this is my bash bible, written by a person with a intermediate knowledge of bash already. bash is Bourne Again SHell

simple shell commands

• sort file, which prints out the lines of file in sorted order.
• uniq file, which eliminates consecutive duplicate lines from file.
• head file and tail file, which respectively print the first and last few lines of file.
• seq print the numbers in between the input and output

complex patterns

grep pattern file finds a pattern in a file. the pattern is a regular expression. -r flag for recursive searches

oh wow sed is a life saver sed is a line editor that changes what you provide line by line in the files provided (can blob files too)

sed -i 's/pattern/replacement/g' file

any pattern can also be a regular expression -i indicates that rewrite the file with the new substituted content

find command finds well files and folders in the given director

# here is a example of finding flac files in the given directory
find . -type f -name "*.flac"

awk parsing data files (like csv)

# parses the 2nd column seperated by a whitespace
awk '{print $2}' file

# parses the 3rd column sperated by a comma
awk -F, '{print $3}' file

-F to change the seperator

variables

variables are like variable=value and note there should be no spaces in between them

var1=value
echo $var1
# output: value

files=$(ls)
echo $files

operators

• > to write std input to a file
• 2> to write std error output to a file
• < feeds the file contents into a commands std input
• >> to append a entry to the file
• && AND operator, only runs next command if previous one is sucessful
• || OR operator, only runs next command if previous one is unsucessful

date > today.txt
cat <today.txt

date >> today.txt
cat <today.txt

awww img $(<~/.cache/ashwal/ashwal) -t none

grep -q 2026 today.txt && echo "Pattern found"
grep -q 2020 today.txt || echo "Not found"

conditionals

if grep 2026 today.txt; then echo "wake up its 2026!"; fi

loops

# for loop
for varname in a b c d; do echo "$varname"; done
for varname in $(seq 1 2 10); do echo "$varname"; done
# while loop
while grep 2026 today.txt; do echo "it's still 2026"; date > today.txt; sleep 60; done

arguments

the shell programs receive a list of arguments when they are executed.

example: the arguments for ls are ['-l', 'folder/']

ls -l folder/

globbing

# all python files in all folders
ls **/*.py

# convert jpg image to png
convert image.{jpg,png}

# move all py and sh files to folder
mv *{.py,.sh} folder

piping

ls | grep .org

environment variables

To change values on the fly or getting some predefined values set by the user or the shell

date

TZ=Asia/Tokyo date

return code

• 0 the process was sucessful
• !0 the process was unsucessful

you can get the error code of the last process using the $? varibale

# sucessful
ls | grep .org > /dev/null
echo $?

# unsucessful
ls amithere 2> /dev/null
echo $?

signals

signals are a special communication mechanism. when a process receives a signal it stops its execution, deals with the signal and potentially changes the flow of execution based on the information that the signal delivered. For this reason, signals are software interrupts.

basic signals

• SIGINT: interrupts the process gracefully using C-c in terminal
• SIGQUIT: quits the process and dumps core using C-\ in terminal
• SIGTSTP: suspends a process using C-z in terminal
• SIGKILL: force kill a process
• SIGCONT: continue a process
• SIGHUP: hangup a process

a python loop to ignore SIGINT

#!/usr/bin/env python
import signal, time

def handler(signum, time):
    print("\nI got a SIGINT, but I am not stopping")

signal.signal(signal.SIGINT, handler)
i = 0
while True:
    time.sleep(.1)
    print("\r{}".format(i), end="")
    i += 1

example session:

$ sleep 1000
^Z
[1]  + 18653 suspended  sleep 1000

$ nohup sleep 2000 &
[2] 18745
appending output to nohup.out

$ jobs
[1]  + suspended  sleep 1000
[2]  - running    nohup sleep 2000

$ kill -SIGHUP %1
[1]  + 18653 hangup     sleep 1000

$ kill -SIGHUP %2   # nohup protects from SIGHUP

$ jobs
[2]  + running    nohup sleep 2000

$ kill %2
[2]  + 18745 terminated  nohup sleep 2000