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Bash

Docs​

man bash
info bash

Bash Script file​

Common usage
#!/bin/bash

(or)

#!/usr/bin/env bash

Execute​

chmod +x script.sh
./script.sh

Variables​

Declare variable​

variableName="value"
export variableName2="value2"
declare -x variableName3="value3"

Access the variable​

echo $variableName
echo $variableName2
echo $variableName3

Grouping​

() -> scope clear
{} -> scope extend

Bash Startup​

.bash_profile # read when bash is invoked as login shell
.bashrc # executed when new shell is started

Typeset​

typeset -i x # x must be an integer

# Types
-i -> Integer

Declare​

declare -a ArrayVariable

# Options
-l -> To lowercase
-u -> To uppercase
-r -> Read only
-a -> To indexed array

While Loop​

while 
    command list 1
do
    command list
done
# loops while command list 1 succeeds


while 
    read a b
do
    echo a is $a b is $b
done <data_file

For Loops​

for <var> in <list>
do
echo i is $i
done

tip

expression inside `` symbol will execute before the loop begin. Ex.

for num in `seq 1 5`
do
echo num is $num
done

Case Statement​

case expression in
pattern 1| pattern 3)
    command list;;
pattern 2)
    command list;;
esac

If Statement​

if 
command list
then
command list
else
command list
fi

Bash Commands​

time find / -name core
source script.sh 
#(or) 
. script.sh
alias ll="ls -l"
unalias ll
read a b <data_file
(or)
read a b c d
seq 1 5
find . -name '*.c'
grep txt    # Find text in result
lsof        # List all port and process names
sed         # Find and replace
wc -l <filename> # get word/line count of the file
test        # Evoluate the epxression. ex. test -f myfile (or) test $x -gt 10
            # Alternatives of test is  [[-gt, -lt]] or ((>, <))
strings 
ps [-ly | -el]
head
tail


Tail​

tail -2 <filename>      # Get last 2 line of the file
tail -n2 -f <filename>  # Watch/Monitor file changes

Sed​

/g in the pattern '/s/old/new/g is repleace all the old value with new value. Otherwise it will replce the first occarance of each line.

p -> Print
d -> delete
G -> Space between line. ex: `sed G f.txt` (or) `sed 'G;G' f.txt`
s -> substitute
g -> 
f -> script file
e -> script
i -> extention
sed 's/old/new/'; 's/demo/prod/'
sed -id 's/old/new/g' file1.txt
sed -d 's/[xX]/Y/' -e 's/b.*n/blue/'
sed -f sedscript -n sed4    # sed scripts file as input
date | sed 's/J/j/'
sed '1,5p'

sed '/alpha/s/beta/gama/'
sed '/apple/,/orange/d'
sed '/important/!s/print/throw_away/'

File all files with multiple replacements​
find . -type f -name '*.txt' -exec sed -f sedfile {} +

(or)

find . -type f -name '*.txt' -exec sed -f sedfile {} \;

awk​


tip

  1. echo $? # Last process exit status

  2. file

  3. Add & end of the expression will run the bash in background.