Bash

1. touch file.ext -- creates a file.ext in current folder
2. which execuatable -- returns where the executeable is being picked up from aka the path of the executable
3. echo "string" -- prints the string
4. chmod u+x file.ext sets the file permission to executable for user

Variable Declaration

#!/bin/bash
    greeting=hellow
    name=tux
    echo $greeting $name
    

Arithmatic Operators

+, -, *, /, **, %

$((expression))

var=$((10**2)) 10 pow 2
    

Above method fractions are not calcualted correctly for that use bc command which stands for bash calculator

echo "scale=2;22/7" | bc # scale sets number of decimal points
    # 3.14
    

Get user input

read variable_name

#!/bin/bash
    echo "Enter a number"
    read a
    
    echo "you've entered $a"
    

Numerical comparision

Conditional

Remember always use single square bracket don't use double even if other tutorials say so as double square bracket isn't compatible with all shells.

if-else-if

Important parts to note are, bash is space sensitive aka remember this structure. if followed by space followed by a square bracket followed by space then statement then space the square bracket close then semicolon, same with elif miss this structure your conditional won't work !!

if [ condition ];
    then
        statement
    elif [ condition ]; then
        statement
    else
        statement
    fi
    

Loops

For

With numbers

!/bin/bash
    for i in {1..5}
    do
        echo $i
    done
    

with strings

#!/bin/bash
    for i in a b c
    do 
        echo $i
    done
    

While

#!/bin/bash
    i=1
    while [ $i -lt 10 ];
    do
        echo $i
    done
    

reading files

cat file.name more file.name

Getting arguments for a script

$@ will contain arguments for the script

for x in $@
    do
        echo $x
    done
    

Cron Jobs

Runs a command based on the intervel provided intervel structure is as follows [minute|Range 0-59] [hour| Range 0-23] [day| range 0-31] [month| range 0-12] [weekday| range 0-6, 0 means sunday] additionally use * to say any

run crontab -l to list all scheduled tasks

Find command

Find is a command that is used to find files in the local directory

find . -type f -name "*.sh" -exec mv {} new/ \; -size +1M -user tux -group tux -perm 777 -mtime +1 -atime -1 -ctime 1

Where parameters are as follows

cut command

Cut is a command that's used to extract substring of a string passed to it. Apart from that it also has features like selecting specific columsn of a csv with any delimiter.

echo "string" | cut -c 1-3 returns str

Here's how to give a range, note that counting starts with 1 aka one indexed

Additional options

Head Command

Head prints first n lines or n bytes based on the input passed

head -c 3 "string" returns str

Additional options

Tail Command

Usage: tail [OPTION]... [FILE]... Tail prints last n lines or n bytes based on the input passed

tail -c 3 "string" returns str

Additional options

Translate command (tr)

Usage: tr [OPTION]... SET1 [SET2] This maps one set of characters to another set of characters and returns it. It also squeezes and deletes characters. Squeeze means if we have repeating characters in a string it keeps only one character

SETs are specified as strings of characters. Most represent themselves.

Interpreted sequences are:

Translation occurs if -d is not given and both SET1 and SET2 appear. -t may be used only when translating. SET2 is extended to length of SET1 by repeating its last character as necessary. Excess characters of SET2 are ignored. Only [:lower:] and [:upper:] are guaranteed to expand in ascending order; used in SET2 while translating, they may only be used in pairs to specify case conversion. -s uses the last specified SET, and occurs after translation or deletion.

Sort command

Write sorted concatenation of all FILE(s) to standard output.

sort [OPTION]... [FILE]... sort [OPTION]... --files0-from=F

With no FILE, or when FILE is -, read standard input. Mandatory arguments to long options are mandatory for short options too. Ordering options:

KEYDEF is F[.C][OPTS][,F[.C][OPTS]] for start and stop position, where F is a field number and C a character position in the field; both are origin 1, and the stop position defaults to the line's end. If neither -t nor -b is in effect, characters in a field are counted from the beginning of the preceding whitespace. OPTS is one or more single-letter ordering options [bdfgiMhnRrV], which override global ordering options for that key. If no key is given, use the entire line as the key. Use --debug to diagnose incorrect key usage.

SIZE may be followed by the following multiplicative suffixes: % 1% of memory, b 1, K 1024 (default), and so on for M, G, T, P, E, Z, Y.

Uniq command

returns unique characters uniq [OPTION]... [INPUT [OUTPUT]]