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The read builtin command

read something about read here!

Synopsis

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

Description

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.

If no <NAME> is given, the whole line read (without performing word-splitting!) is assigned to the shell variable REPLY. The following code will strip leading and trailing whitespace from the input:

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

To preserve leading and trailing whitespace in the result, set IFS to the null string:

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

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.

Options

OptionDescription
-a <ARRAY>read the data word-wise into the specified array <ARRAY> instead of normal variables
-d <DELIM>recognize <DELIM> as data-end, rather than <newline>
-eon interactive shells: use Bash's readline interface to read the data
-i <STRING>preloads the input buffer with text from <STRING>, only works when Readline (-e) is used
-n <NCHARS>reads <NCHARS> characters of input, then quits
-N <NCHARS>reads <NCHARS> characters of input, ignoring any delimiter, then quits
-p <PROMPT>the prompt string <PROMPT> is output (without a trailing automatic newline) before the read is performed
-rraw input - disables interpretion of backslash escapes and line-continuation in the read data
-ssecure input - don't echo input if on a terminal (passwords!)
-t <TIMEOUT>wait for data <TIMEOUT> seconds, then quit (exit code 1). Fractional seconds ("5.33") are allowed since Bash 4. A value of 0 immediately returns and indicates if data is waiting in the exit code. Timeout is indicated by an exit code greater than 128. If timeout arrives before data is read completely (before end-of-line), the partial data is saved.
-u <FD>use the filedescriptor number <FD> rather than stdin (0)

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]

Reading into array elements using the syntax above may cause pathname expansion to occur.

Example: You are in a directory with a file named x1, and you want to read into an array x, index 1 with

read x[1]
then pathname expansion will expand to the filename x1 and break your processing!

Even worse, if nullglob is set, your array/index will disappear.

To avoid this, either disable pathname expansion or quote the array name and index:

read 'x[1]'

Return status

Status Reason
0 no error
0 error when assigning to a read-only variable 1)
2 invalid option
>128 timeout (see -t)
!=0 invalid filedescriptor supplied to -u
!=0 end-of-file reached

read without -r

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 :)

Examples

Rudimentary cat replacement

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).

Press any key...

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)

Reading Columns

Simple Split

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"
Why? because the commands of the pipe run in subshells that cannot modify the parent shell. As a result, the variables col1, col2 and col3 of the parent shell are not modified (see article: Bash and the process tree).

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

Changing The Separator

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"
Here we use the var=value command syntax to set the environment of read temporarily. We could have set IFS normally, but then we would have to take care to save its value and restore it afterward (OLD=$IFS IFS=":"; read ….;IFS=$OLD).

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"
See how the :: in the middle infact defines an additional empty field.

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"

Are you sure?

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

Ask for a path with a default value

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.

Multichar-IFS: Parsing a simple date/time string

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"

Portability considerations

  • 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

See also

1) fixed in 4.2-rc1

Discussion

Dan Douglas, 2011/09/13 08:07

There's something slightly fishy going on with how the read arguments are parsed. In reality it's arbitrary expansions.

$ x='['; y=']'; z='+++'; while read -r "a${x}v${z%+}$y"; do :; done < <(printf '%s\n' {a..d}); echo "${a[@]}";
a b c d

The string is expanded, passed into read, and then eval'd. So expansions are happening twice here assuming it's double not single quoted.

Another example pre-initializing the variable in the arithmetic context:

$ while read -r "var[${x:=5}, x++]"; do :; done < <(printf '%s\n' {a..c}); for i in "${!var[@]}"; do printf '(%d, %s), ' "$i" "${var[$i]}"; done;
(5, a), (6, b), (7, c), (8, ),

However, if that's all there was to it you wouldn't expect this to work:

while read -r "var[$((x++))]"; do ...

There's some unique evaluation mode going on here since Bash knows not to perform arithmetic expansion before passing the resulting string into read otherwise the result would simply be to make all assignments to var[0].

PS... the preview is deleting all the "pluses". Hopefully the above is understandable.

Jan Schampera, 2011/09/20 19:38

Hi,

sorry for the delay :-)

This is not "double-expansion" per se, the behaviour is completely understandable (but I think you know it).

Thanks for this very interesting spotlight, I never thought that much about it!

Andor, 2012/06/06 16:46

Read, has some gotchas.

http://www.redhat.com/mirrors/LDP/LDP/abs/html/gotchas.html

Jan Schampera, 2012/07/01 13:43

Yes, it's not related directly to read, but to Bash's design regarding pipelining. AFAIR, there was a discussion on the mailing list to align the behaviour to what Korn does - running the last element of a pipeline in the current execution environment.

ABS is a good collection and a good work showing how to use the shell, but don't over-estimate it's technical side.

scavenger, 2013/02/18 10:49

thank you bash for running a sub-shell when piping, now we cannot read anymore multiple variables at the same time !

grep -w regexp file | read a b c

there is no solution to replace this KSH functionality. The command: read -r a b c «<$(command) solution is bourne and korn shell (88 & 93) incompatible.

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commands/builtin/read.txt · Last modified: 2013/04/14 14:31 by thebonsai
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