Advanced Bash-Scripting Guide: An in-depth exploration of the art of shell scripting
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# 4.3. Bash Variables Are Untyped

Unlike many other programming languages, Bash does not segregate its variables by "type." Essentially, Bash variables are character strings, but, depending on context, Bash permits arithmetic operations and comparisons on variables. The determining factor is whether the value of a variable contains only digits.

Example 4-4. Integer or string?

       1 #!/bin/bash
       2 # int-or-string.sh
       3
       4 a=2334                   # Integer.
       5 let "a += 1"
       6 echo "a = $a "           # a = 2335
       7 echo                     # Integer, still.
       8
       9
      10 b=${a/23/BB}             # Substitute "BB" for "23".
      11                          # This transforms $b into a string.
      12 echo "b = $b"            # b = BB35
      13 declare -i b             # Declaring it an integer doesn't help.
      14 echo "b = $b"            # b = BB35
      15
      16 let "b += 1"             # BB35 + 1
      17 echo "b = $b"            # b = 1
      18 echo                     # Bash sets the "integer value" of a string to 0.
      19
      20 c=BB34
      21 echo "c = $c"            # c = BB34
      22 d=${c/BB/23}             # Substitute "23" for "BB".
      23                          # This makes $d an integer.
      24 echo "d = $d"            # d = 2334
      25 let "d += 1"             # 2334 + 1
      26 echo "d = $d"            # d = 2335
      27 echo
      28
      29
      30 # What about null variables?
      31 e=''                     # ... Or e="" ... Or e=
      32 echo "e = $e"            # e =
      33 let "e += 1"             # Arithmetic operations allowed on a null variable?
      34 echo "e = $e"            # e = 1
      35 echo                     # Null variable transformed into an integer.
      36
      37 # What about undeclared variables?
      38 echo "f = $f"            # f =
      39 let "f += 1"             # Arithmetic operations allowed?
      40 echo "f = $f"            # f = 1
      41 echo                     # Undeclared variable transformed into an integer.
      42 #
      43 # However ...
      44 let "f /= $undecl_var"   # Divide by zero?
      45 #   let: f /= : syntax error: operand expected (error token is " ")
      46 # Syntax error! Variable $undecl_var is not set to zero here!
      47 #
      48 # But still ...
      49 let "f /= 0"
      50 #   let: f /= 0: division by 0 (error token is "0")
      51 # Expected behavior.
      52
      53
      54 #  Bash (usually) sets the "integer value" of null to zero
      55 #+ when performing an arithmetic operation.
      56 #  But, don't try this at home, folks!
      57 #  It's undocumented and probably non-portable behavior.
      58
      59
      60 # Conclusion: Variables in Bash are untyped,
      61 #+ with all attendant consequences.
      62
      63 exit $?

Untyped variables are both a blessing and a curse. They permit more flexibility in scripting and make it easier to grind out lines of code (and give you enough rope to hang yourself!). However, they likewise permit subtle errors to creep in and encourage sloppy programming habits.

To lighten the burden of keeping track of variable types in a script, Bash does permit declaring variables.

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