shell programming 复习笔记 - advanced topic

advanced topic in shell programming

1. array

modern shell supports types in array.

there are 3 majors ways to create array

declare:
declare -a myarray

initialize value:
area3=([17]=seventeen [24]=twenty-four)
array=( element1 element2 elementN )

#you can use command output to initialize an array
#remember to declare it as array
array1=( `cat "$filename"`)


operations on array

echo ${string[@]}
echo ${#string[@]}
echo ${string[1]} #index can be literal or variable(without $)

2. indirect reference

there are some cases, you need variable name from a variable.

a=b
b=c

you intend to get c with $$a, first $a is transformed to b, then $b get the result of c

there are two ways to handle this.

1. traditional way:

eval f=\$$a

2. bash way

shell use ${!indirect} to get pointer
to solve above issues, you may use
${!a}

3. list operation

there are cases when you often use if and else to run a sequence of commands check previous steps return code
before you proceed.

a good habit to avoid complex if/then/else is using && and || glue.

or operation

command-1 || command-2 || command-3 || ... command-n
Each command executes in turn for as long as the previous command returns false.

and operation

command-1 && command-2 && command-3 && ... command-n
Each command executes in turn, provided that the previous command has given a return value of
true (zero).

4. process substitution

we have some concepts about input redirect and output redirect
but sometimes, it's not enough, we need multiple input from different files.

<(list)
>(list)

to command, above process substitution looks like they are two files. and command acting on the files

for example:

uniq <(sort a)

underlying the scene, shell sort file a and store it into some temp file /proc/pid/n, then
uniq acts on /proc/pid/n

5. options

set

-o verbose or -v

will echo commands

-x

debug mode

-o posix

posix compatable