bash scripting basic string comparison

Question

I have a basic bash scripting question. The output of this script, I expected to be:

a y b x c y

but instead, I get:

a x b x c x

#!/bin/bash                                                                     
for foo in 'a' 'b' 'c'; do
    echo $foo;                                         
    if [ "$foo"=="b" ]; then                                                                        
        echo x;                                                                 
    else                                                                        
        echo y;                                                                 
    fi                                                                                                                                                        
done;

What am I missing?

OK. I figured it out. Having spaces between the operator makes a difference. Argh. — Charles Nobbert
– Charles Nobbert, Commented Nov 1, 2011 at 18:50

glenn jackman · Accepted Answer · 2011-11-01 23:55:03Z

4

Try this script:

#!/bin/bash

for foo in 'a' 'b' 'c'; do
  echo "$foo";

  if [ "$foo" = 'b' ]; then
    echo 'x';
  else
    echo 'y';
  fi
done;

You use = for string comparisons in bash. Also, quote your echo'd strings.

edited Nov 1, 2011 at 23:55

glenn jackman

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answered Nov 1, 2011 at 18:51

Blender

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Comments

Adam Rosenfield · Accepted Answer · 2011-11-02 15:01:15Z

2

You need to add spaces around the == operator, otherwise it gets parsed as the single token a==b (after expansion, for the first iteration). This gets passed to the test builtin (for which [ is an alternative name). Since the single argument a==b is a non-empty string atom, it succeeds and exits with a status of 0, and the then branch gets executed.

edited Nov 2, 2011 at 15:01

answered Nov 1, 2011 at 18:52

Adam Rosenfield

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1 Comment

glenn jackman Over a year ago

Actually, it succeeds and the then branch is executed. With a single argument, test returns true if the argument is non-null, and a==b is a non-null string.

Rohan Dsouza · Accepted Answer · 2017-06-17 12:17:58Z

0

Take a look at the following:

#!/bin/bash

for foo in 'a' 'b' 'c'; do

    echo $foo;

    if [ $foo == "b" ]; then        # Make a note of the spaces
        echo "x";
    else
        echo "y";
    fi
done;

Regards,

Rohan Dsouza

answered Jun 17, 2017 at 12:17

Rohan Dsouza

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