Linux in a Nutshell, Fifth Edition

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Chapter 3: Linux Commands

This is the Title of the Book, eMatter Edition

Copyright © 2006 O’Reilly & Associates, Inc. All rights reserved.

substr string start length

Return a section of string, beginning with start, with a

maximum length of length characters. Return null when given

a negative or nonnumeric start or length.

Examples

Division happens first; result is 10:

expr 5 + 10 / 2

Addition happens first; result is 7 (truncated from 7.5):

expr \( 5 + 10 \) / 2

Add 1 to variable i. This is how variables are incremented in shell

scripts:

i=`expr $i + 1`

Print 1 (true) if variable a is the string “hello”:

expr $a = hello

Print 1 (true) if b plus 5 equals 10 or more:

expr $b + 5 \>= 10

Find the 5th, 6th, and 7th letters of the word character:

expr substr character 5 3

In the examples that follow, variable p is the string “version.100”.

This command prints the number of characters in p:

expr $p : '.*' Result is 11

Match all characters and print them:

expr $p : '\(.*\)' Result is "version.100"

Print the number of lowercase letters at the beginning of p:

expr $p : '[a-z]*' Result is 7

Match the lowercase letters at the beginning of p:

expr $p : '\([a-z]*\)' Result is "version"

Truncate $x if it contains five or more characters; if not, just print

$x. (Logical OR uses the second argument when the first one is 0

or null, i.e., when the match fails.)

expr $x : '\(.....\)' \| $x

In a shell script, rename files to ...