CODEWITHSUNDEEP
bashoperators
bcsteeve
bcsteeve

Reputation: 1001

In bash, what is the -a operator doing?

Complete noob in bash. Trying to edit existing script. I've Googled things like "Bash operators" and I can't find this one.

Here's the snippet:

if [ "${activ_con}" -a ! "${activ_vpn}" ];
then
    nmcli con up id "${VPN_CONNECTION_NAME}"
fi

I just want to know what the "-a !" bit is doing. Thanks. (like always, I'll probably find it in the next few minutes... back to GOogle!)

Upvotes: 1

Views: 138

Answers (3)

MajiK
MajiK

Reputation: 647

-a and -o are so called compound statement. -a is same as logical "&&" operator and -o is the same as logical "||" operator. here is the difference:

if [conditionA -o conditionB]

compared to:

if [conditionA] || [conditionB]

Same case for -a and &&.

Upvotes: 0

Jonathan Leffler
Jonathan Leffler

Reputation: 754190

It (-a) is the 'and' operator, and -o is the 'or' operator.

An alternative is to use:

if [ -n "${activ_con}" ] && [ -z "${activ_vpn}" ]
then
    nmcli con up if "${VPN_CONNECTION_NAME}"
fi

This explicitly tests for a non-zero length of "${activ_con}" and for the zero length of "${active_vpn}". This is more nearly the style recommended by the Autoconf shell style rules (but they don't use [ for test at all because the square brackets are a metacharacter. Some old implementations (in the 1980s and early 1990s, perhaps) had issues misinterpreting the operands to the test command, so the advice given avoids using the -a and -o because these were the options that lead to confusion.

The POSIX specification for test (and [ is a synonym for test; on many systems, there is also a command /bin/[ that is a link to /bin/test, but the shell normally uses a built-in implementation of test) explicitly specifies that -a is used as a connective meaning 'and'. (Note the rationale section of the test page on the POSIX site.)

The GNU bash specification for Conditional Expressions does not list -a as a connective, nor does it list -o at all. That's interesting.

Upvotes: 1

Nathan Grigg
Nathan Grigg

Reputation: 784

It means "and".

From the manual page for test:

 expression1 -a expression2
               True if both expression1 and expression2 are true.

The bracket is a synonym for test.

Upvotes: 4

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