shell在这方面设计，感觉复杂了点，可能有一定历史渊源吧。

一，字符串的判断与比较

在 Shell 中进行条件测试时一定要注意空格问题。使用[]测试时，左方括号右边和右方
括号左边都必须有空格。而且测试的比较符号两边也必须都有空格。

  867  echo $((2-4))
  868  echo $((2*4))
  869  echo $((2**4))
  870  echo $((10%3))
  871  x=2
  872  echo $((x+=2)
  873  )
  874  echo $((x+=2))
  875  echo $((x*=3))
  876  echo $((x%=2))
  877  x=2 ; y=3
  878  echo $((x*y))
  879  echo $x $y
  880  echo $((3>2&&5>3))
  881  echo $((3>2&&5>9))
  882  echo $((1>2||5>8))
  883  echo $((3>2||5>8))
  884  echo $[2+8]
  885  echo $[2**8]
  886  x=3; y=5
  887  echo $[x+y]
  888  echo $[x*y]
  889  echo $[1+2*3]
  890  echo $[(1+2)*3]
  891  echo $[x>y?2:3]
  892  echo $[y>x?2:3]
  893  echo $[y>x?2+2:3*5]
  894  echo $[x>y?2+2:3*5]
  895  let 1+2
  896  x=5
  897  let x++
  898  echo $x
  899  let x*=2; echo $x
  900  let i=1+2*3; echo $i
  901  let i=(1+2)*3; echo $i
  902  let 2.2 + 5.5
  903  x=1
  904  echo $[x+1]
  905  echo $[x++]
  906  echo $x
  907  x=6
  908  echo $[x--]
  909  echo $x
  910  bc
  911  echo "2+3; scale=2; 8/19" |bc
  912  echo {obase=2; 10"|bc
  913  "
  914  echo "obase=2; 10"|bc
  915  test a == a
  916  echo $?
  917  test a == b; echo $?
  918  test a == b ; echo $?
  919  test a != b ; echo $?
  920  test a != b; echo $?
  921  [ $USER == root ]; echo $?
  922  [ $USER == root ] && echo Y || echo N
  923  [ -z $TEST ] && echo Y || echo N
  924  TEST=123456
  925  [ -z $TEST ] && echo Y || echo N
  926  [test == beijing]
  927  [ test==root ]; echo $?
  928  [ 1==2 ]; echo $?
  929  [ -n $TEST ] && echo Y || echo N
  930  [ -n $Jacob ] && echo Y || echo N
  931  [ -n ] && echo Y || echo N
  932  [ -n "$Jacob" ] && echo Y || echo N

二，整数的判断与比较

image.png

933  test 3 -eq 3 && echo Y || echo N
  934  test 3 -ne 3 && echo Y || echo N
  935  [ 6 -gt 4 ] && echo Y || echo N
  936  [ 6 -gt 9 ] && echo Y || echo N
  937  [ 6 -ge 4 ] && echo Y || echo N
  938  [ 6 -ge 6 ] && echo Y || echo N
  939  [ 6 -lt 9 ] && echo Y || echo N
  940  [ 6 -lt 3 ] && echo Y || echo N
  941  [ 6 -lt 6 ] && echo Y || echo N
  942  [ 6 -le 3 ] && echo Y || echo N
  943  [ 6 -le 6 ] && echo Y || echo N
  944  [ $UID -eq 0 ] && echo Y || echo N
  945  cat /proc/meminfo 
  946  grep Available /proc/meminfo |egrep -o "[0-9]"
  947  grep Available /proc/meminfo |egrep -o "[0-9]+"
  948  mem_free = $(grep Available /proc/meminfo |egrep -o "[0-9]+")
  949  mem_free=$(grep Available /proc/meminfo |egrep -o "[0-9]+")
  950  [ $mem_free -le 512000] && echo Y|| echo N
  951  [ $mem_free -le 512000 ] && echo Y|| echo N
  952  ps aux|wc -l
  953  procs=$(ps aux|wc -l)
  954  [ $procs -gt 100 ] &&echo Y ||echo N
  955  cat /etc/passwd|wc -l
  956  num=$(cat /etc/passwd|wc -l)
  957  [ $num -ge 30 ] &&echo Y ||echo N

三，文件属性的判断与比较

image.png

 958  touch ver1.txt
  959  touch ver2.txt
  960  mkdir test
  961  [ -e ver1.txt ] && echo Y || echo N
  962  [ -e test ] && echo Y || echo N
  963  [ ! -e ver1.txt ] && echo Y || echo N
  964  [ -f ver1.txt ] && echo Y || echo N
  965  [ ! -f ver1.txt ] && echo Y || echo N
  966  [ -f test/ ] && echo Y || echo N
  967  [ -d test/ ] && echo Y || echo N
  968  [ -d ver1.txt ] && echo Y || echo N
  969  [ -b /dev/sda ] && echo Y || echo N
  970  [ -b /etc/passwd ] && echo Y || echo N
  971  ls -s /etc/hosts /root/soft
  972  ln -s /etc/hosts /root/soft
  973  ln /etc/hosts /root/hard
  974  ls -l /root/soft 
  975  ls -l /root/hard 
  976  [ -L /root/soft ] && echo Y || echo N
  977  [ -L /root/hard ] && echo Y || echo N
  978  ls -l ver1.txt 
  979  [ -r ver1.txt] && echo Y || echo N
  980  [ -r ver1.txt ] && echo Y || echo N
  981  chmod -r ver1.txt 
  982  [ -r ver1.txt ] && echo Y || echo N
  983  chmod -w ver1.txt 
  984  [ -w ver1.txt ] && echo Y || echo N
  985  ls -l ver1.txt 
  986  [ -x ver1.txt ] && echo Y || echo N
  987  chmod +x ver1.txt 
  988  [ -x ver1.txt ] && echo Y || echo N
  989  [ -s ver1.txt ] && echo Y || echo N
  990  echo "hello" > ver1.txt 
  991  [ -s ver1.txt ] && echo Y || echo N
  992  ls -l ver*
  993  [ ver1.txt -nt ver2.txt ] && echo Y || echo N
  994  [ ver1.txt -ot ver2.txt ] && echo Y || echo N