Between while and do in shell script

Question

I have the following script that works fine:

!/bin/bash

a=12
while [ $a -gt 10 ]
do
    echo "$a"
    a=$(($a-1))
done
echo "done"

If I add line "echo something" above "do", I expect to see a syntax error in that line. It seems that [ $a -gt 10 ] is bypassed, and it becomes an infininte loop. How could that happen?

Answer 1

From the bash manual:

while

The syntax of the while command is:

while test-commands; do consequent-commands;
done

Execute consequent-commands as long as test-commands has an exit status of zero. The return status is the exit status of the last command executed in consequent-commands, or zero if none was executed.

Note: test-commands, plural. You can use multiple commands in the test, and so this is a perfectly valid loop, with the list of commands [ $a -gt 10 ]; echo "$a" as the test:

while [ $a -gt 10 ]
echo "$a"
do
   a=$(($a-1))
done

While the command [ $a -gt 10 ] may or may not fail, the echo will (almost) always succeed (unless it couldn't write the text, or some other error happened), so the final exit status of the test commands will always be success, and the loop will always be run.

Answer 2

From man bash:

while list-1; do list-2; done
The while command continuously executes the list list-2
as long as the last command in the list list-1
returns an exit status of zero.

Which implies that a list could contain several commands, which it does (separated mostly by semicolons or newlines).

So, this works perfectly well:

#!/bin/bash

a=12

while 
    echo something
    echo "a before test =  $a"
    [ a -gt 10 ]
do
    echo "a after test =  $a"
    a=$(($a-1))
done
echo "done"

If the last command before the do is echo the exit code that the do receive is always true (0) and the loop becomes infinite.

Answer 3

As an example of this being taken to extremes, /usr/bin/tzselect typically has about 70 lines of code between the while and the do of the main loop which includes case statements and command substitution, and a single line between the do and the done.

Answer 4

What the other answers here imply but don't explicitly say is that [ is a built-in command, not a syntactical part of the while statement.

Try typing help [ at your command line:

[: [ arg... ]
Evaluate conditional expression.

This is a synonym for the "test" builtin, but the last argument must
be a literal ], to match the opening [.

So your script is exactly as:

!/bin/bash
a=12
while test $a -gt 10
do
  echo "$a"
  a=$(($a-1))
done
echo "done"

Which you change to:

!/bin/bash
a=12
while test $a -gt 10; echo something
do
  echo "$a"
  a=$(($a-1))
done
echo "done"

Answer 5

(as a note to complement other answers).

Using several commands in the condition list is often used to implement a loop similar to C's do { blah; blah; } while (condition), that is where the condition is checked at the end of the loop so the code in the loop is run at least once.

In sh, you'd do it as:

while
  blah
  blah
  condition
do
  continue # or :
done

Though other approaches are possible like:

while true; do
  blah
  blah
  condition || break
done

Or:

continue=true
while "$continue"; do
  blah
  blah
  condition || continue=false
done

end=false
until "$end"; do
  blah
  blah
  condition || end=true
done