Shell Scripting in Linux

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Shell Scripting in Linux
By Kartik N k.narayan_at_in.ibm.com
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Introduction

• Shell
• Interface to the user
• Command interpreter
• Programming features
• Shell Scripting
• Automating command execution
• Batch processing of commands
• Repetitive tasks

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Shells in Linux

• Many shells available
• Examples -sh, ksh, csh, bash
• Bash is most popular

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The Bourne Again Shell

• Abbreviated bash
• Default in most Linux distributions
• Most widely used on all UNIX platforms
• Derives features from ksh, csh, sh, etc.

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BASH Programming features

• Support for many programming features
• - variables, arrays, loops, decision
  operators, functions, positional parameters
• Pipes, I/O re-direction
• Misc. features - expansions, job control
• Built in Commands - read, echo, source, alias

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Starting Shell Scripting
A Simple Shell Script -

• !/bin/bash
• catfile.sh
• Uses the cat command to display
• a file
• FILE1
• echo "Displaying file FILE"
• cat FILE

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Starting Shell Scripting
Running A Shell Script -

• Method1
• chmod x catfile.sh
• ./catfile.sh
• Method 2
• bash catfile.sh

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BASH Features

• Loops
• for
• for VAR in LIST
• do ltsomethinggt
• done
• while
• while ltconditiongt
• do ltsomethinggt
• done
• until

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BASH Features

• Decision operators
• if
• if ltconditiongt
• then
• ltdo somethinggt
• else
• ltdo something elsegt
• fi
• Can use the 'test' command for condition
• if test a b
• then
• echo a
• fi

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BASH Features

• Functions
• Creating
• - function foo ...
• - foo () ...
• Calling
• - foo 5 6
• Using parameters in the function
• - foo () echo "1 2"

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BASH Features

• Arithmetic Evaluation
• Methods of evaluation
• aexpr a 5
• ((aa5))
• a((a5))
• let aa5
• Many operators available
• , -, , /, , , etc

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BASH Features

• I/O Redirection
• Operators - gt, lt, gtgt, gt,
• File Redirection
• command gtfile
• command ltfile
• File Descriptor Redirection
• Opening - exec nltgtfile
• Using - command gtn
• Closing - ngt-
• Output Redirection
• ls sort

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BASH Features
Expansions Pathname expansion , ?,
... Tilde expansions abc or
/abc Arithmetic expansion ((expr)) Paramet
er expansions ...var... Brace
expansions -,-,- Command
substitution (command), command Process
substitution lt(command), gt(command)
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BASH Features

• Environment
• env variables - HOME, PWD, LOGNAME
• Aliases - alias cp'cp -i'
• Use /.bashrc to set env variables and aliases
• Job Control
• Foreground and Background jobs ()
• Ctrl-z, bg, fg, jobs
• kill job

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BASH Features

• Quoting
• Escape character (\)
• echo \hello
• hello
• Partial quoting ("...")
• Escape special characters using \
• Prevents word-splitting
• Full quoting ('...')
• Cannot use escape sequences
• Cannot embed single quotes

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BASH Programmable Completion

• Available in BASH 2.05 and above
• Allows user-defined completions
• Uses the BASH built-ins 'complete' and 'compgen'
• Uses in-built arrays COMPREPLY and COMP_WORDS

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Other Shell-scripting tools

• Basic commands
• ls, cat ,cp, mv, find, xargs, expr
• date, time, etc
• Filters
• grep, sort, uniq, diff, cut, paste, etc.
• Regular Expressions
• Other programs
• sed, awk, lex, yacc
• tty, stty, tset

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Regular Expressions

• Pattern matching rules
• Used in grep, sed, awk , (everywhere??)
• Basic regexps
• . ? ...

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Other Shell-scripting tools

• Sed
• Search and Replace
• Delete
• Awk
• Search and process patterns
• Process input streams

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Usage of Shell-scripts

• Where to use shell-scripting
• System Administration
• -Automate tasks
• -Repeated tasks
• Development
• -Allows testing a limited sub-set of
  functionality
• -Testing tools
• Daily usage
• Simple scripts
• - reminders, diary, mail, etc.

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Usage of Shell-scripts

• Where Not to use shell-scripting
• Resource-intensive tasks
• Cross-platform portability (Use C, JAVA,etc.)
• Complex and mission-critical applications
• Where security is important
• Proprietary, closed-source applications

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Optimising scripts

• Speed improvement
• Use/write programs for slow operations
• e.g use grep to speeden searches and awk for
  mathematical ops
• Optimise loops
• Minimise I/O BASH is slow with files
• Use awk, Perl, etc. where speed is reqd.
• Keeping scripts small
• Modularised scripting

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References

• man bash(1)
• Advanced Bash Scripting Guide
• http//personal.riverusers.com/thegrendel/abs-gu
  ide-0.5.tar.gz
• man pages - awk, sed, etc.

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