The read builtin command

read something about read here!

read [-ers] [-u <FD>] [-t <TIMEOUT>] [-p <PROMPT>] [-a <ARRAY>] [-n <NCHARS>] [-N <NCHARS>] [-d <DELIM>] [-i <TEXT>] [<NAME...>]

The read builtin reads one line of data (text, user input, …) from standard input or a supplied filedescriptor number into one or more variables named by <NAME…>.

Since Bash 4.3-alpha, read skips any NUL (ASCII code 0) characters in input.

If <NAME…> is given, the line is word-split using IFS variable, and every word is assigned to one <NAME>. The remaining words are all assigned to the last <NAME> if more words than variable names are present.

while read -r; do
  printf '"%s"\n' "$REPLY"
done <<<"  a line with prefix and postfix space  "

If a timeout is given, or if the shell variable TMOUT is set, it is counted from initially waiting for input until the completion of input (i.e. until the complete line is read). That means the timeout can occur during input, too.

When both, -a <ARRAY> and a variable name <NAME> is given, then the array is set, but not the variable.

Of course it's valid to set individual array elements without using -a:

read MYARRAY[5]

read x[1]

read 'x[1]'

Essentially all you need to know about -r is to ALWAYS use it. The exact behavior you get without -r is completely useless even for weird purposes. It basically allows the escaping of input which matches something in IFS, and also escapes line continuations. It's explained pretty well in the POSIX read spec.

2012-05-23 13:48:31	geirha	it should only remove the backslashes, not change \n and \t and such into newlines and tabs
2012-05-23 13:49:00	ormaaj	so that's what read without -r does?
2012-05-23 13:49:16	geirha	no, -r doesn't remove the backslashes
2012-05-23 13:49:34	ormaaj	I thought read <<<'str' was equivalent to read -r <<<$'str'
2012-05-23 13:49:38	geirha	# read x y <<< 'foo\ bar baz'; echo "<$x><$y>"
2012-05-23 13:49:40	shbot	geirha: <foo bar><baz>
2012-05-23 13:50:32	geirha	no, read without -r is mostly pointless. Damn bourne
2012-05-23 13:51:08	ormaaj	So it's mostly (entirely) used to escape spaces
2012-05-23 13:51:24	ormaaj	and insert newlines
2012-05-23 13:51:47	geirha	ormaaj: you mostly get the same effect as using \ at the prompt
2012-05-23 13:52:04	geirha	echo \"  outputs a " ,  read x <<< '\"'  reads a "
2012-05-23 13:52:32	ormaaj	oh weird
2012-05-23 13:52:46	 *	ormaaj struggles to think of a point to that...
2012-05-23 13:53:01	geirha	ormaaj: ask Bourne :P
2012-05-23 13:53:20	geirha	(not Jason)
2012-05-23 13:53:56	ormaaj	hm thanks anyway :)

A rudimentary replacement for the cat command: read lines of input from a file and print them on the terminal.

opossum() {
  while read -r; do
    printf "%s\n" "$REPLY"
  done <"$1"
}

Note: Here, read -r and the default REPLY is used, because we want to have the real literal line, without any mangeling. printf is used, because (depending on settings), echo may interpret some baskslash-escapes or switches (like -n).

Remember the MSDOS pause command? Here's something similar:

pause() {
  local dummy
  read -s -r -p "Press any key to continue..." -n 1 dummy
}

Notes:

• -s to suppress terminal echo (printing)
• -r to not interpret special characters (like waiting for a second character if somebody presses the backslash)

Read can be used to split a string:

var="one two three"
read -r col1 col2 col3 <<< "$var"
printf "col1: %s col2: %s col3 %s\n" "$col1" "$col2" "$col3"

Take care that you cannot use a pipe:

echo "$var" | read col1 col2 col3 # does not work!
printf "col1: %s col2: %s col3 %s\n" "$col1" "$col2" "$col3"

If the variable has more fields than there are variables, the last variable get the remaining of the line:

read col1 col2 col3 <<< "one two three four"
printf "%s\n" "$col3" #prints three four

By default reads separates the line in fields using spaces or tabs. You can modify this using the special variable IFS, the Internal Field Separator.

IFS=":" read -r col1 col2 <<< "hello:world"
printf "col1: %s col2: %s\n" "$col1" "$col2"

The default IFS is special in that 2 fields can be separated by one or more space or tab. When you set IFS to something besides whitespace (space or tab), the fields are separated by exactly one character:

IFS=":" read -r col1 col2 col3 <<< "hello::world"
printf "col1: %s col2: %s col3 %s\n" "$col1" "$col2" "$col3"

The fields are separated by exactly one character, but the character can be different between each field:

IFS=":|@" read -r col1 col2 col3 col4 <<< "hello:world|in@bash"
printf "col1: %s col2: %s col3 %s col4 %s\n" "$col1" "$col2" "$col3" "$col4"

asksure() {
echo -n "Are you sure (Y/N)? "
while read -r -n 1 -s answer; do
  if [[ $answer = [YyNn] ]]; then
    [[ $answer = [Yy] ]] && retval=0
    [[ $answer = [Nn] ]] && retval=1
    break
  fi
done

echo # just a final linefeed, optics...

return $retval
}

### using it
if asksure; then
  echo "Okay, performing rm -rf / then, master...."
else
  echo "Pfff..."
fi

Note: The -i option was introduced with Bash 4

read -e -p "Enter the path to the file: " -i "/usr/local/etc/" FILEPATH

The user will be prompted, he can just accept the default, or edit it.

Here, IFS contains both, a colon and a space. The fields of the date/time string are recognized correctly.

datetime="2008:07:04 00:34:45"
IFS=": " read -r year month day hour minute second <<< "$datetime"

• POSIX® only specified the -r option (raw read); -r is not only POSIX, you can find it in earlier Bourne source code
• POSIX® doesn't support arrays
• REPLY is not POSIX®, you need to set IFS to the empty string to get the whole line for shells that don't know REPLY.

  while IFS= read -r line; do
    ...
  done < text.txt

• Internal: The printf builtin command