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up vote 2 down vote favorite

I tried using this:

$ if [$a == 1] then { echo 'yes'; } fi;

but I get an error:

-bash: syntax error near unexpected token `}'

What is the correct format? I tried several with no luck.

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you write bash not csh –  PersianGulf Sep 30 '13 at 6:43
    
Related: using single or double bracket - bash; Understanding quotes in a bash conditional instruction –  Gilles Sep 30 '13 at 20:27

2 Answers 2

up vote 5 down vote accepted

[ is just another character, according to bash; it's not self-delimiting. So you need to put spaces around [ and ]. Although you'd be better off using [[ and ]].

And the command following if (yes, [ is a command) must be terminated with a ; or a newline.

Finally, == (which is not posix, FWIW; posix prefers =) is string equality, not numeric equality.

So you might have meant:

if [[ $a -eq 1 ]]; then echo yes; fi

But you could use arithmetic evaluation instead:

if ((a == 1)); then echo yes; fi

(In arithmetic evalution, equality is ==, and you don't need $ before variable names. I know it's confusing.)

For more information about [: help test. About [[: help [[ (which builds on test). About ((: help let. About bash: man bash

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up vote 2 down vote

[ is a command in Bash, just like any of the other commands such as if, while, etc. You can see this if you double check the man page:

$ man [
NAME
       bash, :, ., [, alias, bg, bind, break, builtin, caller, cd, command, .....

You can also tell it's a real command with this example:

$ type -a [
[ is a shell builtin
[ is /usr/bin/[

The first result is the builtin version of [ that's part of Bash. The second is the version of [ that's included with the GNU coreutils.

On Fedora you can see what RPM it's a part of:

$ rpm -qf /usr/bin/[
coreutils-8.5-7.fc14.x86_64

Given this you need to make sure that there are spaces around any commands so that they get parsed correctly.

Doing this:

$ if [$a == 1] ...

Would be identical to this:

$ lsblah
bash: lsblah: command not found...

The ls command cannot be parsed correctly because it isn't buffered with spaces so that it's parse-able.

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