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|
+++
title = "Mess with bash(1)"
date = 2022-08-25
[taxonomies]
categories = ["code"]
tags = ["bash"]
[extra]
math = false
+++
### Crontab
`crontab` is a useful tool, I really regret that I don't familar it earlier.
It's usage really simple, and these two are my frequent using:
```bash
crontab -e #edit crontab file that reside in /var/spool/cron
crontab -l #list current crontab job
```
It basic syntax as follow, also really simple
example of usage:
1. delete file *foo* every minute
`* * * * * rm foo`
2. delete file *foo* every 15 minutes
`15 * * * * rm foo`
3. delete file *foo* every beginning of hour
`0 * * * * rm foo`
4. delete file *foo* every minute after 3 hours
`* 3 * * * rm foo`
5. delete file *foo* every day at 18:30
`30 18 * * * rm foo`
6. delete file *foo* every beginning of month
`0 0 0 * * rm foo`
7. delete file *foo* on beginning of 1st,10th of month
`0 0 1,10 * * rm foo`
The usage really simple, I now use it to renew my SSL certification and daily update bt-tracker.
---
### tr
`tr` is really useful when encountered situation that needs struggle with string.
Three frequency ways of using `tr`
1. **shrink** multiple characters into single one
`tr -s '[string]'`
e.g. `echo "ssssssspaaaaace" | tr -s 'sa'` would convert "ssssssspace" to "space"
2. **delete** specific character. I usually use it to delete white space or delimiter
`tr -d '[string]'`
e.g. `echo "blog.garhve.com" | tr -d '.o'` would convert url to "blggarhvecm"
3. **convert** specific character to another one.
`tr '[string1]' '[string2]'`
e.g. `echo "woopwon | tr "wo" "fe"` would result "feepfen"
---
### cut
I use `cut` mostly to get word from a string, especially get relative path from absolute path. Because I always want to loop to get same sub-directories file from different main directory, `cut` helps a lot.
for now, I only use it one way
`echo string | cut -d '[character]' -f position`
e.g. `echo path/to/most/inner/file | cut -d '/' -f1` this will give me word before first '/', which is 'path'
Often, `rev` will co-work with `cut` to get last one word
e.g. `echo blog.garhve.com | rev | cut -d '.' -f 1 | rev` this will give word after last '/', which is com.
> *a worth noting here is that the `rev` command needs to appear twice because it usage is not so intuitive, it reverse whole string*
>
> `echo "hello world" | rev` will get 'dlrow olleh`
>
> `echo "hello world" | cut -d ' ' -f1 | rev` will give result of 'dlrow'
>
> `echo "hello world" | rev | cut -d ' ' -f1` will give same result, as 'dlrow'
>
> `echo "hello world" | rev | cut -d ' ' -f1 | rev` will give expected result, as 'world'
### Daily update bt tracker
I already learn shell script for a while.. so I wrote a simple script to test whether I really got used to it, but result is obviously, I need more and more practice to memorize commands.
```bash
#! /bin/sh
#bt-tracker.txt
site=https://raw.githubusercontent.com/ngosang/trackerslist/master/trackers_all.txt
file=/path_to_aria_directory/aria2.conf
Addr=user@addr
# Get bt-tracker and format it to fulfill aria needs, then store in variable
bt=$(curl $site | tr -s '[:space:]' | tr '[:space:]' ',')
# sshpass is a software, that can allow me pass ssh password as argument
# so that I don't need to wait prompt
# 410 is the line of bt tracker resides, I now no other way to replace it.
$(sshpass -p 'password' ssh -T $Addr "sed -i '410d' $file | echo $bt >> $file")
```
> Usage of [tr](https://blog.garhve.com/index.php/archives/23/#tr) and [cut](https://blog.garhve.com/index.php/archives/23/#cut), expansion of 'command tips' is needed
### Ways to find files or specific string in files
In order to look up C definitions, I need to know where linux stores header files or which files store definitions I need. So that here comes up some methods to fulfill this need:
1. `find 'path' -name 'file_name'`
> e.g. `find / -name stdio.h` will return multiple location that stdio.h resides. this could expand to look for others
>
> 
>
> more info could see `man find`
---
2. `grep -rnw 'path' -e 'pattern'`
> e.g. `grep --include=\*.{h,c} -rnw / -e "from_kuid" will return string and filename that contains string.
>
> 
>
> * `-r` or `-R` is recursive,
> * `-n` is line number, and
> * `-w` stands for match the whole word.
> * `-l` (lower-case L) can be added to just give the file name of matching files.
> * `-e` is the pattern used during the search
>
> Along with these, `--exclude`, `--include`, `--exclude-dir` flags could be used for efficient searching:
>
> * This will only search through those files which have .c or .h extensions:
>
> ```bash
> grep --include=\*.{c,h} -rnw '/path/to/somewhere/' -e "pattern"
> ```
>
> * This will exclude searching all the files ending with .o extension:
>
> ```bash
> grep --exclude=\*.o -rnw '/path/to/somewhere/' -e "pattern"
> ```
>
> * For directories it's possible to exclude one or more directories using the `--exclude-dir` parameter. For example, this will exclude the dirs dir1/, dir2/ and all of them matching *.dst/:
>
> ```bash
> grep --exclude-dir={dir1,dir2,*.dst} -rnw '/path/to/somewhere/' -e "pattern"
> ```
>
> more info could see [man grep](https://ss64.com/bash/grep.html).
|
