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10. Misc

10.1 Reading user input with read

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

   #!/bin/bash
   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.

   #!/bin/bash
   echo Please, enter your firstname and lastname
   read FN LN 
   echo "Hi! $LN, $FN !"
   

10.1.1 Reading user input with read

 
User Interaction
echo -n "Prompt: "
read
echo "You typed $REPLY."
echo -n "Prompt: "
read response
echo "You typed $response."
PS3="Choose a fruit: "
select fruit in "apples" "oranges" "pears"
do
if [ -n "$fruit" ]
then
break
fi
echo "Invalid choice"
done
$ dialog --menu "Choose" 10 20 4 1 apples 2 \
oranges 3 pears 4 bananas 2>/tmp/ans
$ fruit=`cat /tmp/ans`
$ echo $fruit
$ zenity --list --radiolist --column "Choose" --column "Fruit" 0 Apples 0 Oranges 0 Pears 0 Bananas > /tmp/ans
$ fruit=`cat /tmp/ans`
$ echo $fruit 

10.1.2 Reading Input from a File

 
exec 6<&0   # 'Park' stdin on #6
exec < temp.txt     # stdin=file "temp.txt"
read   # from stdin
until [ -z "$REPLY" ]
do
echo "$REPLY"       # lists temp.txt
read
done
exec 0<&6 6<&-      # restore stdin
echo

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)

   #!/bin/bash
   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.

   #!/bin/bash
   DBS=`mysql -uroot  -e"show databases"`
   for b in $DBS ;
   do
   mysql -uroot -e"show tables from $b"
   done
   

10.6 Date and Time

   
   Date and Time
   $ start=`date +%s`
   $ end=`date +%s`
   $ echo That took $((end-start)) seconds
   $ date +"%c" -d19540409
   
   Fri 09 Apr 1954 12:00:00 AM GMT

10.7 Preset Variables

   $HOME     User’s home directory
   $HOSTNAME      Name of host
   $HOSTTYPE      Type of host (e.g. i486)
   $PWD      Current directory
   $REPLY    default variable for READ and SELECT
   $SECONDS       Elapsed time of script
   $TMOUT    Max. script elapsed