Shell Programming The UNIX Shell The UNIX shell

Shell Programming

The UNIX Shell § The UNIX shell listens to what you type and executes commands at your request. Command Library User command: lpr file results (on screen) UNIX Shell result or status ls, lpr, mv, rm, telnet, netscape, . . . UNIX Kernel Printers Files Memory

Popular Shells § § § sh csh tcsh bash ksh zsh Bourne shell (the original shell) C-shell (pronounced as “sea shell”) Like csh with more functions “Bourne again” shell Korn shell Z-shell

More Utilities (2) § echo Display command line input to screen $ echo Hi, I am Bill, the President of the US! § date Print the date and time $ date Wed Feb 3 12: 13: 07 HKT 1999

More Utilities (3) § head Display first few lines of file $ head -2 letter 3 Ms. Lewinski: It is getting late. Please order some pizza and stop § tail Display last few lines of file $ tail -2 letter 3 Thanks! Bill § grep Find a pattern in a file $ grep ”some pizza” letter 3 It is getting late. Please order some pizza and stop

More Utilities (4) § sort Sort the lines in lexical order $ sort letter 3 Bill It is getting late. Please order some pizza and stop Ms. Lewinski: Thanks! by my office. We'll tidy up a few more things before calling it a night. $ sort -r letter 3 calling it a night. by my office. We'll tidy up a few more things before Thanks! Ms. Lewinski: It is getting late. Please order some pizza and stop Bill

Input/Output Redirection (2) § Using the “>” character after a command to redirect output to a named file: $ sort names > names. sort $ uniq names. sort Bill Clinton Bill Gates Monica Lewinski § This will create a file test: $ cat > test type line 1 type line 2 <ctrl-d> $ cat test type line 1 type line 2 names sort names. sort uniq display

Input/Output Redirection (3) § Using the “>>” character after a command to redirect output to APPEND to a named file: § Typing to the end of a file $ cat >> test type line 3 type line 4 <ctrl-d> $ cat test type line 1 type line 2 type line 3 type line 4 § Append file 1 to file 2 $ cat file 1 >> file 2

Input/Output Redirection (4) § Using the “<” character after a command to redirect input from a named file: $ uniq < names. sort This is the same as: $ uniq names. sort uniq display § Using input and output redirection together: $ sort < names > names. sort

Pipes (1) § The standard output of a program can be “piped” into the standard input of another program: $ sort names | uniq Bill Clinton Bill Gates Monica Lewinski names sort uniq display

Pipes (2) § Several pipes can be connected: $ sort names | uniq | grep "Bill" Bill Clinton Bill Gates § Pipes and I/O redirection can be used together: $ sort -r names | uniq >names. rev $ cat names. rev Monica Lewinski Bill Gates Bill Clinton

Directory Shorthands § § “. ” is the directory itself “. . ” is the parent directory In most shells “~” means your home directory) ~user means user’s home directory, so: $ more ~jbond/. plan § looks at the file. plan in /home/jbond, which is jbond’s home directory.

Special Directories § “/” (pronounced “slash” and also called “the “root”) is the ancestor of all files in the file system § /bin and /usr/bin contain UNIX utilities (e. g. , cat) § /dev contains special files which describe “devices” such as terminals and printers § /etc has administrative programs like password files § /tmp is for temporary files; periodically deleted § Every directory has at least two entries: “. ” is the directory itself, and “. . ” is the directory’s parent

Security and Access Permissions (1) § There are three types of users: The owner of the file (user) § The group of the file (group) § Anyone else (other) § § There are three types of permission (independent of each other): Read permission § Write permission § Execute permission §

Security and Access Permissions (2) § Use ls -l to see file permissions -rw-r--r-- 1 jbond cs #links Group Permissions User 154 Feb 4 15: 00 letter 3 Byte size Name Last modification § There are four sets of items in the permissions: -rw-r--r-- § § type The type is: user group other “-” regular files, “d” directories , “l” symbolic links. The next nine characters indicate if the file is readable, writable, or executable for the file owner, the file group, or other users, respectively.

Changing Permissions (1) § The chmod command is used to modify permissions. § chmod can only be used by the owner of a file/dir (or the administrator root). § The arguments are: chmod [ugoa] [+-=] [rwxdd] [file/dir] In other words: § Optionally, one of the characters: u (user/owner), g (group), o (other), or a (all). § Optionally, one of the characters: + (add permission), - (remove permission), or = (set permission). § Any combination of the characters r (read), w (write), or x (execute).

Changing Permissions (2) § Another way to change permission is to use numbers representing the permissions. The arguments are: chmod nungno [file/dir] nu - user’s permission code ng - group’s permission code no - other’s permission code -rwxrwxrwx Code table (3 bits) --- 0 r-- 4 --x 1 r-x 5 -w- 2 rw- 6 -wx 3 rwx 7

Permission Example § To let everybody read or write the file letter 1 $ chmod a+rw letter 1 $ ls -l letter 1 -rw-rw-rw 1 jbond (chmod 666 letter 1) cs 154 Feb 4 15: 00 letter 1 § To allow user to execute file letter 1 $ chmod u+x letter 1 $ ls -l letter 1 -rwxrw-rw 1 jbond (chmod 766 letter 1) cs 154 Feb 4 15: 00 letter 1* § To not let “other” to read or write file letter 1 $ chmod o-rw letter 1 $ ls -l letter 1 -rwxrw---1 jbond (chmod 760 letter 1) cs 154 Feb 4 15: 00 letter 1* § To let “group” only read the file letter 1 $ chmod g=r letter 1 $ ls -l letter 1 -rwxr----1 jbond (chmod 740 letter 1) cs 154 Feb 4 15: 00 letter 1*

Alias § The C Shell has the alias command, which allows you to create command shortcuts. $ alias dir "ls -F" $ alias rm "rm -i" $ alias + "chmod u+x *" $ alias - "chmod u-x *" $ alias 111 "cd ~horner/111" $ pwd /bin $ 111 $ pwd /homes/horner/111 § If you put the alias commands in your. cshrc file, you can use them every time you login.

Combining Commands (2) § Commands can be grouped together using parentheses § There are two main reasons to group commands: § To create a “single command” out of a group of commands (especially useful before a pipe): $ (cat letter 1; head -2 names) | sort >list § To run a set of commands in their own subshell (especially when trying to limit the effect of a cd command): $ (cd secret; ls | wc -l); ls | wc -l 3 25 This line has the effect of counting the files in secret, and then counting the files in the current directory.

Shells § A shell can be used in one of two ways: A command interpreter, used interactively § A programming language, to write shell scripts (your own custom commands) §

Shell Scripts § A shell script is just a file containing shell commands, but with a few extras: The first line of a shell script should be a comment of the following form: #!/bin/sh for a Bourne shell scripts are the most common, since C Shell scripts have buggy features. § A shell script must be readable and executable. chmod +rx scriptname § As with any command, a shell script has to be “in your path” to be executed. § – If “. ” is not in your PATH, you must specify “. /scriptname” instead of just “scriptname”

Shell Script Example § Here is a “hello world” shell script: $ ls -l -rwxr-xr-x 1 horner $ cat hello #!/bin/sh 48 Feb 19 11: 50 hello* # comment lines start with the # character echo "Hello world" $ hello Hello world $ § The echo command functions like a print command in shell scripts.

Shell Variables (1) § The user variable name can be any sequence of letters, digits, and the underscore character, but the first character must be a letter. § To assign a value to a variable: number=25 name="Bill Clinton" § There cannot be any space before or after the “=“ § Internally, all values are stored as strings.

Shell Variables (2) § To use a variable, precede the name with a “$”: $ cat test 1 #!/bin/sh number=25 name="Bill Clinton" echo "$number $name" echo '$number $name' $ test 1 25 Bill Clinton $number $name $ § Note the difference of single and double quotes in this example.

User Input (1) § Use the read command to get and store input from the user. $ cat test 2 #!/bin/sh echo "Enter name: " read name echo "How many friends do you have? " read number echo "$name has $number friends!” $ test 2 Enter name: Bill Clinton How many friends do you have? too many Bill Clinton has too many friends!

User Input (2) § reads one line of input from the keyboard and assigns it to one or more user-supplied variables. $ cat test 3 #!/bin/sh echo "Enter name and how many friends: " read name number echo "$name has $number friends!" $ test 3 Enter name and how many friends: Bill Clinton 63 Bill has Clinton 63 friends! $ test 3 Enter name and how many friends: Bill. C 63 Bill. C has 63 friends! $ test 3 Enter name and how many friends: Bill has friends! § Leftover input words are all assigned to the last variable.

Special Symbol : $ (1) § Use a backslash before $ if you really want to print the dollar sign: $ cat test 4 #!/bin/sh echo "Enter amount: " read cost echo "The total is: $$cost" $ test 4 Enter amount: 18. 50 The total is $18. 50

Special Symbol : $ (2) § You can also use single quotes for printing dollar signs. § Single quotes turn off the special meaning of all enclosed dollar signs: $ cat test 5 #!/bin/sh echo "Enter amount: " read cost echo 'The total is: $' "$cost" $ test 5 Enter amount: 18. 50 The total is $ 18. 50

Simple calculation using expr (1) § Shell programming is not good at numerical computation, it is good at text processing. § However, the expr command allows simple integer calculations. § Here is an interactive Bourne shell example: $ i=1 $ expr $i + 1 2 § To assign the result of an expr command to another shell variable, surround it with backquotes: $ i=1 $ i=`expr $i + 1` $ echo "$i" 2

Simple calculation using expr (2) § The * character normally means “all the files in the current directory”, so you need a “” to use it for multiplication: $ i=2 $ i=`expr $i * 3` $ echo $i 6 § expr also allows you to group expressions, but the “(“ and “)” characters also need to be preceded by backslashes: $ i=2 $ echo `expr 5 + ( $i * 3 )` 11

Simple calculation using expr (3) Example $ cat test 6 #!/bin/sh echo "Enter height of rectangle: " read height echo "Enter width of rectangle: " read width area=`expr $height * $width` echo "The area of the rectangle is $area" $ test 6 Enter height of rectangle: 10 Enter width of rectangle: 5 The area of the ractangle is 50 $ test 6 Enter height of rectangle: 10. 1 Enter width of rectangle: 5. 1 expr: non-numeric argument Does not work for floats!

Backquotes – Command Substitution § A command or pipeline surrounded by backquotes causes the shell to: § § Run the command/pipeline Substitute the output of the command/pipeline for everything inside the quotes § You can use backquotes anywhere: $ whoami clinton $ cat test 7 #!/bin/sh user=`whoami` numusers=`who | wc -l` echo "Hi $user! There are $numusers logged on. " $ test 7 Hi clinton! There are 6 users logged on.

Backquote Example (1) $ cat big #!/bin/sh dir=`pwd` big=`ls -l | sort +4 | tail -1 | cut -c 55 -70` size=`ls -l | sort +4 | tail -1 | cut -c 33 -40` echo "The biggest file in $dir is $big. " echo "$big is $size bytes. " $ big The biggest file in /homes/horner/111 is letter 1 is 155 bytes. § sort +4 sorts in increasing order based on the fourth field (fields start from zero, and are separated by whitespace). § cut -cx-y cuts characters from position x to position y (the first position is 1). field 4 field 0 1 2 3 4 5 6 123456789012345678901234567890123 -r--r--r-1 horner cs 155 Feb 12 16: 00 letter 1

Backquote Example (2) $ wc -w letter 1 7 letter 1 $ cat big 1 #!/bin/sh dir=`pwd` big=`ls -l | sort +4 | tail -1 | cut -c 55 -70` nline=`wc -l $big | cut -c 6 -8` nword=`wc -w $big | cut -c 6 -8 ` nchar=`wc -c $big | cut -c 6 -8 ` echo "The biggest file in $dir is $big. " echo "$big has $nlines, $nwords, $ncharacters. " $ big 1 The biggest file in /homes/horner/111 is letter 1 has 7 lines, 29 words, 155 characters. § wc -w counts the number of words in the file (“words” are separated by whitespace). § wc -c counts the number of characters in the file.

Control Flow § The shell allows several control flow statements: § § § if while for