Why does my Bash script generate a "unary operator expected error"?

Question

I trying to make a program that outputs a triangle of stars in Bash.

Here is what it would look like:

Enter image description here

However, I am getting:

line 8: [: 1: unary operator expected error

Here is my code

#! /bin/bash

read -p "Please input a number for a magical surprise diagram: " input

tri1="1"
tri2="1"

while [ $tri1 -le $input ]; do
    while [ $tri2 -le $tri1 ]; do
        echo -n "*"
        tri2=$( ( $tri2 + 1 ) )
    done

    echo -n ""
    tri1=$( ( $tri1 + 1 ) )
    tri2=1
done

I am a beginner at Bash scripting, so please bear with me as I learn.

The canonical may be "unary operator expected" error in Bash if condition — Peter Mortensen
– Peter Mortensen, Commented Oct 5, 2021 at 17:11

Community · Accepted Answer · 2017-05-23 12:03:40Z

2

Here's a revised version that takes full advantage of arithmetic evaluation - ((...)) - and expansion $((...)):

#!/usr/bin/env bash

read -p "Please input a number for a magical surprise diagram: " input

tri1=1
tri2=1

while (( tri1 <= input )); do
  while (( tri2 <= tri1 )); do
    printf '*'
    (( ++tri2 ))
  done
  printf '\n'
  (( ++tri1 ))
  tri2=1
done

Also uses the more portable printf, which makes it clearer what is output.

Update:

Simplified version with for ((...;...;...)) loops in lieu of while:

#!/usr/bin/env bash

read -p "Please input a number for a magical surprise diagram: " input

for (( tri1 = 1; tri1 <= input; tri1++ )); do
  for (( tri2 = 1; tri2 <= tri1; tri2++ )); do
    printf '*'
  done
  printf '\n'
done

Even further simplification, replacing the inner loop with a printf trick (borrowed from here):

#!/usr/bin/env bash

read -p "Please input a number for a magical surprise diagram: " input

for (( i = 1; i <= input; i++ )); do
  printf '*%.s' $(seq $i)
  printf '\n'
done

edited May 23, 2017 at 12:03

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answered Feb 17, 2014 at 0:07

mklement0

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2 Comments

user3317098 Over a year ago

Thanks a lot, I didnt know you could do this, and ill try to implement it!

mklement0 Over a year ago

@user3317098: You're welcome; I should also have mentioned for ((...;...;...)) loops, which are better suited to your task - see my updated answer.

BMW · Accepted Answer · 2014-02-17 00:01:59Z

0

Here is the fix

#! /bin/bash

read -p "Plese input a number for a magical surprise diagram : " input

tri1="1"
tri2="1"

while [ $tri1 -le $input ]; do
   while [ $tri2 -le $tri1 ]; do
   echo -n "*"
   ((tri2=$tri2 + 1 ))    #fixed
   done
  echo -n "" 
    ((tri1= $tri1 + 1 )) #fixed
    tri2=1
  echo ""      # need add a return, otherwise, all output in one line.
done

answered Feb 17, 2014 at 0:01

BMW

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1 Comment

user3317098 Over a year ago

Thank you, this solved it! I didn't realize about the last echo line.

krico · Accepted Answer · 2014-02-16 23:23:08Z

0

Just try using quotes

while [ "$tri1" -le "$input" ]; do
  while [ "$tri2" -le " $tri1" ]; do ...

should fix it. Proably you have spaces on $input.

answered Feb 16, 2014 at 23:23

krico

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1 Comment

user3317098 Over a year ago

Thanks for the reply, but now it says that there is a line 8: [: : integer expression expected

Nik O'Lai · Accepted Answer · 2014-02-16 23:51:06Z

0

See the difference. This works

echo $(( 1+1 ))

while this one does not

echo $( ( 1+1 ) )

answered Feb 16, 2014 at 23:51

Nik O'Lai

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3 Comments

user3317098 Over a year ago

Thanks for the reply, I fixed it and added quotes, but I am still getting line 8: [: : integer expression expected

Nik O'Lai Over a year ago

Once i've corrected this mistake, I stopped getting this error.

Peter Mortensen Over a year ago

Another victim of the Indian space.

John B · Accepted Answer · 2014-02-17 00:56:21Z

0

I think this is a faster and more readable way to print ranging multiples of a character or string.

#!/bin/bash

read -p "Please input a number for a magical surprise diagram : " input

for i in $(seq $input); do
    c+="*"
    echo "$c"
done

It's important to use quotes here as * will otherwise expand to file names in the current directory.

answered Feb 17, 2014 at 0:56

John B

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Comments

Peter Mortensen · Accepted Answer · 2021-10-05 17:19:12Z

0

The two starting parentheses (( must be a single token for an arithmetic expression, so that it cannot be confused with two nested subshells ( ...; ( ... )).

Between (( and )) you can have any amount of whitespace around the operands and operators of the expression.

The first expression in your code should read either

tri2=$(( tri2 + 1 ))  # POSIX

or

(( tri2=tri2 + 1 ))   # Valid in Bash and ksh93

edited Oct 5, 2021 at 17:19

Peter Mortensen

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answered Feb 17, 2014 at 0:18

Henk Langeveld

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