> For the complete documentation index, see [llms.txt](https://bash.netdpi.net/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://bash.netdpi.net/di-er-bu-ji-chu/di-si-zhang-bian-shu-yu-can-shu/4.3-bash-de-bian-shu-mei-you-xing-bie.md).

# 4.3 bash 的變數沒有型別

不同於其他的程式語言，Bash 沒有用型別（type）來區分它的變數。基本上，Bash的變數是字串，但是，根據上下文的語意，Bash 可以讓變數做算術運算和比較，決定因素是變數的值是否只包含數字。

**Example 4-4. 整數或字串？**

{% code lineNumbers="true" %}

```bash
#!/bin/bash
# int-or-string.sh

a=2334                   # Integer.
let "a += 1"
echo "a = $a "           # a = 2335
echo                     # Integer, still.


b=${a/23/BB}             # Substitute "BB" for "23".
                         # This transforms $b into a string.
echo "b = $b"            # b = BB35
declare -i b             # Declaring it an integer doesn't help.
echo "b = $b"            # b = BB35

let "b += 1"             # BB35 + 1
echo "b = $b"            # b = 1
echo                     # Bash sets the "integer value" of a string to 0.

c=BB34
echo "c = $c"            # c = BB34
d=${c/BB/23}             # Substitute "23" for "BB".
                         # This makes $d an integer.
echo "d = $d"            # d = 2334
let "d += 1"             # 2334 + 1
echo "d = $d"            # d = 2335
echo


# What about null variables?
e=''                     # ... Or e="" ... Or e=
echo "e = $e"            # e =
let "e += 1"             # Arithmetic operations allowed on a null variable?
echo "e = $e"            # e = 1
echo                     # Null variable transformed into an integer.

# What about undeclared variables?
echo "f = $f"            # f =
let "f += 1"             # Arithmetic operations allowed?
echo "f = $f"            # f = 1
echo                     # Undeclared variable transformed into an integer.
#
# However ...
let "f /= $undecl_var"   # Divide by zero?
#   let: f /= : syntax error: operand expected (error token is " ")
# Syntax error! Variable $undecl_var is not set to zero here!
#
# But still ...
let "f /= 0"
#   let: f /= 0: division by 0 (error token is "0")
# Expected behavior.


#  Bash (usually) sets the "integer value" of null to zero
#+ when performing an arithmetic operation.
#  But, don't try this at home, folks!
#  It's undocumented and probably non-portable behavior.


# Conclusion: Variables in Bash are untyped,
#+ with all attendant consequences.

exit $?
```

{% endcode %}

無型別的變數有好有壞，這樣可以比較靈活的撰寫 script。然而，這樣子也容易寫出經常出錯的程式碼。為了減少在 script 與蹤變數型別的負擔，Bash 可以宣告變數。<br>


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation dynamically by asking a question.

Perform an HTTP GET request on the current page URL with the `ask` query parameter, and the optional `goal` query parameter:

```
GET https://bash.netdpi.net/di-er-bu-ji-chu/di-si-zhang-bian-shu-yu-can-shu/4.3-bash-de-bian-shu-mei-you-xing-bie.md?ask=<question>&goal=<endgoal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is optional and describes the broader end goal you are ultimately trying to accomplish on behalf of the user. GitBook uses it to tailor the answer towards what is most useful for that goal.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.