CODEWITHSUNDEEP
linuxbashunixksh
Rahul sawant
Rahul sawant

Reputation: 425

What does if [[ $? -ne 0 ]]; mean in .ksh

I have a following piece of code that says if everything is executed mail a person if it fails mail the person with a error message. 

if [[ $? -ne 0 ]]; then
  mailx -s" could not PreProcess files" [email protected]
else            
  mailx -s" PreProcessed files" [email protected]
fi
done

I am new to linux coding I want to understand what if [[ $? -ne 0 ]]; means

Upvotes: 16

Views: 48379

Answers (4)

bishop
bishop

Reputation: 39424

Breaking it down, simple terms:

[[ and ]]

... signifies a test is being made for truthiness.

$?

... is a variable holding the exit code of the last run command.

-ne 0

... checks that the thing on the left ($?) is "not equal" to "zero". In UNIX, a command that exits with zero succeeded, while an exit with any other value (1, 2, 3... up to 255) is a failure.

Upvotes: 27

William Pursell
William Pursell

Reputation: 212374

Presumably, the snippet is part of code that looks something like:

 for var in list of words; do
   cmd $var
   if [[ $? -ne 0 ]]; then
      mailx -s" could not PreProcess files" [email protected]
    else            
      mailx -s" PreProcessed files" [email protected]
   fi
 done

Which could (and should) be re-written more simply as:

for var in list of words; do
  if ! cmd $var; then
    message="could not PreProcess files"
  else
    message="PreProcessed files
  fi
  mailx -s" $message" [email protected]
done

The [[ $? -ne 0 ]] clause is a hackish way to check the return value of cmd, but it is almost always unnecessary to check $? explicitly. Code is nearly always cleaner if you let the shell do the check by invoking the command in the if clause.

Upvotes: -1

Claudio
Claudio

Reputation: 10947

If the previous command returned an error return code.

Upvotes: 2

anubhava
anubhava

Reputation: 785481

if [[ $? -ne 0 ]];

Is checking return code of immediately previous this if condition.

  • $? means return code
  • $? -ne 0 means previous command returned an error since 0 is considered success

Upvotes: 8

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