as an expansion:

[me@linuxbox ˜]$ echo $(ls)
Desktop Documents ls-output.txt Music Pictures Public Templates Videos

One of my favorites goes something like this:

[me@linuxbox ˜]$ ls -l $(which cp)
-rwxr-xr-x 1 root root 71516 2012-12-05 08:58 /bin/cp

Here we passed the results of which cp as an argument to the ls command, thereby getting the listing of the cp program without having to know its full pathname. We are not limited to just simple commands. Entire pipelines can be used (only partial output shown):

[me@linuxbox ˜]$ file $(ls /usr/bin/* | grep zip)
/usr/bin/bunzip2: symbolic link to `bzip2'
/usr/bin/bzip2: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV
), dynamically linked (uses shared libs), for GNU/Linux 2.6.9, stripped
/usr/bin/bzip2recover: ELF 32-bit LSB executable, Intel 80386, version 1
(SYSV), dynamically linked (uses shared libs), for GNU/Linux 2.6.9, stripped
/usr/bin/funzip: ELF 32-bit LSB executable, Intel 80386, version 1 (SYSV
), dynamically linked (uses shared libs), for GNU/Linux 2.6.9, stripped
/usr/bin/gpg-zip: Bourne shell script text executable
/usr/bin/gunzip: symbolic link to `../../bin/gunzip'
/usr/bin/gzip: symbolic link to `../../bin/gzip'
/usr/bin/mzip: symbolic link to `mtools'

In this example, the results of the pipeline became the argument list of the file command.

There is an alternative syntax for command substitution in older shell programs that is also supported in bash. It uses back quotes instead of the dollar sign and parentheses:

[me@linuxbox ˜]$ ls -l `which cp`
-rwxr-xr-x 1 root root 71516 2012-12-05 08:58 /bin/cp

Quoting

Now that we’ve seen how many ways the shell can perform expansions, it’s time to learn how we can control it. For example, take this:

[me@linuxbox ˜]$ echo this is a test
this is a test

Or this:

[me@linuxbox ˜]$ echo The total is $100.00
The total is 00.00

In the first example, word splitting by the shell removed extra whitespace from the echo command’s list of arguments. In the second example, parameter expansion substituted an empty string for the value of $1 because it was an undefined variable. The shell provides a mechanism calledquoting to selectively suppress unwanted expansions.

Double Quotes

The first type of quoting we will look at is double quotes. If you place text inside double quotes, all the special characters used by the shell lose their special meaning and are treated as ordinary characters. The exceptions are $ (dollar sign), \ (backslash), and ` (back tick). This means that word splitting, pathname expansion, tilde expansion, and brace expansion are suppressed, but parameter expansion, arithmetic expansion, and command substitution are still carried out. Using double quotes, we can cope with filenames containing embedded spaces. Say we were the unfortunate victim of a file called two words.txt. If we tried to use this on the command line, word splitting would cause this to be treated as two separate arguments rather than the desired single argument:

[me@linuxbox ˜]$ ls -l two words.txt
ls: cannot access two: No such file or directory
ls: cannot access words.txt: No such file or directory

By using double quotes, we stop the word splitting and get the desired result; further, we can even repair the damage:

[me@linuxbox ˜]$ ls -l "two words.txt"
-rw-rw-r-- 1 me me 18 2012-02-20 13:03 two words.txt
[me@linuxbox ˜]$ mv "two words.txt" two_words.txt

There! Now we don’t have to keep typing those pesky double quotes.

Remember: Parameter expansion, arithmetic expansion, and command substitution still take place within double quotes:

[me@linuxbox ˜]$ echo "$USER $((2+2)) $(cal)"
me 4 February 2012
Su Mo Tu We Th Fr Sa
1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29

We should take a moment to look at the effect of double quotes on command substitution. First let’s look a little deeper at how word splitting works. In our earlier example, we saw how word splitting appears to remove extra spaces in our text:

[me@linuxbox ˜]$ echo this is a test
this is a test

By default, word splitting looks for the presence of spaces, tabs, and newlines (linefeed characters) and treats them as delimiters between words. This means that unquoted spaces, tabs, and newlines are not considered to be part of the text. They serve only as separators. Since they separate the words into different arguments, our example command line contains a command followed by four distinct arguments. If we add double quotes, however, word splitting is suppressed and the embedded spaces are not treated as delimiters; rather, they become part of the argument:

[me@linuxbox ˜]$ echo "this is a test"
this is a test

Once the double quotes are added, our command line contains a command followed by a single argument.

The fact that newlines are considered delimiters by the word splitting mechanism causes an interesting, albeit subtle, effect on command substitution. Consider the following:

[me@linuxbox ˜]$ echo $(cal)
February 2012 Su Mo Tu We Th Fr Sa 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
18 19 20 21 22 23 24 25 26 27 28 29
[me@linuxbox ˜]$ echo "$(cal)"
February 2012
Su Mo Tu We Th Fr Sa
1 2 3 4
5 6 7 8 9 10 11
12 13 14 15 16 17 18
19 20 21 22 23 24 25
26 27 28 29

In the first instance, the unquoted command substitution resulted in a command line containing 38 arguments; in the second, the result was a command line with 1 argument that includes the embedded spaces and newlines.

Single Quotes

If we need to suppress all expansions, we use single quotes. Here is a comparison of unquoted, double quotes, and single quotes:

[me@linuxbox ˜]$ echo text ˜/*.txt {a,b} $(echo foo) $((2+2)) $USER
text /home/me/ls-output.txt a b foo 4 me
[me@linuxbox ˜]$ echo "text ˜/*.txt {a,b} $(echo foo) $((2+2)) $USER"
text ˜/*.txt {a,b} foo 4 me
[me@linuxbox ˜]$ echo 'text ˜/*.txt {a,b} $(echo foo) $((2+2)) $USER'
text ˜/*.txt {a,b} $(echo foo) $((2+2)) $USER

As we can see, with each succeeding level of quoting, more and more expansions are suppressed.

Escaping Characters

Sometimes we want to quote only a single character. To do this, we can precede a character with a backslash, which in this context is called the escape character. Often this is done inside double quotes to selectively prevent an expansion.

[me@linuxbox ˜]$ echo "The balance for user $USER is: \$5.00"
The balance for user me is: $5.00

It is also common to use escaping to eliminate the special meaning of a character in a filename. For example, it is possible to use characters in filenames that normally have special meaning to the shell. These would include $, !, &,(a space), and others. To include a special character in a filename, you can do this:

[me@linuxbox ˜]$ mv bad\&filename good_filename

To allow a backslash character to appear, escape it by typing \\. Note that within single quotes, the backslash loses its special meaning and is treated as an ordinary character.