Bash scripting, multiple conditions in while loop

Question

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.

Solution 1

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.

Solution 2

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.

Comments

[danfuzz]

Actually, [[ is generally the preferred built-in to introduce a test expression. It has several advantages over the older single [ syntax.

[danfuzz]

I recommend [[ over [. See my comment on the other answer.

[drewmm]

That's fair. I used [ ] because that was what the OP was trying to use. I've seen both used with success.

[jake9115]

Fantastic answer, amazingly thorough

[SaxDaddy]

Bravo. a classic way to answer a post. Well done. I'm assuming you can use the same syntax with until as well, yes?

[chepner]

@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.

[msimmer92]

@danfuzz I know this is an old thread, but it would be great if you cite this in case people wanna read exactly why [[ is preferred over [. I know everyone can Google on their own... but still, it's good practice to cite.. makes your comment more robust and the reader's search faster and more efficient.

[uldics]

Seems this does not work when reading incoming data: while [[ read -r line && $inputdata != "" ]]; do. How do I achieve that?

[Admin]

When reading incoming data you can for example use this form for multiple conditions: while read -r theline && [ "$theline" != "" ]

[Arash]

@chepner could you please help me with my condition: while [[ $counter -le 97 -a "$breakCond" -le "$Mu"]]

[chepner]

-a is not recognized inside [[...]]; use &&.