Mess with bash(1) | garhve's gibberish

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Mess with bash(1)

2022-08-25

+ + + /code + + + + + #bash + + +

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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:

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:

delete file foo every minute +* * * * * rm foo
delete file foo every 15 minutes +15 * * * * rm foo
delete file foo every beginning of hour +0 * * * * rm foo
delete file foo every minute after 3 hours +* 3 * * * rm foo
delete file foo every day at 18:30 +30 18 * * * rm foo
delete file foo every beginning of month +0 0 0 * * rm foo
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

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shrink multiple characters into single one
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tr -s '[string]' +e.g. echo "ssssssspaaaaace" | tr -s 'sa' would convert "ssssssspace" to "space"
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delete specific character. I usually use it to delete white space or delimiter
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tr -d '[string]' +e.g. echo "blog.garhve.com" | tr -d '.o' would convert url to "blggarhvecm"
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convert specific character to another one.
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tr '[string1]' '[string2]' +e.g. echo "woopwon | tr "wo" "fe" would result "feepfen"
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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.

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a worth noting here is that the rev command needs to appear twice because it usage is not so intuitive, it reverse whole string
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echo "hello world" | rev will get 'dlrow olleh`
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echo "hello world" | cut -d ' ' -f1 | rev will give result of 'dlrow'
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echo "hello world" | rev | cut -d ' ' -f1 will give same result, as 'dlrow'
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echo "hello world" | rev | cut -d ' ' -f1 | rev will give expected result, as 'world'
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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.

#! /bin/sh
+#bt-tracker.txt
+site=https://raw.githubusercontent.com/ngosang/trackerslist/master/trackers_all.txt
+
+file=/path_to_aria_directory/aria2.conf
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+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 and 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:

find 'path' -name 'file_name'

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e.g. find / -name stdio.h will return multiple location that stdio.h resides. this could expand to look for others
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more info could see man find
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grep -rnw 'path' -e 'pattern'

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e.g. `grep --include=*.{h,c} -rnw / -e "from_kuid" will return string and filename that contains string.
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-r or -R is recursive,
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-n is line number, and
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-w stands for match the whole word.
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-l (lower-case L) can be added to just give the file name of matching files.
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-e is the pattern used during the search
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Along with these, --exclude, --include, --exclude-dir flags could be used for efficient searching:
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This will only search through those files which have .c or .h extensions:
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grep --include=\*.{c,h} -rnw '/path/to/somewhere/' -e "pattern"
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This will exclude searching all the files ending with .o extension:
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grep --exclude=\*.o -rnw '/path/to/somewhere/' -e "pattern"
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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/:
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grep --exclude-dir={dir1,dir2,*.dst} -rnw '/path/to/somewhere/' -e "pattern"
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more info could see man grep.
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