Www.ischool.drexel.edu INFO 320 Server Technology I Week 5 Shell environments and scripting 1INFO 320 week 5

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INFO 320Server Technology I

Week 5

Shell environments and scripting

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Overview

• Time for more practical concerns about how Linux/UNIX works– Shells– Aliases and links– Compound commands– Environment variables– Scripting

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Shells

From (Frisch, 2002), (Petersen,2009), (Nemeth, 2007)

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Shells

• A shell is a text command line interface (CLI) environment– Like ‘cmd’ in Windows or the Terminal utility

in Mac OS X– Ubuntu Server logs you into a bash shell,

instead of using the GNOME GUI

• Each type of shell has its quirks– For example, bash has a prompt of $ or #,

but the C shell has a prompt of %

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Shells

• Linux has several kinds of shells– Korn shell (ksh is the command)– Bourne shell (sh or dash, the original UNIX

shell)• Bourne Again SHell (bash, default in Ubuntu)

– Z shell (zsh)– C shell (csh)

• TCSH shell (tcsh)

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Shells

• In Linux– sh points to bash– csh points to tcsh

• In the GNOME GUI, we’re using a shell emulator

• Key shell files (no games, just files)– /etc/shells lists the shells available– /etc/passwd can specify the default shell

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chsh

• chsh is a command to change shells

• It’s often used with the –s option, which is followed by the shell path & command– The shell used must appear in /etc/shells

• Most shell commands are in /bin/– How else could you find them?

• So typical use of chsh is– chsh –s /bin/sh

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Aliases and links

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Aliases

• We can define a shortcut for a command in the form of an alias – alias alias-name=string

• If I hate typing ls –al all the time I could define– alias z=“ls –al”

• Aliases only apply to the current shell, so they are often defined in the personal initialization files ~/.bash_profile, ~/.bash_login, or ~/.profile

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Unalias

• The alias command by itself gives a list of all current aliases– alias

• To remove an alias, use unalias– unalias alias-name

• For example– unalias z

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Links

• Linux recognizes two forms of links – Symbolic and hard

• Symbolic links are a pointer to the real file– ln –s today weather creates a symbolic

link weather which points to the file today

• Hard links are another name for the file– Delete a hard link and you delete the real file– ln Monday storm creates a hard link

between storm and Monday

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Links

• In ls –l, symbolic links show up with an ‘l’ (ell) in the first column, are very small

• If you erase the real file, symbolic links still exist, and have to be deleted separately– Similarly, to erase a file with hard links, the

real file and all hard links must be deleted

• A Windows Shortcut is a symbolic link, as is the OS X Alias

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Compound commands

From the CommandlineHowto, (Petersen,2009)

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Background processes

• Shell commands are normally interactive– You give a command, and have to wait for it to

finish before giving the next command

• We can override this by running commands in the background with an & after the command– tail –f psout &

• Can use ps –ef to tell when they’re done

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Executing multiple commands

• If you want to execute two or more commands consecutively, then you can use a semicolon between them– command1 ; command2 – Spaces around the semicolon are optional

• Like this– ps -ef;ls

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• It’s possible to make later commands dependent on the earlier commands executing successfully, by using && between the commands– command1 && command2

• Notice the && is a logical AND– If command1 runs, then run command2

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• Or if we want a command to execute only if the first one fails, use a logical OR which is || (two vertical lines)– command1 || command2

• Execute command2 only if command1 fails

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Pipes

• Be careful! A single vertical line between commands is a pipe, which takes the output from one command and uses it as input for the next command– command1 | command2

• For example, the more command forces display of one page of text at a time– So ls –al | more does what?

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Pipes and grep

• Pipes are great for use with grep• By itself, grep string file searches

for a string within the file– grep booker facultylisting.txt

• Using pipes, you can search the output of a command with grep– ls –al | grep apache– ls –al | grep apache > apachefiles

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Pipes into sorting, head or tail

• Pipes can feed sort, head, or tail– sort does what it sounds like– head gives the first ten lines of the file– tail gives the last ten lines of the file

• Great for looking at the end of a log file, for example

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Pipes and disk usage

• The disk usage command, du gives the space used by each directory

• This is a prime candidate for sorting in reverse order– du / | sort –rn | head – du –s /home/mary gives a summary

• df gives file system usage

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Output redirection

• We saw that the default output can be redirected to a file with the > sign– command > filename

• If you want to add (append) to the end of an existing file, then you would use the following syntax: – command >> filename

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Input redirection

• We can also feed a file into a command using input redirection

• We are going to take the input from a file for the command to be executed

• Here is the syntax for this input redirection– command < filename – sort < myfavoriteanimals.txt

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Other wildcards

• We saw that * can represent any number of characters, like info3*.txt

• If you want a one character wildcard use ? – ls –al file0?.txt

• If only a specific range of single characters are allowed, put them in square brackets– ls –al file0[5-8].txt– ls –al file[a-zA-Z].txt– ls –al file[2589].txt

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touch

• The touch command is an oddity– “The touch utility shall change the

modification times, access times, or both of files.”

• But if you touch a file that doesn’t exist, a blank file will be created– So we’ll use touch to create a new file

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Environment variables

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• Environment variables are maintained by the OS to keep track of data

• Some pertain to a login session, some to a shell, some to other things

• Environment variables are in ALL CAPS, and to refer to them put a dollar sign $ before their name– They have an underline between_words

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• For example, your default shell is $SHELL

• To view the current value of an environment variable the echo command can be used– echo $SHELL

• There are also environment variables within scripting, which depend on the type of shell you’re using

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• There are lots of environment variables– $HOME = home directory

•cd ~ returns you to your $HOME directory

– $TERM = the type of terminal mode– $PWD = same as the command pwd– $PS1 = the shell prompt

• To see a list of them, use the env command

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Setting environment variables

• The declare command can set environment variables, or even create new ones

• Some you can set at the command line, like PS1– PS1=‘/w$’– See the PROMPTING section in the bash

manual page for details on prompt characters

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Command completion

• File names, shell variables, and user names can be completed by hitting the tab – echo $HOM<tab>– cat pre<tab> might yield cat preface

• If the command is not unique yet, tab twice to get a list of options– echo $H<tab><tab> shows all $H*

environment variables

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Multiple line commands

• A command can be stretched across more than one line by ending each preliminary line with a backslash \– grep turtle \– /home/username/folder/file.txt

• More often used in scripting, this allows messy commands to be more readable– Notice that \ and / have wildly different

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Scripting

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Bash scripting

• A series of commands can be put in a text file, and executed as though you typed them in manually – that’s the purpose of a shell script– Scripts are used for backups, system

monitoring, and many other purposes

• Here we’re dealing with bash scripts – Syntax differs for other shells

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File to script?

• How does a file become a script?

• We make it executable– chmod u+x filename

• Then to run the script, use the filename like a command– filename

• Or if you’re in a different directory, use the full path name to the script file

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File to script?

• In order for a file to be executed properly as a script, the first line should tell the OS how to execute it– #!/bin/bash

• The #! tells the OS to execute the rest of the line as a command

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Comments

• Oddly enough, by itself a # means the start of a comment, which can appear after commands on a line or from the start of the line– ls –l # here’s a same-line comment– # this is a separate comment– # it’s good to comment what your script does, who wrote it, etc.

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Script basics

• A script can execute normal commands

• Output from the script can be– Create or append files– Provide output to the screen using the echo

command– echo –n keeps text on a single line of output

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Bash environment variables

• There are environment variables which can only be used in the context of a script– $0 is the name of the program/script– $1 through $9 are the values of arguments

passed to the program– $# is the number of arguments– $$ is the process ID of this process

• We can also use the other envir var’s – $HOME, $USER, etc.

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Scripting concepts

• Scripts can use all the tools we’ve seen– Input and/or output redirection– Pipes– Grep

• But we can also have conditionals, typically based on the environment variables

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if and fi

• An if statement exists in bash scripting• The general syntax is

– if [ condition1 ] – then– # commands go here– elif [ condition2 ]– # more commands– else– # more commands– fi

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if and fi

• The if condition often looks cryptic– [ ! -e “$1” ] – To see what the options !, –e, -d, etc. mean,

look for FUNCTIONS in the bash man page– “$1” refers to the environment variable $1

• String comparisons are also allowed, e.g. – string1 == string2 – string1 > string2

Warning! Spaces are critical in the condition!

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Other script features

• We won’t go into them, but other structures can exist in scripts– Loops (do, while, until, etc.)– Case statements

• Scripts can be recursive

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References

• CommandlineHowto, https://help.ubuntu.com/community/CommandlineHowto

• Linux Administration Handbook, by Evi Nemeth et al, Prentice Hall 2007. ISBN 0131480049

• Bash man page, http://manpages.ubuntu.com/manpages/jaunty/man1/bash.1.html

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Www.ischool.drexel.edu INFO 320 Server Technology I Week 5 Shell environments and scripting 1INFO 320 week 5