14 KiB
The read builtin command
read something about read here!
Synopsis
read [-ers] [-u <FD>] [-t <TIMEOUT>] [-p <PROMPT>] [-a <ARRAY>] [-n <NCHARS>] [-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.
<WRAP center round info 90%> If no <NAME> is given, the whole line
read (without performing word-splitting!) is assigned to the shell
variable REPLY. Then, REPLY really contains
the line as it was read, without stripping pre- and postfix spaces and
other things!
while read -r; do
printf '"%s"\n' "$REPLY"
done <<<" a line with prefix and postfix space "
</WRAP>
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
| Option | Description |
|---|---|
-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> |
-e |
on interactive shells: use Bash's readline interface to read the data. Since version 5.1-alpha, this can also be used on specified file descriptors using -u |
-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 |
-r |
raw input - disables interpretion of backslash escapes and line-continuation in the read data |
-s |
secure 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]
<WRAP center round important 90%>
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]'
</WRAP>
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 <"$1"
}
Note: Here, read -r and the default REPLY is used,
because we want to have the real literal line, without any mangeling.
printf is used, because (depending on settings), echo may interpret
some baskslash-escapes or switches (like -n).
Press any key...
Remember the MSDOS pause command? Here's something similar:
pause() {
local dummy
read -s -r -p "Press any key to continue..." -n 1 dummy
}
Notes:
-sto suppress terminal echo (printing)-rto not interpret special characters (like waiting for a second character if somebody presses the backslash)
Reading Columns
Simple Split
Read can be used to split a string:
var="one two three"
read -r col1 col2 col3 <<< "$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" "$col3"
Why? because the commands of the pipe run in subshells that cannot
modify the parent shell. As a result, the variables col1, col2 and
col3 of the parent shell are not modified (see article:
processtree).
If the variable has more fields than there are variables, the last variable get the remaining of the line:
read col1 col2 col3 <<< "one two three four"
printf "%s\n" "$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(r) only specified the
-roption (raw read);-ris not only POSIX, you can find it in earlier Bourne source code - POSIX(r) doesn't support arrays
REPLYis not POSIX(r), you need to setIFSto the empty string to get the whole line for shells that don't knowREPLY.while IFS= read -r line; do ... done < text.txt
See also
- Internal: The printf builtin command
-
fixed in 4.2-rc1 ↩︎