13.2.5 Shell conditionals

Each command returns an exit status which can be used for conditional expressions:

• Success: 0 (True)

• Error: 1–255 (False)

Note that the use here of a 0 value to mean "true" differs from the usual convention in some other areas of computing. Also, `[' is the equivalent of the test command, which evaluates its arguments up to `]' as a conditional expression.

Basic conditional idioms to remember are:

     command && if_success_run_this_command_too || true
     command || if_not_success_run_this_command_instead
     
     if [ conditional_expression ]; then  
      if_success_run_this_command
     else
      if_not_success_run_this_command
     fi

Here || true was needed to ensure this shell script will not exit at this line accidentally when shell is invoked with -e flag.

File comparison operators in the conditional expression are:

     -e file         True if file exists.
     -d file         True if file exists and is a directory.
     -f file         True if file exists and is a regular file.
     -w file         True if file exists and is writable.
     -x file         True if file exists and is executable.
     file1 -nt file2 True if file1 is newer than file2. (modification)
     file1 -ot file2 True if file1 is older than file2. (modification)
     file1 -ef file2 True if they are the same device and inode numbers.

String comparison operators in the conditional expression are:

          -z str    True if the length of str is zero.
          -n str    True if the length of str is non-zero.
     str1 == str2   True if the strings are equal.
     str1 =  str2   True if the strings are equal.
         ("=" should be used in place of "==" for strict POSIX compliance) 
     str1 != str2   True if the strings are not equal.
     str1 <  str2   True if str1 sorts before str2 (locale dependent).
     str1 >  str2   True if str1 sorts after str2 (locale dependent).

Arithmetic integer comparison operators in the conditional expression are -eq, -ne, -lt, -le, -gt, and -ge.