10.1 Reading user input with read

In many ocations you may want to prompt the user for some input, and there are several ways to achive this. This is one of those ways:

echo Please, enter your name
read NAME
echo "Hi $NAME!"

As a variant, you can get multiple values with read, this example may clarify this.

echo Please, enter your firstname and lastname
read FN LN
echo "Hi! $LN, $FN !"

10.2 Arithmetic evaluation

On the command line (or a shell) try this:

echo 1 + 1

If you expected to see ‘2’ you’ll be disappointed. What if you want BASH to evaluate some numbers you have? The solution is this:

echo $((1+1))

This will produce a more ‘logical’ output. This is to evaluate an arithmetic expression. You can achieve this also like this:

echo $[1+1]

If you need to use fractions, or more math or you just want it, you can use bc to evaluate arithmetic expressions.

if i ran echo $[3/4] at the command prompt, it would return 0 because bash only uses integers when answering. If you ran echo 3/4|bc -l, it would properly return 0.75.

10.3 Finding bash

From a message from mike (see Thanks to)

you always use #!/bin/bash .. you might was to give an example of

how to find where bash is located.

locate bash is preferred, but not all machines have locate.

find ./ -name bash from the root dir will work, usually.

Suggested locations to check:

ls -l /bin/bash
ls -l /sbin/bash
ls -l /usr/local/bin/bash
ls -l /usr/bin/bash
ls -l /usr/sbin/bash
ls -l /usr/local/sbin/bash

(can’t think of any other dirs offhand... i’ve found it in

most of these places before on different system).

You may try also ‘which bash’.

10.4 Getting the return value of a program

In bash, the return value of a program is stored in a special variable called $?.

This illustrates how to capture the return value of a program, I assume that the directory dada does not exist. (This was also suggested by mike)

cd /dada &> /dev/null
echo rv: $?
cd $(pwd) &> /dev/null
echo rv: $?

10.5 Capturing a commands output

This little scripts show all tables from all databases (assuming you got MySQL installed). Also, consider changing the ‘mysql’ command to use a valid username and password.

DBS=`mysql -uroot  -e"show databases"`
for b in $DBS ;
        mysql -uroot -e"show tables from $b"

10.6 Multiple source files

You can use multiple files with the command source.