How to make a bash function which can read from standard input?
I have some scripts that work with parameters, they work just fine but i would like them to be able to read from stdin, from a pipe for example, an example, suppose this is called read:
#!/bin/bash
function read()
{
echo $*
}
read $*
Now this works with read "foo" "bar"
, but I would like to use it as:
echo "foo" | read
How do I accomplish this?
Solution 1:
It's a little tricky to write a function which can read standard input, but works properly when no standard input is given. If you simply try to read from standard input, it will block until it receives any, much like if you simply type cat
at the prompt.
In bash 4, you can work around this by using the -t
option to read
with an argument of 0. It succeeds if there is any input available, but does not consume any of it; otherwise, it fails.
Here's a simple function that works like cat
if it has anything from standard input, and echo
otherwise.
catecho () {
if read -t 0; then
cat
else
echo "$*"
fi
}
$ catecho command line arguments
command line arguments
$ echo "foo bar" | catecho
foo bar
This makes standard input take precedence over command-line arguments, i.e., echo foo | catecho bar
would output foo
. To make arguments take precedence over standard input (echo foo | catecho bar
outputs bar
), you can use the simpler function
catecho () {
if [ $# -eq 0 ]; then
cat
else
echo "$*"
fi
}
(which also has the advantage of working with any POSIX-compatible shell, not just certain versions of bash
).
Solution 2:
You can use <<<
to get this behaviour. read <<< echo "text"
should make it.
Test with readly
(I prefer not using reserved words):
function readly()
{
echo $*
echo "this was a test"
}
$ readly <<< echo "hello"
hello
this was a test
With pipes, based on this answer to "Bash script, read values from stdin pipe":
$ echo "hello bye" | { read a; echo $a; echo "this was a test"; }
hello bye
this was a test
Solution 3:
To combine a number of other answers into what worked for me (this contrived example turns lowercase input to uppercase):
uppercase() {
local COMMAND='tr [:lower:] [:upper:]'
if [ -t 0 ]; then
if [ $# -gt 0 ]; then
echo "$*" | ${COMMAND}
fi
else
cat - | ${COMMAND}
fi
}
Some examples (the first has no input, and therefore no output):
:; uppercase
:; uppercase test
TEST
:; echo test | uppercase
TEST
:; uppercase <<< test
TEST
:; uppercase < <(echo test)
TEST
Step by step:
-
test if file descriptor 0 (
/dev/stdin
) was opened by a terminalif [ -t 0 ]; then
-
tests for CLI invocation arguments
if [ $# -gt 0 ]; then
-
echo all CLI arguments to command
echo "$*" | ${COMMAND}
-
else if
stdin
is piped (i.e. not terminal input), outputstdin
to command (cat -
andcat
are shorthand forcat /dev/stdin
)else cat - | ${COMMAND}
Solution 4:
Here is example implementation of sprintf
function in bash which uses printf
and standard input:
sprintf() { local stdin; read -d '' -u 0 stdin; printf "$@" "$stdin"; }
Example usage:
$ echo bar | sprintf "foo %s"
foo bar
This would give you an idea how function can read from standard input.