The Linux Shell and BASH Scripting

The Saigon CTT The Linux Shell and BASH Scripting The Saigon CTT  Objectives Objectives  Identify differrent Linux shell environments  Understand the redirection of input  Understand and utilize command substitution  Write and configure BASH script using variables, flow controls interactive input, functions, arithmetic and arrays The Saigon CTT  Example of Script Example of Script $ more example_script #!/bin/bash /usr/bin/nmbd -D /usr/bin/smbd -D The Saigon CTT  The Linux Shell The Linux Shell  Shells : Bourne (sh), Bourne Again (bash), Korn (ksh), C shell (csh, tcsh)  Programs start from command line have separate environments : parameters, variables , functions. The Saigon CTT  The Linux Shell The Linux Shell  Shells : Bourne (sh), Bourne Again (bash), Korn (ksh), C shell (csh, tcsh)  Programs start from command line have separate environments : parameters, variables , functions. The Saigon CTT  Shell Environment Customize Shell Environment Customize  bash config files : • /etc/profile • ~/.bash_profile, ~/.bash_login, ~/.profile,  Default environment variables : PS1, PS2, HOME, LOGNAME, SHELL, PATH, PAGER, LPDEST, PWD, DISPLAY, MAIL,  set, unset, export, … commands The Saigon CTT  Using the Using the bash bash shell shell  Alt+Fn  gpm : mouse server deamon  up and down keys (~/.bash_history )  Ctrl+Z, Ctrl+C, *, ?, … The Saigon CTT  Redirecting Input and Output Redirecting Input and Output  Input (<) or (<0) #mail admin@sgctt.com < file_name  Output (>) or (1>) # ls –l > file_name ( set –C or set –o noclobber : prevent overwrite )  Append (>>)  Error (2>) The Saigon CTT  Redirecting Input and Output Redirecting Input and Output  Pipe (|) #ls –l *.JPG | tee file_name #ls –al /root | grep error >error_file  Back ticks (`) or “$()” # which passwd /usr/bin/passwd # ls –l `which passwd` -r-sr-xr-x 1 root root 13476 Aug 7 /usr/bin/passwd The Saigon CTT  Redirecting Input and Output Redirecting Input and Output  Sometimes the output of a command is a list (ex: ls) and we wish to execute another command on each of the entries, use xargs #ls | xargs grep README Do Not use : #ls –al | xarg grep README [...]... done The Saigon CTT  The while loop Example count=0 while [$count –lt 4] do echo $count count=$((count +1)) done Output: 0 1 2 3 The Saigon CTT  Return codes/Exit status  The variable $? contains the return code of the previous executed command or application 0 Success ≠0 Failure  The exit n command will cause the script to quit and assign the value of n to the variable $? The Saigon CTT  Tests and. .. commands : for, while, until The Saigon CTT  The for loop  Syntax : for in do #list of commands to do done The Saigon CTT  The for loop Example #!/bin /bash for file in $(ls *.txt) do newname=“$(basename $file txt).html” mv $file $newname done The Saigon CTT  The while and until loop  Syntax : while do #list of commands to do done until do #list of commands... esac shift done The Saigon CTT  Input  We can input the information into script when executing the script ( interactive)  Commands : read select The Saigon CTT  Input - read  Allow to read values into variables  Syntax : read VAR1 VAR2 …  If there is more input than you are looking for, all the extras are put in the last variable Input – read Example The Saigon CTT  #!/bin /bash echo “Enter... : use “[ ]” around expression  if-then-else : if [ ] # include SPACEs then # what to do if the exp1 is true elseif [ ] then # what to do if the exp2 is true else fi The Saigon CTT  Tests and Conditions  Case test: case expression in pattern1 ) action ;; pattern2 ) action ;; … esac The Saigon CTT  case test Example while [ $1 ] do echo –n “$1 hits the “ case $1 in a?c | ab? ) echo “first... called postion parameter $0 The name of script $1-$9 Parameters being passed to script $* String contains ALL parameters passed to script separated by the first chacracter in IFS $@ A list of ALL as separate string $# Number of parameters on included the command line  The shift command will shift the positional parameters one or more position to the left or right Flow control The Saigon CTT   Loops.. .The Saigon CTT  Background jobs  Job #ls –l *.JPG | tee file_name #ls –al /root | grep error >error_file  Backround jobs : - Append with (&) or - Ctrl+Z and #bg %job_id The Saigon CTT  Variables  Naming : not begin with a digit  Assigning : VAR=value VAR=$(command) VAR=`command` Note : not SPACES around “=” Ex: # VAR=“Hello World” The Saigon CTT  Variables  Quotes and Command Substitution... add them” read VAR1 VAR2 echo “$VAR1 + $VAR2 =$(($VAR1+$VAR2))” The Saigon CTT  Input - select  It is great for creating menu  Syntax : select VAR [in list] do #commands that use $VAR done The Saigon CTT  Functions Syntax: function function_name { } Or function_name () { } The Saigon CTT  Functions  Functions can be called in the main script by function’s name, parameters are given on the command... VAR1=`ls /var/log | wc –l` # echo $VAR1 65 The Saigon CTT  Variable Notation  Expand variables : use ${VAR} # VAR1=“This is a String1” # VAR2=“$VAR1xyz” # VAR3=“${VAR1}xyz” # VAR4=‘${VAR1}xyz’ # echo $VAR1; echo $VAR2; echo $VAR3 # echo $VAR4 This is a String1 Nothing # default This is a String1xyz ${VAR1}xyz The Saigon CTT  Passing Info to Sript  On the command line, info can be passed to script through... are given on the command line  It inherits ALL variables in main script  We can change the return code of function by using return n command The Saigon CTT  Advanced bash Concepts  Arithmetic Expression :  Arrays : var[subscript]  More thrilling Examples for file in $(ls file*) do mv $file ${file%html}txt done The Saigon CTT  Summary Step 1 : create script file (cat, vi, mc, …), enter script codes . The Saigon CTT The Linux Shell and BASH Scripting The Saigon CTT  Objectives Objectives  Identify differrent Linux shell environments  Understand the. variables , functions. The Saigon CTT  The Linux Shell The Linux Shell  Shells : Bourne (sh), Bourne Again (bash) , Korn (ksh), C shell (csh, tcsh)  Programs