1. Note that
$variable is actually a simplified form of
${variable}.
2. An
uninitialized variable has no value, however
it acts as if it were 0 in an arithmetic operation. This is undocumented (and probably non-portable) behavior, and should not be used in a script.(P31)
3. 此处a的值为21，
使用let对变量赋值时，会对等号后面的算术式进行算术运算，将运算后的值赋给变量。
-
let a=16+5
-
echo "The value of \"a\" is now $a."
4. 注意代码中的注释。Variable assignment using the
$(...) mechanism (a newer method than backquotes).
-
a=`ls -l` # Assigns result of 'ls -l' command to 'a'
-
echo $a # Unquoted, however, it removes tabs and newlines.
-
echo
-
echo "$a" # The quoted variable preserves whitespace.
5. If a script sets environmental variables, they need to be "exported", that is, reported to the environment local to the script. This is the function of the
export command. A script can export variables only to
child processes.
6. Arguments passed to the script from the command line: $0, $1, $2, ...,
${10}, ${11},... $0 is the name of the script itself. The special variables
$* and $@ denote all the positional parameters.
$# Number of args passed (不包括脚本本身）.
7. The
shift command reassigns the positional parameters, in effect shifting them to the left one notch.
The old $1 disappears, but $0 (the script name) does not change. The shift command works in a similar fashion on parameters passed to a function.
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