This question is a sequel of sorts to my earlier question. The users on this site kindly helped me determine how to write a bash for loop that iterates over string values. For example, suppose that a loop control variable fname iterates over the strings "a.txt" "b.txt" "c.txt". I would like to echo "yes!" when fname has the value "a.txt" or "c.txt", and echo "no!" otherwise. I have tried the following bash shell script:


for fname in "a.txt" "b.txt" "c.txt"
 echo $fname
 if [ "$fname" = "a.txt" ] | [ "$fname" = "c.txt" ]; then
 echo "yes!"
 echo "no!"

I obtain the output:







Why does the if statement apparently yield true when fname has the value "a.txt"? Have I used | incorrectly?

  • 3
    In bash, 'or' operator is '||' (C style). – Marius Cotofana Sep 8 '12 at 20:47
  • 4
    You can also use -o within the same [ ]. – Thor Sep 8 '12 at 20:53
  • 8
    @Thor I'd prefer || and separate [ ] over -o for portability simply because [ is not guaranteed to support more than 4 arguments. Of course if the target language is bash, no one should be using [ anyways because bash's [[ is superior in many ways. – jw013 Sep 9 '12 at 0:46
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    @jw013 Thanks. Does this mean that I should be using if [[ "$fname" = "a.txt" ]] || [[ "$fname" = "c.txt" ]] rather than if [ "$fname" = "a.txt" ] || [ "$fname" = "c.txt" ]? – Andrew Sep 9 '12 at 17:14
  • 5
    @Andrew That is correct, if as you are declaring the shebang as bash, as you are already doing. One advantage of [[ is that it doesn't do word splitting (special case) so [[ $unquoted_var = string ]] is safe. – jw013 Sep 10 '12 at 2:06

If you want to say OR use double pipe (||).

if [ "$fname" = "a.txt" ] || [ "$fname" = "c.txt" ]

(The original OP code using | was simply piping the output of the left side to the right side, in the same way any ordinary pipe works.)

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  • 6
    Furthermore, || doesn't do a standard logic "OR" - it short-circuits, and the second command is run only if the first fails. – holdenweb Dec 12 '16 at 19:33
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    @holdenweb I'm pretty sure most modern optimized languages work the same way. No need to spend CPU cycles evaluating the second condition of OR if the first condition evaluates true. – bahamat Dec 12 '16 at 20:50
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    I thought bash liked == but after seeing this answer, I decided to look it up. Apparently, "it can be used but isn't standard". I thought I'd put this here for others if your curious: stackoverflow.com/a/2237103 – harperville Sep 17 '18 at 15:55
  • This is what the test man page recommends too – cdosborn Apr 21 '19 at 18:13
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    You can also use double bracket tests - if [[ "$fname" = "a.txt" ]] || [[ "$fname" = "c.txt" ]] (If you want or need to have the extra functionality associated with [[ ]]). – HankCa Apr 24 '19 at 5:45

The accepted answer is good but since you're using bash I'll add the bash solution:

if [[ "$fname" == "a.txt" || "$fname" == "c.txt" ]]; then

This is documented in the bash reference manual Conditional Constructs

expression1 && expression2

True if both expression1 and expression2 are true.

expression1 || expression2

True if either expression1 or expression2 is true.

The && and || operators do not evaluate expression2 if the value of expression1 is sufficient to determine the return value of the entire conditional expression.

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You can use or condition like this

 if [ "$fname" = "a.txt" -o "$fname" = "c.txt" ]
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How about something like:

if [ a = b ] || [ c = d && e = f]

Syntax can be tricky...

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  • 1
    Syntax can be tricky, indeed. Do consider revisiting this answer to fix the syntax errors (spacing). Also consider testing it with variables containing spaces rather than just using static strings ([ a = b ] in your code would always be false, for example). – Kusalananda Mar 5 at 7:09

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