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up vote 14 down vote favorite
6

I'm trying to get a simple while loop working in bash that uses two conditions, but after trying many different syntax from various forums, I can't stop throwing an error. Here is what I have:

while [ $stats -gt 300 ] -o [ $stats -eq 0 ]

I have also tried:

while [[ $stats -gt 300 ] || [ $stats -eq 0 ]]

... as well as several others constructs. I want this loop to continue while $stats is > 300 or if $stats = 0.

share|improve this question

3 Answers 3

up vote 37 down vote accepted

The correct options are (in increasing order of recommendation):

# Single POSIX test command with -o operator (not recommended anymore).
# Quotes strongly recommended to guard against empty or undefined variables.
while [ "$stats" -gt 300 -o "$stats" -eq 0 ]

# Two POSIX test commands joined in a list with ||.
# Quotes strongly recommended to guard against empty or undefined variables.
while [ "$stats" -gt 300 ] || [ "$stats" -eq 0 ]

# Two bash conditional expressions joined in a list with ||.
while [[ $stats -gt 300 ]] || [[ $stats -eq 0 ]]

# A single bash conditional expression with the || operator.
while [[ $stats -gt 300 || $stats -eq 0 ]]

# Two bash arithmetic expressions joined in a list with ||.
# $ optional, as a string can only be interpreted as a variable
while (( stats > 300 )) || (( stats == 0 ))

# And finally, a single bash arithmetic expression with the || operator.
# $ optional, as a string can only be interpreted as a variable
while (( stats > 300 || stats == 0 ))

Some notes:

  1. Quoting the parameter expansions inside [[ ... ]] and ((...)) is optional; if the variable is not set, -gt and -eq will assume a value of 0.

  2. Using $ is optional inside (( ... )), but using it can help avoid unintentional errors. If stats isn't set, then (( stats > 300 )) will assume stats == 0, but (( $stats > 300 )) will produce a syntax error.

share|improve this answer
    
+1 for good answer + (close to ?) exhaustive – Gilles Quenot Mar 20 '13 at 22:37
    
Fantastic answer, amazingly thorough – jake9115 Mar 21 '13 at 14:49
    
Bravo. a classic way to answer a post. Well done. I'm assuming you can use the same syntax with until as well, yes? – SaxDaddy Aug 14 '14 at 21:05
    
@SaxDaddy More or less, as long as you take care to negate the conditions correctly: while [ foo -o bar ] becomes until ! [ foo -o bar ], but while foo || bar becomes until ! foo && ! bar. – chepner Aug 14 '14 at 21:19
up vote 1 down vote

Try:

while [ $stats -gt 300 -o $stats -eq 0 ]

[ is a call to test. It is not just for grouping, like parentheses in other languages. Check man [ or man test for more information.

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1  
I recommend [[ over [. See my comment on the other answer. – danfuzz Mar 20 '13 at 21:50
    
That's fair. I used [ ] because that was what the OP was trying to use. I've seen both used with success. – drewmm Mar 20 '13 at 21:59
up vote 0 down vote

The extra [ ] on the outside of your second syntax are unnecessary, and possibly confusing. You may use them, but if you must you need to have whitespace between them.

Alternatively:

while [ $stats -gt 300 ] || [ $stats -eq 0 ]
share|improve this answer
    
Actually, [[ is generally the preferred built-in to introduce a test expression. It has several advantages over the older single [ syntax. – danfuzz Mar 20 '13 at 21:49

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