Syntax for a single-line while loop in Bash

Question

I am having trouble coming up with the right combination of semicolons and/or braces. I'd like to do this, but as a one-liner from the command line:

while [ 1 ]
do
    foo
    sleep 2
done

Solution 1

while true; do foo; sleep 2; done

By the way, if you type it as a multiline (as you are showing) at the command prompt and then call the history with arrow up, you will get it on a single line, correctly punctuated.

$ while true
> do
>    echo "hello"
>    sleep 2
> done
hello
hello
hello
^C
$ <arrow up> while true; do    echo "hello";    sleep 2; done

Solution 2

It's also possible to use sleep command in while's condition. Making one-liner looking more clean imho.

while sleep 2; do echo thinking; done

Solution 3

Colon is always "true":

while :; do foo; sleep 2; done

Solution 4

You can use semicolons to separate statements:

$ while [ 1 ]; do foo; sleep 2; done

Solution 5

You can also make use of until command:

until ((0)); do foo; sleep 2; done

Note that in contrast to while, until would execute the commands inside the loop as long as the test condition has an exit status which is not zero.

---

Using a while loop:

while read i; do foo; sleep 2; done < /dev/urandom

---

Using a for loop:

for ((;;)); do foo; sleep 2; done

---

Another way using until:

until [ ]; do foo; sleep 2; done

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