Did I Execute? - Akash Trehan

Did I Execute?


I’ll be discussing about a small detail, about executing multiple commands on a terminal, which I realised most people don’t know about.

There are three ways in which commands are combined:-

  1. ; (semicolon)
  2. && (Double ampersand)
  3. || (Double pipe)

What people don’t realise is that all of them are meant for completely different use cases.

Let me elaborate.

Semicolon ;

Semicolon is the legit command separator. Let us see some examples.

$ false ; echo "OK"
$ true ; echo "OK"

Thus A ; B implies Run A and then B. Very straight-foward. No tricks. Lets move to && and ||.

Double ampersand && and Double pipe ||

&& and || are Logical Binary Operators and as you might have guessed && is Logical AND, || is Logical OR.

The operands they take are boolean values - true or false, 1 or 0.

But where are the boolean values in echo hello && echo world! ?

These values are derived from the Exit status of the commands!

Note that a logical AND checks the second operand only when the first one is true. Similarly logical OR checks the second operand only when the first one is false.

Thus in A && B, B executes only when A is a success( exit status 0 ) while in A || B, B executes only when A returns an error. Thus both commands may not always execute! Lets see some examples.

$ false && echo "OK"
$ true && echo "OK"
$ false || echo "OK"
$ true || echo "OK"

This is unlike A ; B where B always executes irrespective of exit status of A. Both methods have their uses. A simple case where I use this little detail is while compiling programs.

g++ -o hack hack.cpp && ./hack

Here the binary is not executed if the compilation is not successful.

test_command && echo success || echo fail

We can get the proper control flow depending on whether test_command was a success or not.

$ true && echo success || echo fail
$ false && echo success || echo fail

Hope this helped :) In case you have any doubts comment below. If you are an Infosec person, don’t forget to checkout my Write-ups

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Akash Trehan

Akash Trehan


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