# 3/9 Shell

## 選取＆檔案處理相關指令
grep usage:
-n: print line number in file
-l: print filename that contains your arguments
cut(select column):
diff
wc
uniq
tail -f xxx.log(可以監控某個檔案的變化)
grep -v total(invert,反向選取)＋(開頭是total)＝list 除了開頭是total的所有東西
sort
sed
awk
tr usage:(replace): grep xxx /aaa/bbb | tr ":" "\n"
-d:delete ex:tr -d "\t"< file1
-s:delete multiple

xargs usage(use the output as args for the next command): ls | xargs echo
-n1: ls | xargs -n1 echo
-I %(replace every % in the next command):ls | xargs -I % -n1 echo % here %
-J %(replace onely one % in the next command):ls | xargs -J % -n1 echo % here %
!command history usage:
!str:find the latest command start with str
!?str:find the latest command start with str

## 組合技例子

## Shell Script
* $$= shell PID
* ＃!(shebang):tell the system which interpreter to use (e.g.:#!/usr/bin/env python )
* {1..10}=`seq 1 10` 比較方便！

* ${var:=value}
* ${var:+value}
* ${var:-value}
* ${var:?value}

## Lab 02