How to make if not true condition

Question

I would like to have the echo command executed when cat  etc passwd   grep  sysa  is not true   What am I doing wrong   if       cat  etc passwd   grep  sysa      then         echo  ERROR - The user sysa could not be looked up          exit 2 fi

User · Answer

Here is an answer by way of example   In order to make sure data loggers are online a cron script runs every 15 minutes that looks like this      bin bash   if   ping -c 1 SOLAR  amp  gt  dev null then   echo  SUBJECT   SOLAR is not responding to ping    ssmtp abc def com   echo  SOLAR is not responding to ping    ssmtp 4151112222 txt att com else   echo  SOLAR is up  fi   if   ping -c 1 OUTSIDE  amp  gt  dev null then   echo  SUBJECT   OUTSIDE is not responding to ping    ssmtp abc def com   echo  OUTSIDE is not responding to ping    ssmtp 4151112222 txt att com else   echo  OUTSIDE is up  fi        and so on for each data logger that you can see in the montage at http   www SDsolarBlog com montage    FYI  using  amp  gt  dev null redirects all output from the command  including errors  to  dev null    The conditional only requires the exit status of the ping command   Also FYI  note that since cron jobs run as root there is no need to use sudo ping in a cron script

User · Answer

This one if         cat  etc passwd   grep  quot sysa quot      Then echo  quot  something quot  exit 2 fi

User · Answer

On Unix systems that supports it  not macOS it seems    if getent passwd   username   gt  dev null  then     printf  User  s exists n    username  else     printf  User  s does not exist n    username  fi    This has the advantage that it will query any directory service that may be in use  YP NIS or LDAP etc   and the local password database file     The issue with grep -q   username   etc passwd is that it will give a false positive when there is no such user  but something else matches the pattern  This could happen if there is a partial or exact match somewhere else in the file   For example  in my passwd file  there is a line saying  build   21 21 base and xenocara build  var empty  bin ksh   This would provoke a valid match on things like cara and enoc etc   even though there are no such users on my system   For a grep solution to be correct  you will need to properly parse the  etc passwd file   if cut -d     -f 1  etc passwd   grep -qxF   username   then       found else       not found fi       or any other similar test against the first of the  -delimited fields

User · Answer

I think it can be simplified into   grep sysa  etc passwd          echo  ERROR - The user sysa could not be looked up      exit 2     or in a single command line    grep sysa  etc passwd      echo  ERROR - The user sysa could not be looked up   exit 2

User · Answer

try  if   grep -q sysa  etc passwd   then   grep returns true if it finds the search target  and false if it doesn t   So NOT false    true   if evaluation in shells are designed to be very flexible  and many times doesn t require chains of commands  as you have written     Also  looking at your code as is  your use of the          form of cmd-substitution is to be commended  but think about what is coming out of the process  Try echo   cat  etc passwd   grep  sysa   to see what I mean  You can take that further by using the -c  count  option to grep and then do if       grep -c  sysa   etc passwd  -eq 0     then which works but is rather old school   BUT  you could use the newest shell features  arithmetic evaluation  like  if        grep -c  sysa   etc passwd     0      then        which also gives you the benefit of using the c-lang based comparison operators      lt   gt   gt    lt     and maybe a few others   In this case  per a comment by  Orwellophile  the arithmetic evaluation can be pared down even further  like  if        grep -c  sysa   etc passwd       then        OR  if        grep -c  sysa   etc passwd       then        Finally  there is an award called the Useless Use of Cat  UUOC    -  Some people will jump up and down and cry gothca  I ll just say that grep can take a file name on its cmd-line  so why invoke extra processes and pipe constructions when you don t have to   -    I hope this helps

User · Answer

What am I doing wrong           holds the value  not the exit status  that is why this approach is wrong  However  in this specific case  it does indeed work because sysa will be printed which makes the test statement come true  However  if       true     then echo false  fi would always print false because the true command does not write anything to stdout  even though the exit code is 0   That is why it needs to be rephrased to if   grep      then   An alternative would be cat  etc passwd   grep  sysa     echo error  Edit  As Alex pointed out  cat is useless here  grep  sysa   etc passwd    echo error   Found the other answers rather confusing  hope this helps someone

[bash] How to make "if not true condition"?

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