What is the purpose of in a shell command

Question

As far as I know  using  amp  after the command is for running it in the background   Example of  amp  usage  tar -czf file tar gz dirname  amp   But how about  amp  amp    look at this example  https   serverfault com questions 215179 centos-100-disk-full-how-to-remove-log-files-history-etc answer-215188

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amp  amp  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

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A quite common usage for   amp  amp   is compiling software with autotools   For example     configure --prefix  usr  amp  amp  make  amp  amp  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  amp  amp    cd package    configure  amp  amp  make  amp  amp  sudo make install    amp  amp  rm package -rf   I do this when for example making a linux from scratch box

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command 1  amp  amp  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  amp  amp  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

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See the example   mkdir test  amp  amp  echo  Something   gt  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

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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  amp  amp  echo  Things went well  Things went well    false  amp  amp  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

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command-line - what is the purpose of  amp  amp     In shell  when you see     command one  amp  amp  command two   the intent is to execute the command that follows the  amp  amp  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  amp  amp  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           amp  amp                               And the Grammar  also quoted from the documentation   which shows that any number of AND IFs   amp  amp   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   amp  amp   and             A list is a sequence of one or more AND-OR lists separated by the   operators     and   amp   and optionally terminated by        amp    or          The operators   amp  amp   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  amp  amp  echo foo    echo bar   true    echo foo  amp  amp  echo bar     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  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

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amp  amp  lets you do something based on whether the previous command completed successfully - that s why you tend to see it chained as do something  amp  amp  do something else that depended on something

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amp  amp  or  Logical AND                         first command  1  two command  2   if      first command    1   amp  amp    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

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It s to execute a second statement if the first statement ends succesfully  Like an if statement    if  1    1  amp  amp  2    2    echo  test     Its first tries if 1  1  if that is true it checks if 2  2

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

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