Emulating a do-while loop in Bash

问题

What is the best way to emulate a do-while loop in Bash?

I could check for the condition before entering the while loop, and then continue re-checking the condition in the loop, but that\'s duplicated code. Is there a cleaner way?

Pseudo code of my script:

while [ current_time <= $cutoff ]; do
    check_if_file_present
    #do other stuff
done

This doesn\'t perform check_if_file_present if launched after the $cutoff time, and a do-while would.

回答1:

Two simple solutions:

Execute your code once before the while loop

actions() {
   check_if_file_present
   # Do other stuff
}

actions #1st execution
while [ current_time <= $cutoff ]; do
   actions # Loop execution
done

Or:

while : ; do
    actions
    [[ current_time <= $cutoff ]] || break
done

回答2:

Place the body of your loop after the while and before the test. The actual body of the while loop should be a no-op.

while 
    check_if_file_present
    #do other stuff
    (( current_time <= cutoff ))
do
    :
done

Instead of the colon, you can use continue if you find that more readable. You can also insert a command that will only run between iterations (not before first or after last), such as echo "Retrying in five seconds"; sleep 5. Or print delimiters between values:

i=1; while printf '%d' "$((i++))"; (( i <= 4)); do printf ','; done; printf '\n'

I changed the test to use double parentheses since you appear to be comparing integers. Inside double square brackets, comparison operators such as <= are lexical and will give the wrong result when comparing 2 and 10, for example. Those operators don't work inside single square brackets.

回答3:

Instead of the colon, you can use continue if you find that more readable

This method mimics the way of doing Do-While loops that I was looking for - to have the condition test after the actions. I also found that I can place an index increment (if needed) after the do statement, instead of a continue statement.

while 
    check_if_file_present
    #do other stuff
    (( current_time <= cutoff ))
do
    (( index++ ));
done

回答4:

We can emulate a do-while loop in Bash with while [[condition]]; do true; done like this:

while [[ current_time <= $cutoff ]]
    check_if_file_present
    #do other stuff
do true; done

For an example. Here is my implementation on getting ssh connection in bash script:

#!/bin/bash
while [[ $STATUS != 0 ]]
    ssh-add -l &>/dev/null; STATUS="$?"
    if [[ $STATUS == 127 ]]; then echo "ssh not instaled" && exit 0;
    elif [[ $STATUS == 2 ]]; then echo "running ssh-agent.." && eval `ssh-agent` > /dev/null;
    elif [[ $STATUS == 1 ]]; then echo "get session identity.." && expect $HOME/agent &> /dev/null;
    else ssh-add -l && git submodule update --init --recursive --remote --merge && return 0; fi
do true; done

It will give the output in sequence as below:

Step #0 - "gcloud": intalling expect..
Step #0 - "gcloud": running ssh-agent..
Step #0 - "gcloud": get session identity..
Step #0 - "gcloud": 4096 SHA256:XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX /builder/home/.ssh/id_rsa (RSA)
Step #0 - "gcloud": Submodule '.google/cloud/compute/home/chetabahana/.docker/compose' (git@github.com:chetabahana/compose) registered for path '.google/cloud/compute/home/chetabahana/.docker/compose'
Step #0 - "gcloud": Cloning into '/workspace/.io/.google/cloud/compute/home/chetabahana/.docker/compose'...
Step #0 - "gcloud": Warning: Permanently added the RSA host key for IP address 'XXX.XX.XXX.XXX' to the list of known hosts.
Step #0 - "gcloud": Submodule path '.google/cloud/compute/home/chetabahana/.docker/compose': checked out '24a28a7a306a671bbc430aa27b83c09cc5f1c62d'
Finished Step #0 - "gcloud"

来源：https://stackoverflow.com/questions/16489809/emulating-a-do-while-loop-in-bash

标签

bash

loops

do-while