What command means do nothing in a conditional in Bash

Question

Sometimes when making conditionals  I need the code to do nothing  e g   here  I want Bash to do nothing when  a is greater than  10   print  1  if  a is less than  5   otherwise  print  2    if     a  -ge 10   then elif     a  -le 5   then     echo  1  else     echo  2  fi   This makes an error though  Is there a command which will do nothing and also not slow down my script

User · Accepted Answer

The no-op command in shell is    colon    if     a  -ge 10   then       elif     a  -le 5   then     echo  1  else     echo  2  fi   From the bash manual         a colon    Do nothing beyond expanding arguments and performing redirections  The return status is zero

User · Answer

You can probably just use the true command   if     a  -ge 10    then     true elif     a  -le 5    then     echo  1  else     echo  2  fi   An alternative  in your example case  but not necessarily everywhere  is to re-order your if else   if     a  -le 5    then     echo  1  elif     a  -lt 10    then     echo  2  fi

User · Answer

Although I m not answering the original question concering the no-op command  many  if not most  problems when one may think  in this branch I have to do nothing  can be bypassed by simply restructuring the logic so that this branch won t occur   I try to give a general rule by using the OPs example     do nothing when  a is greater than  10   print  1  if  a is less than  5   otherwise  print  2    we have to avoid a branch where  a gets more than 10  so  a  lt  10 as a general condition can be applied to every other  following condition   In general terms  when you say do nothing when X  then rephrase it as avoid a branch where X  Usually you can make the avoidance happen by simply negating X and applying it to all other conditions   So the OPs example with the rule applied may be restructured as   if     a  -lt 10    amp  amp      a  -le 5   then     echo  1  elif     a  -lt 10   then     echo  2  fi   Just a variation of the above  enclosing everything in the  a  lt  10 condition   if     a  -lt 10   then     if     a  -le 5       then         echo  1      else         echo  2      fi fi    For this specific example  Flimzys restructuring is certainly better  but I wanted to give a general rule for all the people searching how to do nothing

[bash] What command means "do nothing" in a conditional in Bash?

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