Linux with Operating System Concepts

command string params
‘
. 
There is another command we could use called match. But before we look at match, there 

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Linux with Operating System Concepts
is a simpler way to obtain a substring. We do this through 
${string:start:length}
where 
start
and 
length
are integer numbers or variables storing integer numbers. This 
operation returns the substring of 
string
starting at 
start
with 
length
characters. As with 
the array, but unlike using the 
expr substr
command, the first character of a string is 
at location 0 so that 
${string:2:4}
would actually provide the third, fourth, fifth, and 
sixth characters of the string, or the equivalent of 
‘expr substr $string 3 4‘
.
Imagine that we have three variables, First, Middle, and Last, which are a person’s first 
name, middle name, and last name. We want to form a variable, Initials, to store the initials 
of the three names. We could do this in one of two ways.
Initials
=
"‘expr substr $First 1 1‘
‘expr substr $Middle 1 1‘‘expr substr $Last 1 1‘“
or
Initials
=
${First:0:1}${Middle:0:1}${Last:0:1}
When using the 
${string:start:length}
notation, if the value for 
start
is greater 
than the length of the string, nothing is returned but it does not yield an error. If the value 
for 
length
is larger than the remaining length of the string, it returns the rest of the string. 
For instance, if 
name
=
"Frank Zappa"
then 
${name:6:15}
returns “Zappa.” Note 
that if you omit length, it defaults to the remainder of the string, for instance 
${name:0}
returns “Frank Zappa” and 
${name:3}
returns “nk Zappa.”
You can also use a negative value for 
start
. This indicates that counting will start from 
the right end of the string rather than the left end. For this to work, you must either place 
the negative value in parentheses or add a space after the colon. For instance, 
${name: 
-3}
returns “ppa” and 
${name:(-7)}
returns “k Zappa.” If you fail to include the space or 
parenthesize the negative number, the entire string is returned. You can combine the nega-
tive number with a length, which operates as you would expect; it returns 
length
characters 
starting from the right side as indicated by the negative start value. For instance, 
${name: 
-7:3}
returns “k Z” and 
${name:(-4:3)}
returns “app.”
Just as the notation 
${string:start:length}
is a shortcut for 
‘expr substr‘
, 
we can employ a shortcut for 
‘expr length $string‘
. In this case, the notation is 
${#string}
, which is similar to the notation used to obtain the number of items in an 
array. Using name from the previous examples, echo 
${#name}
will return 11 (11 charac-
ters in the string “Frank Zappa”).
7.8.2 String Regular Expression Matching
Let us return to the expr command and look at another string operation known as 
match
. 
This allows us to compare a string to a regular expression. The expr operation then returns 
the portion of the string that matched the regular expression. Unlike grep, match will only 
begin matching from the left hand side of the string. The notation is

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Shell Scripting
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‘
expr match "string" '\(