Monday, December 1, 2014

Prompting for user input with read in bash script

Following bash script demonstrates two different ways to get user's input. One is echo & read and other combines both with read -p option.


# echo and read
echo -n "Enter your name: "
read name
echo "How are you, $name?"

# read -p option
read -p "Enter your name: " name
echo "How are you, $name?"

# read -p with multiple input values
read -p "Enter three numbers: " x y z
echo "$x + $y + $z = $(( x + y + z ))"

Following is the "read" documentation as reference:

read [-ers] [-u fd] [-t timeout] [-a aname] [-p prompt] [-n nchars] [-d delim] [name ...]
One line is read from the standard input, or from the file descriptor fd supplied as an argument to the -u option, and the first word is assigned to the first name, the second word to the second name, and so on, with leftover words and their intervening separators assigned to the last name.  If there are fewer words read from the input stream than names, the remaining names are assigned empty values. The characters in IFS are used to split the line into words. The backslash character (\) may be used to remove any special meaning for the next character read and for line continuation. Options, if supplied, have the following meanings:

-a aname
   The words are assigned to sequential indices of the array variable aname, starting at 0. aname is unset before any new values are assigned. Other name arguments are ignored.

-d delim
   The first character of delim is used to terminate the input line, rather than newline.

-e If the standard input is coming from a terminal, readline (see READLINE above) is used to obtain the line.

-n nchars
   read returns after reading nchars characters rather than waiting for a complete line of input.

-p prompt
   Display prompt on standard error, without a trailing newline, before attempting to read any input. The prompt is displayed only if input is coming from a terminal.

-r Backslash does not act as an escape character. The backslash is considered to be part of the line. In particular, a backslash-newline pair may not be used as a line continuation.

-s Silent mode. If input is coming from a terminal, characters are not echoed.

-t timeout
   Cause read to time out and return failure if a complete line of input is not read within timeout seconds.  This option has no effect if read is not reading input from the terminal or a pipe.

-u fd Read input from file descriptor fd.

If no names are supplied, the line read is assigned to the variable REPLY. The return code is zero, unless end-of-file is encountered, read times out, or an invalid file descriptor is supplied as the argument to -u.

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