Advanced UNIX CIS 218 Advanced UNIX 6 . The Bourne and Bash Shells - PowerPoint PPT Presentation

Advanced UNIX CIS 218 – Advanced UNIX 6 . The Bourne and Bash Shells  Objectives – explain how to write Bourne and Bash Shell scripts 1 CIS 218 Advanced UNIX

Overview 1 . Making a File Executable 2 . Combining Commands 3 . Redirecting Output 4 . Executing Scripts 5 . Variables 6 . Control Flow continued 2 CIS 218 Advanced UNIX

7 . Functions 8 . Other Commands 9 . Here Documents 10 . Debugging 11 . More Information 3 CIS 218 Advanced UNIX

1 . Making a File Executable  $ cat whoson date echo Users currently logged on who  Wrong: $ whoson whoson: Permission denied 4 CIS 218 Advanced UNIX

Right:  $ ls -lg whoson -rw-r--r-- 1 ad pubs 42 Jun 17 10 : 55 whoson $ chmod u+x whoson $ ls -lg whoson -rwxr--r-- 1 ad pubs 42 Jun 17 10 : 55 whoson  $ whoson Tue Nov 7 13 : 21 : 34 ICT 2000 Users currently logged in ad consol Nov 7 08: 26 jenny tty 02 Nov 7 10:04 5 CIS 218 Advanced UNIX

Possible PATH Problem  $ whoson whoson: Command not found  Due to PATH shell variable (see later)  Quick fixes: $ ./whoson or $ sh whoson 6 CIS 218 Advanced UNIX

2 . Combining Commands  Sequencing: $ a ; b ; c  same as: $ a $ b $ c 7 CIS 218 Advanced UNIX

Process Creation (&)  $ a & b & c 14271 (PID for a) 14272 (PID for b)  $ a & b & c & 14290 14291 14292 $  $ a | b | c & 14302 (PID for piped commands) 8 CIS 218 Advanced UNIX

Processes in Action $ cat a echo - n “aaaaaaaaaaaaaaaaaaaaa” echo - n “aaaaaaaaaaaaaaaaaaaaa” sleep 2 echo - n “aaaaaaaaaaaaaaaaaaaaa” echo - n “aaaaaaaaaaaaaaaaaaaaa” On calvin there  Similarly for b and c isn't much variation unless the machine is  Try the following a few times: loaded. $ a & b & c & 9 CIS 218 Advanced UNIX

3 . Redirecting Output  1 > redirect standard output (stdout) 2 > redirect standard error (stderr) $ cat a b 1 > out 2 > err Files stdout out cat a b err stderr 10 CIS 218 Advanced UNIX

>&  redirect one stream into another: – 2>&1 redirect stderr into stdout $ cat a b 1 > theLot 2>&1 stdout theLot cat a b stderr 11 CIS 218 Advanced UNIX

4 . Executing Scripts  Make sure that a script is executed by the Bourne Shell: (no need for chmod ) $ sh whoson  or: $ cat boss #!/bin/sh echo Definitely Bourne Shell Script continued 12 CIS 218 Advanced UNIX

 On Linux machines (e.g. calvin ), the Bourne shell has been replaced by Bash – sh means the Bash shell 13 CIS 218 Advanced UNIX

5 . Variables 5 . 1 . User-defined Variables 5 . 2 . Environment Variables 5 . 3 . Readonly Shell Variables 14 CIS 218 Advanced UNIX

5 . 1 . User-defined Variables No spaces  $ person=alex around the = $ echo person person $ echo $ person alex  $var returns the value stored in var – called substitution 15 CIS 218 Advanced UNIX

5 . 1 . 1 . Switch off Substitution  Swich off substitution with ' var ' or \var  $ echo '$person' $person $ echo \$person $person 16 CIS 218 Advanced UNIX

5 . 1 . 2 . Switch off Special Chars (")  "" switches off the special meaning of characters used in filename generation (e.g. *, ?)  $ ls // directory contents ad.report ad.summary  $ memo=ad* * means only * $ echo "$memo" ad* $ echo $memo * means any number ad.report ad.summary of characters 17 CIS 218 Advanced UNIX

5 . 1 . 3 . Exporting Variables  Normally a variable is local to the running script (the process).  It is sometimes useful if running scripts (processes) can access another script’s variables. we want to calls use cheese  e.g. extest subtest in subtest cheese=english 18 CIS 218 Advanced UNIX

No Exporting: extest 1 & subtest Using " is  $ cat extest 1 a good habit, cheese=english see later echo "extest 1 1 : $cheese" subtest echo "extest 1 2 : $cheese" $cat subtest echo "subtest 1 : $cheese" cheese=swiss echo "subtest 2 : $cheese" continued 19 CIS 218 Advanced UNIX

subtest does not  $ extest 1 see extest 1 's extest 1 1 : english cheese value subtest 1 : subtest 2 : swiss extest 1 2 : english extest 1 is not affected by subtest 's setting of cheese 20 CIS 218 Advanced UNIX

Exporting: extest 2 & subtest  $ cat extest 2 export cheese cheese=english echo “extest 2 1 : $cheese” subtest echo “extest 2 2 : $cheese”  $ extest 2 extest 2 1 : english cheese value passed in subtest 1 : english subtest 2 : swiss change not exported extest 2 2 : english from subtest 21 CIS 218 Advanced UNIX

5 . 1 . 4 . Reading read inputs everything up to the newline  $ cat readln echo - n “Type: ” read ln echo “You entered: $ln”  $ readln Type: The Wizard of Oz You entered: The Wizard of Oz 22 CIS 218 Advanced UNIX

No Quotes  $ cat readlnnq echo - n “Type: ” read ln echo You entered: $ln directory  $ ls ad.report summary 1 contents $ readlnnq Type: * You entered: ad.report summary 1 23 CIS 218 Advanced UNIX

5 . 1 . 5 . Executing Commands A very simple shell  $ cat proc_cmd echo - n “Enter a command: ” read command $command echo Thanks  $ proc_cmd Enter a command: echo Display this Display this Thanks 24 CIS 218 Advanced UNIX

5 . 1 . 6 . Splitting Input Text is split based on white  $ cat split 3 space. echo - n “Enter something: ” read word 1 word 2 word 3 echo “Word 1 is: $word 1 ” echo “Word 2 is: $word 2 ” echo “Word 3 is: $word 3”  $ split 3 Enter something: this is something Word 1 is: this Word 2 is: is Word 3 is: something 25 CIS 218 Advanced UNIX

 $ split 3 Enter something: this is something else, x Word 1 is: this Word 2 is: is Word 3 is: something else, x The last variable gets everything that is left in the input. 26 CIS 218 Advanced UNIX

5 . 1 . 7 . Command Subsitution Must use ‘  $ cat mydir this_dir=‘pwd‘ echo “Using the $this_dir directory.” this_date=$(date) A Bash echo "Today's date: $this_date" addition  $ mydir Using /home/ad/teach/adv-unix/bourne directory Today's date: Tue Nov 7 13 : 52 : 46 ICT 2000 $ 27 CIS 218 Advanced UNIX

5 . 2 . Environment Variables  Most environment variables get their values from the shell at login.  The values of some of the variables can be set by editing the .profile file in your home directory – Bash uses .bash_profile and .bashrc 28 CIS 218 Advanced UNIX

5 . 2 . 1 . Examples  HOME pathname of your home directory  $ pwd /home/ad/planning $ echo $HOME /home/ad cd uses HOME $ cd to return to your $ pwd home directory /home/ad continued 29 CIS 218 Advanced UNIX

 PATH – directories where executable can be found – represented as a string of pathnames separated by ‘:’ s  $ echo $PATH Extend the /usr/local/bin:/usr/bin:/bin: default PATH $ PATH=SPATH":/home/ad/bin:." $ echo $PATH /usr/local/bin:/usr/bin:/bin:/home/ad/bin:. 30 CIS 218 Advanced UNIX

Note for SysAdmins  If you are the system administrator (superuser, root) for your machine, do not extend your path with "." – it opens you to potential attack by hackers – e.g. 'fake' UNIX utilities placed in the current directory 31 CIS 218 Advanced UNIX

5 . 2 . 2 . Typical .profile  $ cat .profile TERM=vt 100 PATH=$PATH":/home/ad/bin:." PS 1 =“ad: “ CDPATH=:$HOME export TERM PATH PS 1 CDPATH stty kill ^u export needed  $ . .profile in the Bourne shell 32 CIS 218 Advanced UNIX

5 . 2 . 3 . Typical .bash_profile  On calvin , .bash_profile simply invokes your .bashrc (if it exists): umask 002 if [ -f ~/.bashrc -a PS 1 ="\$ " ]; then . ~/.bashrc fi These shell commands will be explained later continued 33 CIS 218 Advanced UNIX

Typical .bashrc  PS 1 ="\u@\h$ " # PS 1 ="\w[\#]$ " No export needed PATH=$PATH":." alias ls='/bin/ls -F' alias dir='ls -ba' alias cls="clear" These features : will be explained : later. psgrep() { ps aux | grep $1 | grep -v grep } 34 CIS 218 Advanced UNIX

5 . 2 . 4 . set  The current settings for the environment variables can be listed with set : $ set | more BASH=/bin/bash : PATH=/usr/local/bin:/usr/bin:/bin:. : PS 1 ='\u@\h$ ' : 35 CIS 218 Advanced UNIX

5 . 3 . Readonly Shell Variables  These are environment variables that cannot have their values changed.  Most of them relate to the arguments supplied to a script when it is called. 36 CIS 218 Advanced UNIX

5 . 3 . 1 . Script Name ( $0 )  $ cat abc echo The name of this script is $0  $ abc The name of this script is abc 37 CIS 218 Advanced UNIX

5 . 3 . 2 . Script Arguments ( $1, $2,..., $9 )  $ cat display_ 5 args echo The first five command line echo arguments are $1 $2 $3 $4 $5  $ display_ 5 args jenny alex helen The first five command line arguments are jenny alex helen If the variable has no value, then nothing is printed. 38 CIS 218 Advanced UNIX

5 . 3 . 3 . All Arguments ( $* )  $ cat display_all echo $*  $ display_all a b c de fg hi jk mno pqr stu w x y z a b c de fg hi jk mno pqr stu w x y z  $@ is like $* but puts “...” around each printed argument 39 CIS 218 Advanced UNIX

5 . 3 . 4 . Number of Arguments ( $# )  $ cat num_args echo “This script has $# arguments.”  num_args helen alex jenny This script has 3 arguments 40 CIS 218 Advanced UNIX