Comparing variables in shell scripts

Question

I have got a project that involves shell scripts and comparing values/variables within them. I have looked here and elsewhere on comparing variables and I have tried all the various examples given but I am running into something that is not as advertised. OS is Solaris10 I have created the following script as a learning experience- ``` #!/bin/ksh stest() { if $X = $Y then echo they're the same else echo they're notthe same fi } X=a Y=a stest echo completed ``` I keep getting some variation of the following- using shell sh or ksh- ``` #./test.sh ./test.sh[2]: a: not found completed ``` using shell bash- ``` #./test.sh ./test.sh: line 5: a: command not found completed ``` I have tried enclosing the `if $X = $Y` line in brackets and double brackets and I get back ``` [a: not found ``` or ``` [[a: not found ``` If I change the variables X and Y to the numeral "1" I get the same thing- ``` ./test.sh[2]: 1: not found ``` I have tried enclosing things in single quotes, double quotes & backwa…

Accepted Answer

After `if`, you need a shell command, like anywhere else. `$X = $Y` is parsed as a shell command, meaning `$X` is interpreted as a command name (provided that the value of the variable is a single word).

You can use the `[` command (also available as `test`) or the `[[ … ]]` special syntax to compare two variables. Note that you need spaces on the inside of the brackets: the brackets are a separate token in the shell syntax.

```
if [ "$X" = "$Y" ]; then …
```

or

```
if [[ "$X" = "$Y" ]]; then …
```

`[ … ]` works in any shell, `[[ … ]]` only in ksh, bash and zsh.

Note that you need double quotes around the variables¹. If you leave off the quotes, then the variable is split into multiple words and each word is interpreted as a wildcard pattern. This doesn't happen inside `[[ … ]]`, but the right-hand side of `=` is interpreted as a wildcard pattern there too. Always put double quotes around variable substitutions (unless you want the value of the variable to be used as a list of filename matching patterns, rather than as a string).

¹ Except on `$X` the `[[ … ]]` syntax.

Comparing variables in shell scripts

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