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.

3

4 Answers

Two simple solutions:

1. 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

2. Or:

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

5

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.

6

This implementation:

• Has no code duplication
• Doesn't require extra functions()
• Doesn't depend on the return value of code in the "while" section of the loop:

do=true
while $do || conditions; do do=false # your code ...
done

It works with a read loop, too, skipping the first read:

do=true
while $do || read foo; do do=false # your code ... echo $foo
done

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' (: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"

2

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