Ytdyklly

I was wonder with someone can help me:

if [ -z $1 ]; then                                                                                                                                                                                                                                                             

  user=$(whoami)                                                                                                                                                                                                                                                         

else                                                                                                                                                                                                                                                                           

  if [ ! -d "/home/$1" ]; then                                                                                                                                                                                                                                           

    echo "Requested $1 user home directory doesn't exist."                                                                                                                                                                                                         

    exit 1                                                                                                                                                                                                                                                         

  fi                                                                                                                                                                                                                                                                     

  user=$1                                                                                                                                                                                                                                                                

fi 

I was studying some bash commands when I saw two commands: -z and -d. I know what they do (first check for blank variable and the second check for existence directory). My question is how I can find descriptions about these commands (i.g man page -d/-z). They can be only used with if-else statement?

The -d and -z are not commands but options to the test and [ utilities. These utilities are built into bash and documented in the bash manual. These utilities and these flags also happens to be standardized by POSIX, so they are available in any POSIX shell, not just bash.

If you're in an interactive bash session, you may get documentation for the built-in variants of these utilities by typing help test (help [ works too, but its text just refers to the documentation for test).

man test and man [ should work too. These manuals describe the external utilities, probably /bin/test and /bin/[, not the ones you use by default in bash.

So for example,

! test -z "$dir" && test -d "$dir" && printf '%s is a directory' "$dir"

is exactly equivalent to

! [ -z "$dir" ] && [ -d "$dir" ] && printf '%s is a directory' "$dir"

or, if you will,

if ! test -z "$dir" && test -d "$dir"; then

    printf '%s is a directory' "$dir"

fi

and

if ! [ -z "$dir" ] && [ -d "$dir" ]; then

    printf '%s is a directory' "$dir"

fi

(! [ -z "$dir" ] would probably be more commonly written [ ! -z "$dir" ] or [ -n "$dir" ], and I've only used the -z test above because it was mentioned in the question, the -d test on an empty string would fail anyway).

See also:

• POSIX documentation for the test utility