doubt on while loop

[laiju.c.babu]

Hi Gurus,

In a script i saw a while loop  having below condition

while [[ -a $LOCK_FILE ]]

do { ===== done }

could you please tell me what this while condition will do. This  LOCK_FILE is decalred as /tmp/audit.lock

Regards

[Matti_Kurkela]

The expression with double square brackets is a conditional expression that is parsed by the shell that is executing the script, so you should identify the shell that is used to run the script. Is it sh, ksh or something else?

For example, if the script is run using /sbin/sh or /usr/bin/sh in HP-UX, the sh-posix(1) man page lists all the possible conditions for a double-square-bracket conditional expression in a chapter titled Conditional Expressions.

In this case, -a <filename> means "the condition is true if <filename> exists". The test completely ignores the type of <filename>: it might actually be a directory, a symbolic link, a named pipe, or something else.

The loop will run as long as /tmp/audit.lock exists.

You might have been confused because in a single-square-bracket expression, the -a has a completely different meaning: it appears as <condition> -a <condition> and means that the expression is true only if both conditions are true.

[Dennis Handly]

Be careful if replacing it with -e, that's only for sh, not ksh.

[laiju.c.babu]

Hi MK,

Thanks for the reply, the shell on which this script is executing is /usr/bin/ksh.

Regards