• Kernal
  • linux
  • android
• Find
• Sed
• Bash
  • String operation
  • Loop
  • IF-ELSE

Kernal

Linux

To find out what version of Linux (distro) you are running, try these 3 commands:

$ cat /etc/*-release
$ uname -a
$ cat /proc/version

Android

How to check android CPU is 32 or 64

$ cat /proc/cpuinfo

the result will looks like this:

Processor : AArch64 Processor rev 4 (aarch64)
processor : 0
processor : 1
processor : 2
processor : 3
Features : fp asimd evtstrm aes pmull sha1 sha2 crc32
CPU implementer : 0x41
CPU architecture: AArch64
CPU variant : 0x0
CPU part : 0xd03
CPU revision : 4

Hardware : Amlogic
Serial : adsf

Find

find . -type [f/d] -name prefix*suffix

Replace all file’s extention from js to jsx if file is start with Capital letter

find . -type f -name "[[:upper:]]*.js" -exec sh -c 'mv "$1" "${1}x"' _ {} \;

Sed

sed -i '' 's/\(regex\)/\1, extra string/g' app/models/*.rb

node: '' is needed for mac

Find all rb files and replace all FactoryGirl to FactoryBot

$ find ./ -type f -name *.rb -exec sed -i '' 's/FactoryGirl/FactoryBot/g'

Bash-ish

String Operation

() vs {}

(): Parentheses cause the commands to be run in a subshell.

{}: Braces cause the commands to be grouped together but not in a subshell.

With and without $

#!/bin/bash
var1="A B  C   D"
echo $var1   # A B C D
echo "$var1" # A B  C   D

String length

export stringZ=abcABC123ABCabc
echo ${#stringZ}                 # 15
echo `expr length $stringZ`      # 15
echo `expr "$stringZ" : '.*'`    # 15

Substring

export stringZ=abcABC123ABCabc
echo ${stringZ:1}          # bcABC123ABCabc
echo ${stringZ:7:3}        # 23A
                           # Three characters of substring.

Substring replacement

${parameter/pattern/string}

• Replace first match with string;
• If pattern begins with ‘/’, all matches of pattern are replaced with string
• If pattern begins with ‘#’, it must match at the beginning
• If pattern begins with ‘%’, it must match at the end

• If string is null, delete match(es)

  export stringZ=abcABC123ABCabc
  echo ${stringZ/abc/xyz}       # xyzABC123ABCabc
                                # Replaces first match of 'abc' with 'xyz'.
  
  echo ${stringZ//abc/xyz}      # xyzABC123ABCxyz
                                # Replaces all matches of 'abc' with # 'xyz'.
  
  echo ${stringZ/#abc/XYZ}      # XYZABC123ABCabc
                                # Replaces front-end match of 'abc' with 'XYZ'.
  
  echo ${stringZ/%abc/XYZ}      # abcABC123ABCXYZ
                                # Replaces back-end match of 'abc' with 'XYZ'.
  

Params with default:

# ${parameter-default}, ${parameter:-default}
# var is not declared
echo ${var-'1'}   # 1
# var is declared, but null
echo ${var:-'2'}  # 2

arithmetic expansion

The arithmetic expansion can be performed using the double parentheses ((…)) and $((…))

i=0
echo "$((i + 1))" # 2

Loop

Loop lines over a file

while IFS= read -r line; do
  echo "fetch daily kdata for $line"
  # extra command # python daily_kdata.py --code $line --start 1989-01-01 --end 2020-09-08
done < filepath

Loop file over find

Find all filename contains index.less, and rename pattern index.less to style.less

for file in $(find src -type f -name *index.less*); do mv $file "${file%%index.less}style.less"; done

Resize all image with png to 300x300 and rename to -300.png

for f in *; do convert "$f" -resize 300x300 "${f%%.png}-300.png";done

Loop over folder for csv files: i=0 for filename in ./baostock/csv/day/*.csv do i=$((i + 1)) # number operation echo “dumping $filename… [$i]” cat $filename done

Loop with different IP:

for i in {1..8}; do curl --header "X-Forwarded-For: 1.2.3.$i" [url]; done

IF-ELSE

Syntax check man test to check all the possible test operation

#!/bin/bash
if [ <some test> ]
then
  <commands>
fi

Note:

test 001 = 1
echo $? # 1
test 001 -eq 1
echo $? # 0

# 0       expression evaluated to true.
# 1       expression evaluated to false or expression was missing.
# >1      An error occurred.