Linux Notes

Wednesday. June 19, 2019 - 4 mins

Alias

create
```
 alias foo = "cd ..; ls; cd ..; ls"
```
delete
```
 unalias foo
```
Alias will be gone if the terminal is closed. To make these alias persistent, store them in bashrc/zshrc.
```
 nano .zshrc
```

History

history of inputs
```
 history
 history | grep ls
```
control + r to enter query mode, and control + c to exit.

Details of a command

type of a command
```
 type ls
```
manual page of a command
```
 man ls
```
whatis
```
 whatis ls
```

I/O Redirection

store output, if there is error, error will show in terminal and output.txt will not be written
```
 ls -l > output.txt
```
> is used to overwrite a file and >> is used to append a file
```
 ls -l >> output.txt
```

Error Redirection

Types of I/O:

Standard Input - stdin - 0
Standard Output - stdout - 1
Standard Error - stderr - 2

Error messages are printed to standard error. > only redirects standard output, so standard error is still shown on the terminal. To redirect them:

stdout to one file and stderr to another file
```
 command >output.txt 2>error.txt
```
redirect stderr to stdout(&1)
```
 command >output.txt 2>&1
```
redirect both to a file
```
 command &> output.txt
```

cat

open a file
```
 cat file.txt
```
create and write to a file, type control + d twice to exit.
```
 cat > test.txt
```

pipelines and grep

the pipeline makes the last output as the input for next command.
```
 stdin | stdout
```
uniq
```
 ls | uniq
```
sort
```
 ls | uniq | sort
```

wc - word count

 wc -l : prints the number of lines in a file.
 wc -w : prints the number of words in a file.
 wc -c : prints the count of bytes in a file.
 wc -m : prints the count of characters from a file.
 wc -L : prints only the length of the longest line in a file.

grep

 grep "pattern" sample.txt
 ls | grep txt

head/tail -n shows first/last n lines
```
 head -n 5
 tail -n 5
```

Echo

Echo will print matching files in current dir

 echo * : prints all files under current dir.
 echo D* : prints all files starting with D.
 echo [[:upper:]]* : all files starting with UpperCase.
 echo Five Divided By Two is $((5 / 2)) : Five Divided By Two is 2.

{}
```
 echo here {1..9}
 echo hey {Z..A}
```
$
```
 echo $(ls)
```
More about Dollar Sign $

Quotes and Exceptions

Exceptions
```
 `
 \
 $
```

"" and ''

 echo example ~/P* $(echo test) $((2 + 2)) $USER
 Output: example /Users/yiyang/Parallels /Users/yiyang/Pictures /Users/yiyang/Public test 4 yiyang

 echo "example ~/P* $(echo test) $((2 + 2)) $USER"
 Output: example ~/P* test 4 yiyang

 echo 'example ~/P* $(echo test) $((2 + 2)) $USER'
 Output: example ~/P* $(echo test) $((2 + 2)) $USER

Processes

list all processes

 ps     : prints all processes running in the terminal
 ps x   : prints all processes in the machine
 ps aux : prints all processes in the machine with detailed info

top lists top processes sorted by %CPU
```
 top
```
kill the specified pid
```
 kill 45461
```

Environment Variables

printenv shows environment variables
export
```
 export intern=yy
 echo $intern
```
unset removes the env var
```
 unset intern
```