Talk Unix Shell Script 1

UNIX Shell Script (1) Dr. Tran, Van Hoai Faculty of Computer Science and Engineering HCMC Uni. of Technology [email_address]

What is a shell script?
A series of OS commands for execution
Stored in a text file
#!/bin/sh rm -f /tmp/listing.tmp > /dev/null 2>&1 touch /tmp/listing.tmp ls -l [a-z]*.doc | sort > /tmp/listing.tmp lpr -Ppostscript_1 /tmp/listing.tmp rm -f /tmp/listing.tmp

Components of a script #!/bin/sh rm -f /tmp/listing.tmp > /dev/null 2>&1 touch /tmp/listing.tmp # This is a comment ls -l [a-z]*.doc | sort > /tmp/listing.tmp lpr -Ppostscript_1 /tmp/listing.tmp rm -f /tmp/listing.tmp Shell in use (sh, bash, csh) Comment Command

How to invoke a script
Correct way
$ /bin/bash my_script arg_1 arg_2
Simple way
$ my_script arg_1 arg_2
The first line specifying the shell must be provided in simple way

How to be helped
Man pages
hoai@moon:~> man bash
...
hoai@moon:~> man sh
...

Definitions
Blank = chunk of tab or space
Name = sequence of ASCII letters , digits , underscores , beginning with a letter or underscore
Argument = string supplied on command-line

Example (argument) (1)
#!/bin/sh
##############################
echo "Script name is [$0]"
echo "First argument is [$1]"
echo "Second argument is [$2]"
echo "This process ID is [$$]"
echo "This argument count is [$#]"
echo "All arguments [$@]"

Example (argument) (2)
hoai@moon:~> my_script.sh hoai 1 university
Script name is [my_script.sh]
First argument is [hoai]
Second argument is [1]
This process ID is [5401]
This argument count is [3]
All arguments [hoai 1 university]

Filename metacharacters Current working directory ( $PWD ) ~+ Previous working directory ( $OLDPWD ) ~- Home directory of user name ~ name Home directory of the current user ~ Match any character not enclosed as above [! abc ...] Match any one of the enclosed characters; a hyphen can specify a range (e.g., a-z, A-Z, 0–9) [ abc ...] Match any single character ? Match any string of zero or more characters *

Simple regular expressions (Korn shell)
Pattern = sequence of patterns separated by “|”
Match any strings that don't match pattern !( pattern ) Match exactly one instance of pattern @( pattern ) Match one or more instances of pattern +( pattern ) Match zero or more instances of pattern *( pattern ) Match zero or one instance of pattern ?( pattern )

Example (metacharacters)
List files having prefix new
$ ls new*
Cat files having prefix ch and one more letter
$ cat ch?
Vi files starting by letters from D to E
$ vi [D-R]*
Print files not *.o and core (Korn shell)
$ pr !(*.o|core) | lp

Quoting Charater following taken literally Everything taken literally ''
Everything taken literally, except
$ (variable substitution)
` (command substitution)
“ (ending mark)
""

Example (quoting)
$ echo 'my class is "unix and tools"'
My class is "unix and tools"
$ echo "Well, isn't that "good" ?"
Well, isn't that "good" ?
$ echo "You have `ls | wc –l` files in `pwd`"
You have 34 files in /home/hoai
$ echo "The value of $x is $x"
The value of $x is 100

Variables (1) Use var if set, otherwise, print value and exit ${ var :? value } Use value of var is set, otherwise use nothing ${ var :+ value } Use the length of var ${# var } Use the number of positional arguments ${#*} or ${#@} Use var if set, otherwise, user value and assign it to var ${ var := value } Use var if set, otherwise, use value ${ var :- value } Use value of var ${ var } Set variable to value var=value …

Variables (2) Same as # pattern . Remove longest matching ${ var ## pattern } Same as %pattern . Remove longest matching ${ var %% pattern } Use value of var after removing pattern from the right. Remove shortest matching ${ var % pattern } Use value of var after removing pattern from the left. Remove shortest matching ${ var # pattern }

Command forms OR cmd1 || cmd2 AND; cmd1, then cmd2 if (cmd succeeds) cmd1 && cmd2 POSIX shell arithmetic substitution cmd $((expression)) POSIX Command substitution (nesting is allowed) cmd1 $(cmd2) Command substitution cmd1 `cmd2` Pipe cmd1 | cmd2 NOT; change exit status ! cmd Commands as a group in a subshell ( cmd1 ; cmd2 ) Commands as a group in current shell { cmd1 ; cmd2 } Multiple commands on the same line cmd1 ; cmd2

Example (command forms)
$ nroff file > file.txt &
Format in the background
$ cd; ls
Execute sequentially
$ (date; who; pwd) > logfile
All output is redirected
$ sort file | pr -3 | lp
Sort file, page output, then print
$ vi 'grep -l ifdef *.c'
Edit files found by grep
$ grep XX file && lp file
Print file if it contains the pattern;
$ grep XX file || echo "XX not found"
otherwise, echo an error message

Simple commands Create date strings date Evaluate variables eval Transform characters tr 'a' 'b' Simple arithmetic processor expr Predicate or conditional processor [( test )] Access lines in files head/tail Chop up a text by strings or characters cut Get directory name from path string dirname Get file name from path string basename Search for regular expressions grep Sort lines sort

Example (script) (1)
#!/bin/bash
alphabet="a b c d e" # Initialise a string
count=0 # Initialise a counter
for letter in $alphabet # Set up a loop control
do # Begin the loop
count=`expr $count + 1` # Increment the counter
# Display the result
echo "Letter $count is [$letter]"
done

Example (script) (2)
alphabet="a b c d e" # Initialise a string
count=0 # Initialise a counter
while [ $count -lt 5 ] # Set up a loop control
do # Begin the loop
count=`expr $count + 1` # Increment the counter
# Position of next letter
position=`bc $count + $count - 1`
letter=`echo "$alphabet" | cut -c$position-$position` # Get next letter
# Display the result
echo "Letter $count is [$letter]"
done

Homeworks (1)
Write a script for C compiler
Objective: use gcc by default, if gcc is not availablle, find another compiler (ending with cc ) instead.
Write a script to convert filenames to lowercase letters
Input: a directory path string
Output: all filenames in the directory in lowercase

Homeworks (2)
Write a script to warn users using too much space
Objective: find all users of the system (/etc/passwd, cut), check if someone uses > 1GB, mail him a warning message

Conditional structure
Loop structure
Function
File input/output
Array
are NEXT

Talk Unix Shell Script 1

by Dr.Ravi on Jul 18, 2008

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