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.

return value from shell script

ref: http://tldp.org/LDP/abs/html/exit-status.html

是用 exit 指令。

exit 123

然後 caller 用 $? 就可以取得上一個 script exit 的值。

example : disk usage 超過90% , 刪除Android folder

---

ref: http://stackoverflow.com/questions/19703621/get-free-disk-space-with-df-to-just-display-free-space-in-kb

刪除括號 (和括號內的文字)

$ echo '0 [123] 456' | sed 's/\[.*\]//' 
0  456

就是.. 把 [.*] switch 成 empty.
因為 [,] 要 escapre，所以變成 \[.*\]

把他放到 s/pattern/replace/action 裡的 pattern.