Why test for equality in sh scripts in an indirect way?

Question

I often see this construct in sh scripts: ``` if [ "z$x" = z ]; then echo x is empty; fi ``` Why don't they just write it like this? ``` if [ "$x" = "" ]; then echo x is empty; fi ```

Accepted Answer

TL;DR short answer

In this construct:

```
if [ "z$x" = z ]; then echo x is empty; fi
```

the `z` is a guard against funny content of `$x` and many other problems.

If you write it without the `z`:

```
if [ "$x" = "" ]; then echo x is empty; fi
```

and `$x` contains the string `-x` you will get:

```
if [ "-x" = "" ]; then echo x is empty; fi
```

and that confuses the hell out of some older implementations of `[`.

If you further omit the quotes around `$x` and `$x` contains the string `-f foo -o x` you will get:

```
if [ -f foo -o x = "" ]; then echo x is empty; fi
```

and now it silently checks for something completely different.

the guard will prevent these maybe honest human errors maybe possibly malicious attacks to fall through silently. with the guard you either get the correct result or an error message. read on for an elaborate explanation.

Elaborate explanation

The `z` in

```
if [ "z$x" = z ]; then echo x is empty; fi
```

is called a guard.

To explain why you want the guard I first want to explain the syntax of the bash conditional `if`. It is important to understand that `[` is not part of the syntax. It is a command. It is an alias to the `test` command. And in most current shells it is a builtin command.

The grammar rule for `if` is roughly as follows:

```
if command; then morecommands; else evenmorecommands; fi
```

(the `else` part is optional)

`command` can be any command. Really any command. What bash does when it encounters an `if` is roughly as follows:

- Execute `command`.

- Check the exit status of `command`.

- If exit status is `0` then execute `morecommands`. If exit status is anything else, and the `else` part exists, then execute `evenmorecommands`.

Let's try that:

```
$ if true; then echo yay; else echo boo; fi
yay
$ if wat; then echo yay; else echo boo; fi
bash: wat: command not found
boo
$ if echo foo; then echo yay; else echo boo; fi
foo
yay
$ if cat foo; then echo yay; else echo boo; fi
cat: foo: No such file or directory
boo
```

Let's try the `test` command:

```
$ if test z = z; then echo yay; else echo boo; fi
yay
```

And the alias `[`:

```
$ if [ z = z ]; then echo yay; else echo boo; fi
yay
```

You see `[` is not part of the syntax. It is just a command.

Note that the `z` here has no special meaning. It is just a string.

Let's try the `[` command outside of an `if`:

```
$ [ z = z ]
```

Nothing happens? It returned an exit status. You can check the exit status with `echo $?`.

```
$ [ z = z ]
$ echo $?
0
```

Let's try unequal strings:

```
$ [ z = x ]
$ echo $?
1
```

Because `[` is a command it accepts parameters just like any other commands. In fact, the closing `]` is also a parameter, a mandatory parameter which must come last. If it is missing the command will complain:

```
$ [ z = z
bash: [: missing `]'
```

It is misleading that bash does the complaining. Actually the builtin command `[` does the complaining. We can see more clearly who does the complaining when we invoke the system `[`:

```
$ /usr/bin/[ z = z
/usr/bin/[: missing `]'
```

Interestingly the system `[` doesn't always insist on a closing `]`:

```
$ /usr/bin/[ --version
[ (GNU coreutils) 7.4
...
```

You need a space before the closing `]` otherwise it will not be recognized as a parameter:

```
$ [ z = z]
bash: [: missing `]'
```

You also need a space after the `[` otherwise bash will think you want to execute another command:

```
$ [z = z]
bash: [z: command not found
```

This is much more obvious when you use `test`:

```
$ testz = z
bash: testz: command not found
```

Remember `[` is just another name for `test`.

`[` can do more than just compare strings. It can compare numbers:

```
$ [ 1 -eq 1 ]
$ [ 42 -gt 0 ]
```

It can also check for the existence of files or directories:

```
$ [ -f filename ]
$ [ -d dirname ]
```

See `help [` or `man [` for more information about the capabilities of `[` (or `test`). `man` will show you the documentation for the system command. `help` will show you the documentation for the bash builtin command.

Now that I have covered the bases I can answer your question:

Why do people write this:

```
if [ "z$x" = z ]; then echo x is empty; fi
```

and not this:

```
if [ "$x" = "" ]; then echo x is empty; fi
```

For brevity I will strip off the `if` because this is only about `[`.

The `z` in this construct:

```
[ "z$x" = z ]
```

is a guard against funny content of `$x` in combination with older implementations of `[`, and/or a guard against human error like forgetting to quote `$x`.

What happens when `$x` has funny content like `-f`?

This

```
[ "$x" = "" ]
```

will become

```
[ "-f" = "" ]
```

Some older implementations of `[` will get confused when the first parameter starts with a `-`. The `z` will make sure that the first parameter never starts with a `-` regardless of content of `$x`.

```
[ "z$x" = "z" ]
```

will become

```
[ "z-f" = "z" ]
```

What happens when you forgot to quote `$x`? Funny content like `-f foo -o x…

Why test for equality in sh scripts in an indirect way?

Solution

Problem