Bash Shell Scripting: Getting Started SELF2015 Beau Sanders Greenville Technical College https://beausanders.org/self15 150609 SELF2015 • 1 About me • Academic Program Director Computer Technology Department Greenville Technical College Greenville, South Carolina • Instructor for 13 years • RHCE, RHCSA, RHCT • Certified Red Hat Academy Instructor • Teach Linux Essentials, Administration, Network Applications, Security, and Scripting 150609 SELF2015 • 2 Assumptions • You can handle the Linux command prompt • You understand the need to make administration easier and more manageable • You know a little programming • You know some Linux commands • You understand command structure in Linux 150609 SELF2015 • 3 Some Basic Linux Commands • cat • less • ls • more • ps • chmod • pwd • chown • cp • chgrp • mv • useradd • rm • usermod • mkdir • logger • head • wget • tail • mail 150609 SELF2015 • 4 Linux Command Structure 150609 SELF2015 • 5 Bash Shell Scripting: Getting Started Overview of BASH 150609 SELF2015 • 6 The bash shell • The shell is the most commonly used program in Linux • The shell is what you see when you log in or open a terminal, and is what you use to start most every command • The most commonly used shell in Linux is the bash shell • Bourne Again SHell 150609 SELF2015 • 7 Interactive Shells vs. Shell Scripts • The bash shell is an interactive shell • The bash shell is also designed to be a powerful scripting language – Bash shell scripts are short programs written using the same syntax used on the command line – Shell scripts allow users to automate often repeated actions by combining a series of commands – Many of the features of the bash shell provide programming logic (such as branches and loops) for writing sophisticated scripts 150609 SELF2015 • 8 An Introduction to Shell Scripts • A bash shell script is just a text file containing a list of ordinal Linux commands • The commands are sent through a specified program, called an interpreter, which runs each command in turn • The interpreter will be the bash shell (/bin/bash or /bin/sh). • Other interpreters allow you to use more powerful programming languages like Perl, Python and Ruby 150609 SELF2015 • 9 An Introduction to Shell Scripts • The first line of your script must specify which interpreter to send instructions to – This is done with a special string called a "shebang" which looks like this: #! – The shebang is followed by the name of the interpreter for this script – For example, to use bash as your interpreter you would use #!/bin/sh or #!/bin/bash 150609 SELF2015 • 10 An Introduction to Shell Scripts • Before you can run a script, you must enable the executable permission on it – otherwise, it's just a text file • A typical command to enable execute permissions on a script is: chmod 775 scriptname 150609 SELF2015 • 11 Default Path • A fixed default set of directories in which Linux looks for commands • These directories are referred to collectively as your PATH • For security reasons, your PATH seldom includes the current directory 150609 SELF2015 • 12 Default Path • To solve this problem you have two choices: – You can explicitly specify the location of the script by typing ~/foo.sh or ./foo.sh ( . always refers to the current directory) – You can place the script in a directory that is part of your PATH – Non-root users will need to create their personal bin: $mkdir ~/bin 150609 SELF2015 • 13 Bash Shell Scripting: Getting Started Variables and Redirecting Data 150609 SELF2015 • 14 Shell Variable Basics • The bash shell allows users to set and reference shell variables • A shell variable is simply a named value that the shell remembers • Shell variables • Can be used in commands • Can be used in shell scripts • Can also be referenced by programs as configuration options 150609 SELF2015 • 15 Shell Variable Basics • There are two types of shell variables: local variables and environmental variables • A local variable exists only within the shell in which it is created • Environmental variables are inherited by child shells such as when a graphical terminal is launched after logging in • Setting local variables is quite simple 150609 SELF2015 • 16 Shell Variable Basics • Make sure that you do not put any spaces on either side of the = sign • The shell will "hang on" to this association for as long as the shell exists or until it is unset 150609 SELF2015 • 17 Environmental Variables • Environmental variables are a part of the process stored in the kernel, like the process id, user id, and current working directory are part of the process • Whenever a process starts another process, environmental variables are inherited by the child process • This allows users to use the bash shell to create or modify an environmental variable, and then all commands started by the shell will inherit that variable 150609 SELF2015 • 18 Environmental Variables • How do we create environmental variables within the bash shell? – First, a shell variable is created, and then the shell variable is "promoted" to an environmental variable using the export command – The variable will then be exported to any future child processes 150609 SELF2015 • 19 Standard In and Standard Out • Command line programs (terminal based programs) read information from one source, and write information to one destination • The source programs read from is referred to as Standard In (stdin), and is usually connected to a terminal's keyboard • The destination programs write to is referred to as Standard Out (stdout), and is usually connected to a terminal's display • When using the bash shell, stdout can be redirected using > or >>, and stdin can be redirected using < 150609 SELF2015 • 20 Redirecting stdout Writing Output to a File • The bash shell uses > to redirect a process's stdout stream to a file • 150609 SELF2015 • 21 Appending Output to a File • To append a command's output to a file, rather than clobbering it, bash uses >> • Often >> is used to create a log file in a script 150609 SELF2015 • 22 Redirecting stdin • Bash uses < to read input from somewhere other than the keyboard • Often used to read in a file as input to script 150609 SELF2015 • 23 Bash Shell Scripting: Getting Started Commands and Tools 150609 SELF2015 • 24 echo • The echo command takes whatever text is typed as part of the command and echoes it to stdout • The echo command, together with redirection, can be used to create text files 150609 SELF2015 • 25 echo • Usage: echo [OPTIONS] [STRING...] • Echoes STRING(s) to standard output 150609 SELF2015 • 26 grep • The name grep stands for general regular expression parser • A regular expression is simply a way of describing a pattern, or template, to match some sequence of characters 150609 SELF2015 • 27 grep 150609 SELF2015 • 28 grep • The pattern argument supplies the template characters for which grep is to search • The pattern is expected to be a single argument – If the pattern contains any spaces, or other characters special to the shell, you must enclose the pattern in quotes to prevent the shell from expanding or word splitting it 150609 SELF2015 • 29 grep • By default, grep shows only the lines matching the search pattern • Sometimes you are interested in the lines that do NOT match the pattern • The -v command line switch inverts grep's operation • 150609 SELF2015 • 30 Pipes • When two commands are joined by a pipe, the stdout stream of the first process is tied directly to the stdin sequence of the second process • In order to create a pipe using bash, the two commands are joined with a vertical bar | • All processes that are joined in a pipe are referred to as a process group • Many commands in Unix are designed to operate as a filter, reading input from stdin and sending output to stdout 150609 SELF2015 • 31 Filtering output using grep • The traditional Unix grep command is commonly used in pipes to reduce data to only useful data; sometimes called a filter • The grep command is used to search for and extract lines which contain a specified string of text 150609 SELF2015 • 32 Filtering output using grep • The first argument to the grep command is the string of text to be searched for • Any remaining arguments are files to be searched for the text • If the grep command is called with only one argument, it looks to Standard In as its source of data on which to operate 150609 SELF2015 • 33 Extracting and Assembling Text: cut and paste • The cut command extracts texts from text files, based on columns specified by bytes, characters, or fields • The paste command merges two text files line by line 150609 SELF2015 • 34 The cut Command - Extracting Text with cut • The cut command extracts columns of text from a text file or data stream • The cut command interprets any command line arguments as names of files on which to operate, or operates on the standard in stream if none are provided • The cut command can extract by bytes, characters, or fields 150609 SELF2015 • 35 The cut Command - Extracting Text with cut • The list arguments are actually a comma-separated list of ranges • Each range can take one of the following forms: 150609 SELF2015 • 36 Extracting text by Character Position with cut -c • With the -c command line switch, the list specifies a character's position in a line of text • The first line character is character number 1 150609 SELF2015 • 37 Extracting Fields of Text with cut -f • The cut command can also be used to extract text that is structured not by character position, but by some delimiter character, such as a TAB or : 150609 SELF2015 • 38 The paste Command