boolean type for while loop in bash?

Question

I have a cron script on a shared web host that occasionally gets killed. I'd like to make a loop in bash that tries again if it gets killed, because most of the time it will make it. I'm having trouble with the syntax for storing a boolean value :P

#!/bin/bash
VAR=0;

while [ $VAR ]; do
    if nice -19 mysqldump -uuser -ppassword -h database.hostname.com --skip-opt --all --complete-insert --add-drop-table database_name > ~/file/system/path/filename.sql; then
        VAR=1;
    fi
done

So the script recovers from a killed process okay, but once it's run properly, the new VAR value doesn't kill the while loop.

What am I doing wrong?

kennytm · Accepted Answer · 2018-09-02 10:03:11Z

10

Try

while [ "$VAR" -eq 0 ]; do

0 and 1 are both considered True because they are not null strings.

edited Sep 2, 2018 at 10:03

answered Apr 22, 2010 at 19:17

kennytm

526k111 gold badges1.1k silver badges1k bronze badges

Sign up to request clarification or add additional context in comments.

1 Comment

user151841 Over a year ago

That's right, but for how I wrote it in my script, it should actually be -ne 1 :)

Jürgen Hötzel · Accepted Answer · 2010-04-22 20:16:28Z

8

No need to you a helper variable, consider using:

while ! mysqldump .....; do :;done

answered Apr 22, 2010 at 20:16

Jürgen Hötzel

20k3 gold badges44 silver badges59 bronze badges

2 Comments

user151841 Over a year ago

What does the colon mean after do?

Jürgen Hötzel Over a year ago

No operation: Just to satisfy bash syntax.

amertune · Accepted Answer · 2010-04-22 23:36:32Z

4

You could use /bin/true and /bin/false. Although while ! mysqldump .....; do :;done (Jürgen Hötzel's solution) would be better in this case, there are other cases when using true and false are useful.

#!/bin/bash
VAR=true

while $VAR; do
  if nice -19 mysqldump -uuser -ppassword -h database.hostname.com --skip-opt --all --complete-insert --add-drop-table database_name > ~/file/system/path/filename.sql; then
    VAR=false
  fi
done

answered Apr 22, 2010 at 23:36

amertune

6905 silver badges9 bronze badges

1 Comment

Pierre-Olivier Vares Over a year ago

I was going to say "mmmm. Shouldn't it be VAR=true AND var=false ? Because - unless error - VAR=true means assigning the string 'true' to VAR. and VAR=false means assigning 'false' to VAR."... but No, because while $VAR executes $VAR.

Dennis Williamson · Accepted Answer · 2010-04-22 22:26:00Z

3

In Bash, you can do a numeric conditional using double parentheses rather than a string conditional, but the logic is inverted:

#!/bin/bash
VAR=1;

while (( VAR )); do
    if nice -19 mysqldump -uuser -ppassword -h database.hostname.com --skip-opt --all --complete-insert --add-drop-table database_name > ~/file/system/path/filename.sql; then
        VAR=0;
    fi
done

answered Apr 22, 2010 at 22:26

Dennis Williamson

364k95 gold badges386 silver badges446 bronze badges

Comments

Aarif · Accepted Answer · 2020-08-22 07:19:42Z

1

I like until better for this... :

i=0
until [ $_done ]; do
   echo stuff...;
   sleep 1;
   (( i++ ));
   if [ "$i" -gt 9 ]; then
      _done=1
   fi
done

edited Aug 22, 2020 at 7:19

Aarif

1,6833 gold badges19 silver badges29 bronze badges

answered Aug 8, 2017 at 23:59

Rondo

3,75631 silver badges28 bronze badges

Collectives™ on Stack Overflow

boolean type for while loop in bash?

5 Answers 5

1 Comment

2 Comments

1 Comment

Comments

Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

1 Comment

2 Comments

1 Comment

Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related