This is about basics of shell scripting

Note: #!/bin/bash or #!/usr/bin/env bash ==> shebang it refers to execve() function. When this is run kernel passes path to original program as command line argument. so ./testing.sh gets executed as /bin/bash ./testing.sh On other hand, env will look for bash in your path.

#variables

created when we assign a value or with export command
remove it with unset command
reference it with $myvar
for a shell script to get copy of shell variable it need to be exported: export mynewvar or declare -x mynewvar
for function we can use export -f myfunc.
typing just export tells which variables are part of shell environment.

Example: echoes.sh

#enable

we can get list of bash bullitins using enable command
prefers bullitins, keywords, aliases and functions over commands in FS.

#compgen -k

it list keywords

#.bash_profile

it is invoked when bash shell is started
it is used to customize settings in bash
.bashrc is executed with every bash shell.
exporing an exported variable is usiually done in .bas_profile but not in .bashrc since it will grow with each nested shell invokation.
alias and functions are not exported so if we want it to be available in every shell it should be in .bashrc

source .sh

sourcing the script to import set of variables and fucntions in current shell
. ./.sh OR source .sh ==> produce same results

#echo

doesn't start new process
options used are -n, -e, -E etc.

Example: echoes.sh

#Assigning variables:

typeset command makes variable local.
declare -l myvar ==> if myvar has uppercase values it would be converted to lowercase automatically.
declare -u myvar ==> to convert to uppdercase automatically.
declare -r myvar ==> variable is read only.

Example: typeset.sh, declare.sh

reading input with read command.

read a b ==> reads first word to a and second to b
can combine with while or for loop too.

Example: while_1.sh, while_2.sh,while_read.sh

#for loop

command handy with for loop is seq command. seq 1 5 ==> it prints 1 2 3 4 5

Example: for_1.sh, for_2.sh

#function()

it executes within shell and not in new process
it gives names to sequence of statements.
using exit in function will terminates whole shell program but not just the function
exit terminates the shell process. we can get exit status value with special variable called '$?'
export -f functionname ==> this will export function to new shell

#redirection for file

0 ==> stdin, 1==> stdout , 2==>stderr
command > stdout-here
command 2 > stderr-here
command < stdin-from-here
command &> file #file gets stdout and stderr from command. file is created or overwritten.
command1 | command2 ==> stdout of command1 is stdin of command2
command 2>&1 | command2 ==> gets stdout and stderr from command.
command |& command2 ==> alternative way to get stdout and stderr from command
command >> file ==> file is created or appended if exists.
command &>> file ==> appends sdout and stderr at end of file.

Examples: sort.sh

#open and close file descriptors

exec N< myFile ==> open file descriptor N for reading from file myFile
exec N>myFile ==> open file descriptor N for writing to file myFile
exec <>N myFile ==> open file descriptor N for both reading and writing with myFile
exec N>&- or execN<&- ==> closes file descirptor N
lsof ==> to see what file descriptors for a process is open.
lsof -p $$ ==> $$ is process id of shell. it gives which file descriptor and which process are open in shell.

Examples: here.sh

case, if-else

above are some of other loop commonly used in bash

tests in bash

builtin test is used to check various conditions and set the return code with the result.
loops and conditionals often use result of test
an alternative to test is [[]] or (())
if [[-f afile]] ==> if afile exists
if test $x -gt 5

Examples: case_1.sh,compare.sh,ifthen.sh, exercise.sh

#Filter

a program is a filter if it reads from stdin and writes to stdout
filter can be used in pipes

head and tail commands

head -5 , tail -200 etc.
wc -l , wc -w etc. ==> word count for lines, words etc.

Examples: makeoutput.sh,

#sed

works great as filter
its a stream editor which means its non interactive.
-i option to change file
sed 's/old/new/' myfile ==> substitue the first occurence of old on each line for new in myfile and display result on stdout.

awk

pattern matchin language
it works as a filter.
good for report writing.
processes a line at a time like sed
breaks each line into fields, $1, $2 etc.
$0 is entire line
fields are delimitted in values by variable FS, normally white space.

Examples: sed_1.sh,awk1

Script parameters and {}

parameters to shell program : $1, $2 .. are called positional parameters.
$0 is path to program itself.
Shift is used to move positional arguments.

#indirection

x=abc abc=def echo ${!x} ===> prints def,this is called indirection. prints value of variable assigned to x.

#unset or null variables

${variable value} x=${var:- Hotdog} ==> if var unset/null return value, otherwise return value of var. So if var is set it will be value assigned to var. := ==> if var is unset/null var is assigned value and returned. :? ==> displays error and exit script if var unset/null. :+ ==> if var unset/null then return nothing, otherwise return value. ${var:offset} ==> value of var starting at offset. ${var:offset:len} ==> value of var starting at offset upto length len. ${#var} ==> length of variable.

Examples: unsetnull.sh,pos.sh, indirect.sh, string.sh,prepost.sh,challenge.pos.sh

#Advanced Bash:

njain51/bash_scripting