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UNIX Shell Script
Dr. Tran, Van Hoai
Faculty of Computer Science and Engineering HCMC Uni. of Technology
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What is a shell script?
A series of OS commands for execution Stored in a text file
#!/bin/shrm -f /tmp/listing.tmp > /dev/null 2>&1touch /tmp/listing.tmpls -l [a-z]*.doc | sort > /tmp/listing.tmplpr -Ppostscript_1 /tmp/listing.tmprm -f /tmp/listing.tmp
Dr. Tran, Van Hoai2007
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Components of a script
#!/bin/shrm -f /tmp/listing.tmp > /dev/null 2>&1touch /tmp/listing.tmp# This is a commentls -l [a-z]*.doc | sort > /tmp/listing.tmplpr -Ppostscript_1 /tmp/listing.tmprm -f /tmp/listing.tmp
Shell in use (sh, bash, csh)
Comment
Command
Dr. Tran, Van Hoai2007
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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
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How to be helped Man pages
hoai@moon:~> man bash
...
hoai@moon:~> man sh
...
Dr. Tran, Van Hoai2007
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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
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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 [$#]"echo "All arguments [$@]"
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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]
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Filename metacharacters* Match any string of zero or more characters
? Match any single character
[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 character not enclosed as above
~ Home directory of the current user
~name Home directory of user name
~+ Current working directory ($PWD)
~- Previous working directory ($OLDPWD)
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Simple regular expressions (Korn shell)?(pattern) Match zero or one instance of pattern
*(pattern) Match zero or more instances of pattern
+(pattern) Match one or more instances of pattern
@(pattern) Match exactly one instance of pattern
!(pattern) Match any strings that don't match pattern
Pattern = sequence of patterns separated by “|”
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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
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Quoting
"" Everything taken literally, except $ (variable substitution) ` (command substitution) “ (ending mark)
'' Everything taken literally
\ Charater following \ taken literally
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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
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Variables (1)
var=value… Set variable to value
${var} Use value of var
${var:-value} Use var if set, otherwise, use value
${var:=value} Use var if set, otherwise, user value and assign it to var
${var:?value} Use var if set, otherwise, print value and exit
${var:+value} Use value of var is set, otherwise use nothing
${#var} Use the length of var
${#*} or ${#@} Use the number of positional arguments
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Variables (2)
${var#pattern} Use value of var after removing pattern from the left. Remove shortest 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} Same as %pattern. Remove longest matching
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Command formscmd1 ; cmd2 Multiple commands on the same line
{cmd1 ; cmd2} Commands as a group in current shell
(cmd1 ; cmd2) Commands as a group in a subshell
cmd1 | cmd2 Pipe
cmd1 `cmd2` Command substitution
cmd1 $(cmd2) POSIX Command substitution (nesting is allowed)
cmd $((expression))
POSIX shell arithmetic substitution
cmd1 && cmd2 AND; cmd1, then cmd2 if (cmd succeeds)
cmd1 || cmd2 OR
! cmd NOT; change exit status
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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
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Simple commands
sort Sort lines
grep Search for regular expressions
basename Get file name from path string
dirname Get directory name from path string
cut Chop up a text by strings or characters
[(test)] Predicate or conditional processor
tr 'a' 'b' Transform characters
expr Simple arithmetic processor
eval Evaluate variables
date Create date strings
head/tail Access lines in files
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Example (script) (1)
#!/bin/bash
alphabet="a b c d e" # Initialise a stringcount=0 # Initialise a counterfor letter in $alphabet # Set up a loop controldo # Begin the loop count=`expr $count + 1` # Increment the counter # Display the result echo "Letter $count is [$letter]"done
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Example (script) (2)
alphabet="a b c d e" # Initialise a stringcount=0 # Initialise a counterwhile [ $count -lt 5 ] # Set up a loop controldo # 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
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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
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Homeworks (2)
Write a script to warn users using too much spaceObjective: find all users of the system
(/etc/passwd, cut), check if someone uses > 1GB, mail him a warning message
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t Conditional structure Loop structure Function File input/output Array
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