if 比較，字串與數值 的 operator 不同

ref: How to Compare Numbers, Strings and Files in Bash Shell Script

就是 if [ A ... 裡面 用的 operator，在數值字串會不一樣..
以上面的 link 中的說明：

if [2 -gt 3]
     then
     print "2 is greater"
     else
     print "2 is not greater"
fi

其他的 數值用 operator:

• num1 -eq num2 : check if 1st number is equal to 2nd number
• num1 -ge num2 : checks if 1st number is greater than or equal to 2nd number
• num1 -gt num2 : checks if 1st number is greater than 2nd number
• num1 -le num2 : checks if 1st number is less than or equal to 2nd number
• num1 -lt num2 : checks if 1st number is less than 2nd number
• num1 -ne num2 : checks if 1st number is not equal to 2nd number

字串的話...

name=linuxtechi
if [ $USER = $name ]
        then
                echo "User exists"
        else
                echo "User not found"
fi

• var1 = var2 checks if var1 is the same as string var2
• var1 != var2 checks if var1 is not the same as var2
• var1 < var2 checks if var1 is less than var2
• var1 > var2 checks if var1 is greater than var2
• -n var1 checks if var1 has a length greater than zero
• -z var1 checks if var1 has a length of zero

bookmark : shell script 中的各種括號..

• bash script parenthesis explained

為避免這摸好的說明不見，所以轉貼一下.. (其實就是 copy 人家的內容...)

---

雙 (( 用在運算上..

((var++))
((var = 3))
for ((i = 0; i < VAL; i++))
echo $((var + 2))

用說括號 (( 刮起來的變數，不必加 $

方括號 [ 用在判斷

$ VAR=2
$ if [ $VAR -eq 2 ]
> then
> echo 'yes'
> fi
yes

接著的這個說明我不了解...
兩格個方括號用在 ? extended function ? 在regular expression 的 =~ 上使用.

$ VAR='some string'
$ if [[ $VAR =~ [a-z] ]]; then
> echo 'is alphabetic'
> fi
is alphabetic

大括號 { 用在區分變數名稱

$ foo='stage'
$ echo $fooone
           ... returns empty line
$ echo ${foo}one
stageone

大括號同時還有處理變數內容的功能..

$ var="abcdefg"; echo ${var%d*}
abc

• 有關這種變數字串處理可以參考操作字串

程式輸出轉為變數

其實這跟一般shell script call shell command (ls. find ..etc) 然後處理輸出是一樣的。
不會因為是自己寫的程式而有不同，無腦的example 來看一下...

echo10.c:
#include <stdio.h>

int main()
{
 printf("%d",10);

 return 0;
}

然後 shell script 吃這個輸出..

#!/bin/bash

LOOPEND=$(./echo10)
echo "LOOPEND: ${LOOPEND}"

i=1
while [[ $i -le $LOOPEND ]]
do
 echo "$i"
 ((i = i + 1))
done

就很直覺使用..

---

反而是括號使用有點...參考一下..

• bash script parenthesis explained

check soft link exist

-L 用來檢查 softlink 在不在。

#!/bin/bash
if [ -L bbb ]; then
   echo OK
fi

如果 ls .. 中..

bbb --> aaa
..

不管 aaa 在不在，都會 OK。

eval

有時候在 shell script 中，有沒有 用 eval，姊果好像都一樣。
例如 f_list = $(eval ls "$1") 其中，如果 eval 刪掉 f_list = $(ls "$1") 姊果 f_list 的內容相同。

看到 這一篇 說明，決的得很清楚：

1) foo=10 x=foo
2) y='$'$x
3) echo $y
4) $foo
5) eval y='$'$x
6) echo $y
7) 10

ffmpeg, rotate all the videoes in folder

#!/bin/bash
for f in *.MP4; do
 ffmpeg -i "$f" -vf "transpose=1" "r${f%.MP4}.MP4"
done

操作字串

ref: http://www.linuxtopia.org/online_books/advanced_bash_scripting_guide/string-manipulation.html

就直接 copy ...

Substring Removal

${string#substring}
Strips shortest match of $substring from front of $string.

${string##substring}
Strips longest match of $substring from front of $string.

stringZ=abcABC123ABCabc
#       |----|
#       |----------|

echo ${stringZ#a*C}      # 123ABCabc
# Strip out shortest match between 'a' and 'C'.

echo ${stringZ##a*C}     # abc
# Strip out longest match between 'a' and 'C'.

反過來：

${string%%substring}
Strips longest match of $substring from back of $string.

stringZ=abcABC123ABCabc
#                    ||
#        |------------|

echo ${stringZ%b*c}      # abcABC123ABCa
# Strip out shortest match between 'b' and 'c', from back of $stringZ.

echo ${stringZ%%b*c}     # a
# Strip out longest match between 'b' and 'c', from back of $stringZ.