unary operator expected error in Bash if condition

Question

I ve been trying to figure out whats wrong with this but just can t figure it out    This is the part seems to be getting an error    elif    operation    man     then     if    aug1    add     then            lt - Line 75     echo  Man Page for  add      echo        echo  Syntax  add  number 1   number 2       echo        echo  Description       echo  Add two different numbers together       echo        echo  Info       echo  Added in v1 0      echo    elif   -z  aug1    then     echo  Please specify a command to read the man page   else     echo  There is no manual page for that command   fi   I get this error   calc 1 2  line 75        unary operator expected

User · Answer

Took me a while to find this but note that if you have a spacing error you will also get the same error:

[: =: unary operator expected

Correct:

if [ "$APP_ENV" = "staging" ]

vs

if ["$APP_ENV" = "staging" ]

As always setting -x debug variable helps to find these:

set -x

User · Answer

Try assigning a value to  aug1 before use it in if   statements  the error message will disappear afterwards

User · Answer

You can also set a default value for the variable  so you don t need to use two      which amounts to two processes      is actually a program  instead of one   It goes by this syntax    VARIABLE -default    The whole thing has to be thought in such a way that this  default  value is something distinct from a  valid  value content   If that s not possible for some reason you probably need to add a step like checking if there s a value at all  along the lines of  if   -z  VARIABLE      then echo  the variable needs to be filled    or  if     -z  VARIABLE     then  everything is fine  proceed with the rest of the script

User · Answer

If you know you re always going to use bash  it s much easier to always use the double bracket conditional compound command            instead of the Posix-compatible  single bracket version          Inside a           compound  word-splitting and pathname expansion are not applied to words  so you can rely on  if     aug1     and        to compare the value of  aug1 with the string and   If you use          you always need to remember to double quote variables like this   if     aug1     and       If you don t quote the variable expansion and the variable is undefined or empty  it vanishes from the scene of the crime  leaving only  if      and        which is not a valid syntax   It would also fail with a different error message if  aug1 included white space or shell metacharacters    The modern    operator has lots of other nice features  including regular expression matching

[bash] "unary operator expected" error in Bash if condition

Examples related to bash

Examples related to shell