What is the purpose of "&&" in a shell command?

As far as I know, using & after the comman开发者_运维技巧d is for running it in the background.

Example of & usage: tar -czf file.tar.gz dirname &

But how about &&? (example)

Furthermore, you also have || which is the logical or, and also ; which is just a separator which doesn't care what happend to the command before.

$ false || echo "Oops, fail"
Oops, fail

$ true || echo "Will not be printed"
$  

$ true && echo "Things went well"
Things went well

$ false && echo "Will not be printed"
$

$ false ; echo "This will always run"
This will always run

Some details about this can be found here Lists of Commands in the Bash Manual.

&& lets you do something based on whether the previous command completed successfully - that's why you tend to see it chained as do_something && do_something_else_that_depended_on_something.

command-line - what is the purpose of &&?

In shell, when you see

$ command one && command two

the intent is to execute the command that follows the && only if the first command is successful. This is idiomatic of Posix shells, and not only found in Bash.

It intends to prevent the running of the second process if the first fails.

You may notice I've used the word "intent" - that's for good reason. Not all programs have the same behavior, so for this to work, you need to understand what the program considers a "failure" and how it handles it by reading the documentation and, if necessary, the source code.

Your shell considers a return value of 0 for true, other positive numbers for false

Programs return a signal on exiting. They should return 0 if they exit successfully, or greater than zero if they do not. This allows a limited amount of communication between processes.

The && is referred to as AND_IF in the posix shell grammar, which is part of an and_or list of commands, which also include the || which is an OR_IF with similar semantics.

Grammar symbols, quoted from the documentation:

%token  AND_IF    OR_IF    DSEMI
/*      '&&'      '||'     ';;'    */

And the Grammar (also quoted from the documentation), which shows that any number of AND_IFs (&&) and/or OR_IFs (||) can be be strung together (as and_or is defined recursively):

and_or           :                         pipeline
                 | and_or AND_IF linebreak pipeline
                 | and_or OR_IF  linebreak pipeline

Both operators have equal precedence and are evaluated left to right (they are left associative). As the docs say:

An AND-OR list is a sequence of one or more pipelines separated by the operators "&&" and "||" .

A list is a sequence of one or more AND-OR lists separated by the operators ';' and '&' and optionally terminated by ';', '&', or .

The operators "&&" and "||" shall have equal precedence and shall be evaluated with left associativity. For example, both of the following commands write solely bar to standard output:

$ false && echo foo || echo bar
$ true || echo foo && echo bar

1. In the first case, the false is a command that exits with the status of 1

   $ false
   $ echo $?
   1
   

   which means echo foo does not run (i.e., shortcircuiting echo foo). Then the command echo bar is executed.

2. In the second case, true exits with a code of 0

   $ true
   $ echo $?
   0
   

   and therefore echo foo is not executed, then echo bar is executed.

A quite common usage for '&&' is compiling software with autotools. For example:

./configure --prefix=/usr && make && sudo make install

Basically if the configure succeeds, make is run to compile, and if that succeeds, make is run as root to install the program. I use this when I am mostly sure that things will work, and it allows me to do other important things like look at stackoverflow an not 'monitor' the progress.

Sometimes I get really carried away...

tar xf package.tar.gz && ( cd package; ./configure && make && sudo make install ) && rm package -rf

I do this when for example making a linux from scratch box.

&& strings commands together. Successive commands only execute if preceding ones succeed.

Similarly, || will allow the successive command to execute if the preceding fails.

See Bash Shell Programming.

It's to execute a second statement if the first statement ends succesfully. Like an if statement:

 if (1 == 1 && 2 == 2)
  echo "test;"

Its first tries if 1==1, if that is true it checks if 2==2

See the example:

mkdir test && echo "Something" > test/file

Shell will try to create directory test and then, only if it was successfull will try create file inside it.

So you may interrupt a sequence of steps if one of them failed.

command_1 && command_2: execute command_2 only when command_1 is executed successfully.

command_1 || command_2: execute command_2 only when command_1 is not successful executed.

Feels similar as how an 'if' condition is executed in a mainstream programming language, like, in if (condition_1 && condition_2){...} condition_2 will be omitted if condition_1 is false and in if (condition_1 || condition_2){...} condition_2 will be omitted if condition_1 is true. See, it's the same trick you use for coding :)

####################### && or (Logical AND) ######################
first_command="1"
two_command="2"

if [[ ($first_command == 1) && ($two_command == 2)]];then
 echo "Equal"
fi

When program checks if command, then the program creates a number called exit code, if both conditions are true, exit code is zero (0), otherwise, exit code is positive number. only when displaying Equal if exit code is produced zero (0) that means both conditions are true.