比较bash shell脚本中的两个字符串变量

Question

The output I get is sh21.sh: 5: [: xhi: unexpected operator no match

My code is as follows:

#!/bin/bash
s1="hi"
s2="hi"
s3="hello"
if [ "x$s1" == "x$s2" ]
then
  echo match
else
  echo no match
fi

Please explain to me what I am doing wrong.

Answer 1

If you are going to use bashisms in your script, it is important to use bash. Your code works fine with bash:

$ bash sh21.sh
match

It fails with dash (which is the sh on debian-like systems):

$ sh sh21.sh
sh21.sh: 5: [: xhi: unexpected operator
no match

== is a bashism, meaning it only works under bash or similar shells. If you want a POSIX compatible script, use = . If not, run the script under bash.

Answer 2

if [ "x$s1" == "x$s2" ]

should be

if [ "x$s1" = "x$s2" ]

There is only 1 equal sign when using test or [ in shell programming. Bash allows == with [[ , but it should not be used with [ . Both test and [ are equivalent and are the POSIX test utility. Bash has the [[ operator that is not the same. There are subtle differences in syntax, quoting requirements and available operators between them.

Answer 3

Maybe you are using Debian based distros, and the default shell is dash , not bash

Check your shell

ls -l /bin/sh /bin/bash

Run the script with bash

bash sh21.sh