Ubuntu Unleashed 2019 Edition: Covering 18.04, 18.10, 19.04

execution of a command to be stored in a variable. For example, if you want
to count the number of lines in a file called test.txt in the current
directory and store the result in a variable called var, you can use the
following commands:

Command Environment

var=`wc -l  test.txt` pdksh and bash

set var =   `wc -l  test.txt`tcsh

Comparison of Expressions in pdksh and bash

Comparing values or evaluating the differences between similar bits of data—
such as file information, character strings, or numbers—is a task known as
comparison of expressions. Comparison of expressions is an integral part of
using logic in shell programs to accomplish tasks. The way the logical
comparison of two operators (numeric or string) is done varies slightly in
different shells. In pdksh and bash, a command called test can be used to
achieve comparisons of expressions. In tcsh, you can write an expression to
accomplish the same thing.

This section covers comparison operations using the pdksh or bash shells.
Later in the chapter, you learn how to compare expressions in the tcsh shell.

The pdksh and bash shell syntax provide a command named test to
compare strings, numbers, and files. The syntax of the test command is as
follows:

Click here to view code image
test expression
or
[ expression ]

Both forms of the test commands are processed the same way by pdksh
and bash. The test commands support the following types of comparisons:

    String  comparison

    Numeric comparison

    File    operators

    Logical operators