2006-Jan-23 1

Shell Scripts

Jacob Morzinski jmorzins@mit.edu

http://sipb.mit.edu/iap/2006/shell/http://web.mit.edu/sipb-iap/www/2006/shell/

2006-Jan-23 3

Why use shell scripts?

They simplify your life. Store complicated commands in a script, to

save effort and reduce typing errors.

athrun ops rdesktop -ujmorzins -f –N \-a16 vash.mit.edu:4884

ldapsearch -u -LLL -h ldap.mit.edu \-b dc=mit,dc=edu uid=jmorzins 2>&1 | \egrep -v '^(SASL[ /]|objectClass:)'

2006-Jan-23 4

Basic: setting up a script

1. Edit the file. Emacs, vi, jedit, gedit, whatever you prefer. Run with: bash /path/to/file

2. (Make Unix happy.) Make sure the file begins with #!/bin/bash chmod a+rx file Make sure file is in your $PATH (or $path).

3. Run the script. file (or /path/to/file )

2006-Jan-23 5

Basic: making Unix happy

The #! line tells Unix what program to run the file through. We want to use bash.

The chmod command tells Unix to make the file readable and executable.

Your $PATH is a list of directories that Unix looks through, when trying to find a program whose name matches the word you just typed at the prompt.

2006-Jan-23 6

Basic: setting the PATH

If your shell is sh-based (like bash):PATH="$PATH":/mit/jmorzins/bin

export PATH

Sometimes you can combine these:export PATH="$PATH":/mit/jmorzins/bin

If your shell is csh-based (like tcsh):set path = ( $path:q /mit/jmorzins/bin )

2006-Jan-23 7

Basic: fixed-text example

$ cat user1

#!/bin/sh

vos exa user.jmorzins

kvno jmorzins

$ ./user1

user.jmorzins

APHRODITE.MIT.EDU /vicepb

Creation Wed Aug 30 09:50:43 1995

jmorzins@ATHENA.MIT.EDU: kvno = 20

2006-Jan-23 8

Basic: variables allow flexebility$ cat user2

#!/bin/sh

vos exa user."$1"

kvno "$1"

$ ./user2 boojum

user.boojum

COCYTUS.MIT.EDU /vicepb

Creation Sun Aug 10 00:08:35 1997

boojum@ATHENA.MIT.EDU: kvno = 78

2006-Jan-23 9

Review of basic knowledge

At this point, you know: How to set up simple shell scripts How to pass one word into the script, to have the

script run commands using that word ("$1").

2006-Jan-23 10

What the shell really does

You: type a command The shell does:

Variable substitution (interpolation)* File name substitution (globbing)* Parses the line into arguments* Finally, runs the command

* Can be influenced by quoting.

2006-Jan-23 11

Runs the command

Searches through $PATH to find the executable file, if not an absolute path.

You can run a sequence of commands if you separate them with semicolons ( ; )

Commands can return an exit status. Status = 0 means success Status != 0 means some sort of failure

You can use “&&” and “||” to chain commands in an “and” or “or” fashion.

2006-Jan-23 12

Parses the line into arguments The shell splits the command line on white

space. The first word is the command name The rest of the words are “arguments,” and

are passed to the command.

2006-Jan-23 13

File globs

If files match your pattern, the file names are filled into the command line.

Pattern Meaning

* Match zero or more characters

? Matches just one character

[abc] [x-z] Match just one character, from a range

[!0-9] Match one character, that is not in the range

2006-Jan-23 14

Variable substitution

A variable can be a word, a number, or a special symbol: Some symbols: $$, $?, $*, “$@” Numbers 0, and 1-9: $0, $1, …, $9 Words: $PATH, $var, $longer_name

Note that variable substitution happens before file substitution and before the commandline is split into words.

2006-Jan-23 15

Quoting

Single quote: '...' The shell ignores all enclosed characters

Double quote: "..." The shell ignores most enclosed charactecters The shell does interpret $, `, and \

Backtick: `...` Run a command, insert the output

Backslash: \ Special treatment for the following character.

2006-Jan-23 16

Input/Output redirection

Commands usually print to standard output, sometimes print to standard error.

Commands read from standard input.

Stdin is 0 Stdout is 1 Stderr is 2 Higher numbers are possible

2006-Jan-23 17

Table of I/O redirection

cmd1 | cmd2 pipe output from cmd1 into cmd2

commad > file put text into file, truncate file

command >> file append text to file

command < file read from file

command 2>&1 send stderr to where stdout is

command 1>&2 send stdout to where stderr is

cmd1 `cmd2` capture output of cmd2 for command-line of cmd1

2006-Jan-23 18

Setting Variables

Setting variables var=value ENVVAR=value export ENVVAR

Note: variables don’t affect parent shells only environmental variables affect child shells

2006-Jan-23 19

Using variables

$* - list of all command-line parameters. Bad. “$@” – Good list of all command-line

parameters. “$1”, “$2”, …, “$9” – individual command line

parameters shift – remove “$1”, shift all other variables

down one set – set new “$1”, “$2”, etc variables

2006-Jan-23 20

Utilities – test

test – runs a test, returns true or false Good for if statements, or

while loops also called “[”, but in that

case it must be used with a matching “]”

-f is a file

-d is a directory

-z is zero length

-n is non-zero length

=, != compare strings

-a, -o and, or

! negate next part

-gt, -lt-ge, -le

compare numbers

2006-Jan-23 21

Utilities – grep

Pattern searching Uses regular expressions Powerful, but hard to

learn

a letter “a”

. any one char

* zero or more…

[abc][^abc]

char in a range,or inverted

^ start of line

$ end of line

\(z\)\1

save a match,

re-use a match

2006-Jan-23 22

Shell Loops - if

if expr ; then cmd1 ; else cmd2 ; fi cmd1 and cmd2 can be complex the else can be omitted

if [ x“$var” = x“yes” ] ; then echo good ; fi

2006-Jan-23 23

Shell Loops – while, for

while expr ; do cmd ; done

for var in a b c d e ; do cmd ; done

2006-Jan-23 24

Shell functions

name () { cmd1 ; cmd2 “$@” ; }

Act like mini-shell scripts. Can set variables in the current shell.