Intro. to Unix Environment
Xiaolan Zhang
Spring 2015
1
Outline
 Shell: interactive mode and batch mode
 Unix commands: syntax and some basic useful commands
 File systems and commands, emacs or vi
 Users and groups
 Environment variables and configuration files
 Standard input/output, redirection, pipeline
 grep command basics
 Shell scripting: control structures (branch and loop)
2
A Quick Start
 Terminal
 PuTTy, a telnet/ssh client
 a free and open source terminal emulator application
 a window in your desktop that works like old time terminal
 Enter your ID and password
3
Your first encounter: shell
•
Shell: a special-purpose program, command line
interpreter, read commands typed by a user and execute
programs in response to entered commands
•
Many different shells:
Bourne Shell (sh): oldest,
•
•
•
•
•
4
I/O redirection, pipelines, filename generation (globbing), variables,
environment variables, command substitution, background command
execution, function
C Shell (csh): syntax of flow-control similar to C, command
history, command-line editing, job control, aliases
Korn Shell (ksh): “csh”, compatible with sh
Bourne again Shell (bash): GNU’s reimplementation of Bourne
shell, supports features added in C shell, and Korn shell
Check/Change Login Shell
 To check the shell you are using
 echo $SHELL
 echo $0
 login shell: default shell for a user, specified in /etc/passwd
 To change your login shell, use command
 chsh
5
Shell: interactive mode
•
A shell session (a dialog between user and shell)
1.
2.
3.
4.
6
Displays a prompt character, and waits for user to type in a
command line
 Prompt depends on shell: sh, ksh, bash: $ csh: % tcsh: >
 May be customized (with current directory, host, ...)
On input of a command line, shell extracts command name
and arguments, searches for the program, and runs it.
When program finishes, shell continues to step 1
The loop continues until user types “exit” or “ctrl-d” to end
UNIX command line
• Command name and arguments:
command [ [ - ] option (s) ] [ option argument (s) ] [ command argument (s) ]
– Command arguments are file or path names
• cp prog1.cpp prog1.cpp.bak
– Options: used to control behavior of the command
• head -20 lab1.cpp
• wc –w lab2.cpp // count how many words
• Some options come with option argument
–
–
7
sort –k 1 data.txt
// use the first column of data.txt as the key to sort
The most important command !!!
man ls
• man: displaying online manuals
– Press q to quit, space to scroll down, arrow keys to roll up/down
8
Correcting type mistakes
• Shell starts to parse command line only when Enter key is
pressed
• Delete the whole line (line-kill): C-u
• Erase a character: C-h or backspace key
• Many more fancy functionalities:
– Auto-completion: press Tab key to ask shell to auto-complete
command, or path name
– History (repeat command): use arrow (up and down) keys to
navigate past commands
–…
9
Shell: batch/scripting mode
 In batch mode, shell can interpret and execute shell scripts
#!/bin/bash
# count number of files/directories in curr. directory
ls –l | wc –l
 Shell constructs:
 variables,
 Loop and conditional statements
 I/O commands (read from keyboard, write to terminal)
 Function, arrays …
10
Outline
 Shell: interactive mode and batch mode
 Unix commands: syntax and some basic useful commands
 File systems and commands, emacs or vi
 Users and groups
 Environment variables and configuration files
 Standard input/output, redirection, pipeline
 grep command basics
 Shell scripting: control structures (branch and loop)
11
Unix File
 Files: store information
 a sequence of 0 or more bytes containing arbitrary information
 What's in a filename?
 Case matters, no longer limited to 14 chars
 Special characters such as -, spaces are allowed, but you shouldn’t use
them in filename
 Can you think of the reason ?
 Dot files are hidden, i.e., normally not listed by command ls
 To display all files, including hidden files, use ls -a
12
What’s in a file ?
 So far, we learnt that files are organized in a hierarchical
directory structure
 Each file has a name, resides under a directory, is associated with
some admin info (permission, owner)
 Contents of file:
 Text (ASCII) file (such as your C/C++ source code)
 Executable file (commands): e.g., ls, vi, …
 A link to other files, …
 Virtual file:
 /proc: a pseudo-filesystem, contains user-accessible objects on runtime state
of kernel and executing processes
 To check the type of file: “file <filename>”
13
File Viewing Commands
• cat: concatenate files and display on standard output (i.e., the
terminal window)
– cat [option] … [file] …
[ ] means the argument is optional
… means there can be multiple
– cat proj1.cpp
arguments of this type
– cat proj1.cpp proj2.cpp
– cat –n proj1.cpp // display the file with line #
• More: file perusal filter (i.e., displaying file one screen at a time)
– more proj1.cpp
• head, tail: display the beginning or ending lines of a file
– tail -f output // display the file, append more
lines as the file grows
14
File manipulation commands
• rm: remove one or multiple files or directories
– rm [option] … FILE …
– rm temp
– rm temp1 temp2
• Wildcards (metacharacter) can be used in command line
– Letter * matches with any string
• rm *.o: remove all .o files
– ?: match any one character
– [abc]: match with letter a or b or c
• rm –r: remove directories and their sub-dirs recursively
• rm –i : confirm with user before removing files
15
File manipulation commands (2)
 cp: copy file or directory
 cp [OPTION] SOURCE DESTINATION
 To make a backup copy of your program before dramatic
change
 cp proj1.cpp proj1.cpp.bak
 To make a backup copy of a whole directory
 cp –r lab1_dir lab1_dir_backup
 -R, -r, --recursive: copy directories recursively
16
File manipulation commands (3)
 mv: move (rename) files/directories
 mv [OPTION] SOURCE DEST
 Rename SOURCE to DEST
 mv proj1.cpp lab1.cpp
 mv [OPTION]… SOURCE… DIRECTORY
 Move SOURCE to DIRECTORY
 mv lab1.cpp lab2.cpp CISC3130
17
Hierarchical file system
• Directory: a file that can hold other files
 Advantages of hierarchical file system:
• Files can have same names, as long as they are under
different directories
• Easier for protection
• Organized files
dev
cdrom
tty24
/ (root)
bin
home
staff
zhang
18
etc
lib
group passwd
Absolute pathname, path
/ (root)
dev
cdrom
tty24
bin
home
staff
etc
lib
passwd
zhang
 Pathname of a file/directory: location of file/directory in the file
19
system
 How do you tell other where your prog. Is located ?
 Absolute pathname: path name specified relative to root, i.e.,
starting with the root (/)
 e.g., /home/staff/zhang
 What’s the absolute pathname for the “passwd” file?
Home directory
 Every user has a home directory created for him/her
 When you log in, you are in your home directory
 In home directory, a user usually has permission to create
files/directories, remove files ..
 ~ to refer to current user’s home directory
 ~username to refer to username’s home directory
20
Current directory & Relative Pathname
 Tiring to specify absolute pathname each time
 To make life easier: working directory
 User can move around the file system, shell remembers where
the user is (i.e., current directory)
 To check your current directory, use command:
 pwd
21
Getting around in the file system
• To create a subdirectory:
– mkdir [option]… directory…
– cd
– mkdir CISC3130
– cd CISC3130
– mkdir lab1
• To remove a directory:
– rmdir [option]… directory…
– Report failure if directory is not empty
• Can use rm –rf to remove non-empty directory
22
Command for change current directory
(move around)
 cd [directory]
[zhang@storm Work]$ cd
[zhang@storm ~]$ pwd
/home/staff/zhang
[zhang@storm ~]$ cd Work
[zhang@storm Work]$ pwd
/home/staff/zhang/Work
[zhang@storm Work]$ cd ..
[zhang@storm ~]$ pwd
/home/staff/zhang
[zhang@storm ~]$
23
23
Relative pathname
 Absolute pathname: specified relative to root
 Relative pathname: specified relative to current directory
 . (current directory), .. (parent directory, one level up)
 If current directory is at /home/staff/zhang, what is the
relative pathname of the file passwd?
 ../../../etc/passwd: go one level up, go one level up, go one level
up, go to etc, passwd is there
/ (root)
dev
cdrom
tty24
bin
home
staff
24
zhang
etc
lib
passwd
Relative pathname
 For all commands that take file/directory name as
arguments, you can use pathnames of the file/directory
 Example:
 cd /home/staff/zhang/public_html
 pico CISC3130/index.html
 cd .. (go up one level to parent directory)
 cp ../prog2.cpp prog2.cpp
25
Getting around in the file system
 ls: list directory contents
 ls [OPTION] … [FILE]
ls: list files/directories under current directory
ls –l: long listing,
[zhang@storm CISC1600]$ ls -l
total 59180
-rw-r--r-- 1 zhang staff 509952 Sep 7 13:02 3_types.ppt
-rw-r--r-- 1 zhang staff 593408 Sep 14 23:38 4_computation.ppt
-rw-r--r-- 1 zhang staff 1297 Sep 2 12:18 account.html
-rw-r--r-- 1 zhang staff 3304448 Nov 7 18:24 ArrayVector1.ppt
drwxr-xr-x 2 zhang staff 4096 Dec 8 22:36 Codes
26
Long listing
 To get more information about each file
[zhang@storm Demo]$ ls -al
total 32
Total disc space taken in blocks (1024 Byte)
drwxr-xr-x 5 zhang staff 4096 2008-01-16 16:01 .
drwxr-xr-x 41 zhang staff 4096 2008-01-16 16:01 ..
drwxr-xr-x 2 zhang staff 4096 2008-01-16 15:55 CCodes
-rw-r--r-- 1 zhang staff 38 2008-01-16 16:01 .HiddenFile
-rw-r--r-- 1 zhang staff 53 2008-01-16 15:57 README
drwxr-xr-x 2 zhang staff 4096 2008-01-16 15:55 SampleCodes
drwxr-xr-x 4 zhang staff 4096 2008-01-16 15:56 ShellScriptes
d means
directory
27
User name of the owner and its group
Who has permission to read/write the file
Long listing explained
drwxr-xr-x 4 zhang staff 4096 2008-01-16 15:56 ShellScriptes
 field 1
 1st Character: specifies the type of the file.
 - normal file, d directory, s socket file, l link file
 next 9 characters – File Permissions






28
field 2: specifies the number of links for that file
field 3 : specifies owner of the file
field 4: specifies the group of the file
field 5 : specifies the size of file.
field 6: date and time of last modification of the file
field 7: File name
File permissions
drwxr-xr-x 4 zhang staff 4096 2008-01-16 15:56 ShellScriptes
 Each file is associated with permission info.
 Differentiate three type of users: owner user, users from same
group as owner, others
 Three type of access: - in the field means no permission
 Read (r): use “cat” to open a file to read, use “ls” to list files/directories under
a directory
 Write (w): modify the contents of the file, create/remove files from the
directory
 Execute (x): execute the file, or “cd” or “search” the directory for file
 Trying to list other’s directory
29
[zhang@storm ~]$ ls ../roche/
ls: cannot open directory ../roche/: Permission denied
Outline
 Shell: interactive mode and batch mode
 Unix commands: syntax and some basic useful commands
 File systems and commands, emacs or vi
 Users and groups
 Environment variables and configuration files
 Standard input/output, redirection, pipeline
 grep command basics
 Shell scripting: control structures (branch and loop)
30
User and Group
 Each user has unique login name (user name), and
corresponding numeric ID
 Group id, home directory, login shell:
 Stored in file /etc/passwd
 Groups: each user can belong to multiple groups
 For collaboration …
 Group name, group id, user list info stored in /etc/group
 Superuser
 ID: 0, username: root
 Bypass all permission checks …
31
More to play with
 who: show who is logged on
 who am I
 write: write to another user’s terminal (IMS?)
 which: show the full path name of a command
$ which bash
/bin/bash
 How does shell find a command ?
 Environment variable PATH stores a list of paths to search for programs: “set
| grep PATH” or “echo $PATH”, “set” to show all variable settings
 PATH=$PATH:$HOME/bin:.
 Built-in commands: history, set, echo, etc.
 mail: send email from command line
 mail –s “graded project” zhang < proj1.cpp
32
Outline
 Shell: interactive mode and batch mode
 Unix commands: syntax and some basic useful commands
 File systems and commands, emacs or vi
 Users and groups
 Environment variables and configuration files
 Standard input/output, redirection, pipeline
 grep command basics
 Shell scripting: control structures (branch and loop)
33
SHELL Variables
 Different types of variables
 Environment variables: HOME, PATH, PS1, PS2 …
 Parameter variables: $1, $*, …
 User defined variables: student, file, x, ..
 PATH variable: define the list of directories that shell
searches for programs
PATH=$PATH:.:~zhang/bin
export PATH # make PATH an environment variable, which is inherited by all
subprocesses
34
Variables
 A variable is a name that you give to a particular piece of
information.
 Shell variable names: start with a letter or underscore, and
may contain any number of following letters,digits,or
underscores.
 Shell variables hold string values, there is no limit on
length of string value
 variable values can be, and often are, empty—that is, they
contain no characters.
 Empty values are referred to as null
35
Variable assignment
 Assign value to variable: writing variable name, immediately
followed by an = character, and new value, without any
intervening spaces.
myvar=this_is_a_long_string_that_does_not_mean_much
first=isaac middle=bashevis last=singer ##Multiple assignments
allowed on one line
 Shell variable values are retrieved by prefixing the variable’s
name with a $ character.
echo $myvar ## display the value of myvar
this_is_a_long_string_that_does_not_mean_much
36
Environment Variables
 Environment variables:
 a list of name-value pairs available to all programs running from
the shell
 A child process inherits environment from parent process
 Variables not in environment not inherited
 export command: a bulit-in command
 Puts given variable into environment, i.e., makes the variable an
environment variables
 Will learn how to access environment from C/C++ program
 When setting PATH, needs to put it into environment, unless
only for current script
37
echo command
 echo: produce output, prompting or to generate data for
further processing.
 printed its arguments to standard output, with each one
separated from next by a space and terminated with a
newline
$ echo Now is the time for all good men
Now is the time for all good men
$ echo to come to the aid of their country.
to come to the aid of their country.
 Example usage: to see the settings of environment variables …
 echo $PATH
 echo $HADOOP_CONF_DIR
38
Bash: startup initialization
 For login shell, run ~/.bash_profile
 For non-login shell, run ~/.bashrc
 Suggested way to configure your environment:
 Define your environment variables, and alias in ~/.bashrc
 Call ~/.bashrc in the ~/.bash_profile file
 For example:
# .bash_profile
39
# Get the aliases and functions.
if [ -f ~/.bashrc ]; then
. ~/.bashrc //source command, or dot command
fi
Source command
 A shell builtin command
 Usage:
. filename [arguments]
source filename [arguments]
Read and execute commands from filename in current shell
environment, and return exit status of last command executed from
filename.
 Demo: difference of running a script directly and source it
$./CountFiles
$source CountFiles
 Why?
 When running a script directly, a new shell (non-login, non-interactive shell)
40
is started to batch processing script …
Outline
 Shell: interactive mode and batch mode
 Unix commands: syntax and some basic useful commands
 File systems and commands, emacs or vi
 Users and groups
 Environment variables and configuration files
 Standard input/output, redirection, pipeline
 grep command basics
 Shell scripting: control structures (branch and loop)
41
Standard I/O
 All programs should have a data source, a data sink (where data
goes),and a place to report problems. These are standard input,
standard output, standard error.
• Standard input, by default is linked to keyboard
• Standard output, by default is linked to terminal window
• Standard error, by default linked to terminal window
 A program should neither know, nor care, what kind of device lies
behind its input and outputs: disk files,terminals, tape
drives,network connections,or even another running program!
 A program can expect these standard places to be already open
and ready to use when it starts up.
42
Simple example
 A very simple C program
#include <stdio.h>
main() {
char yourName[256];
printf ("Your name ?\n"); // Similar to cout
if (fgets (yourName,256,stdin)==NULL) //similar to cin
fprintf (stderr,"No input");
else
printf("hello, %s\n", yourName);
}
43
Input/Output Redirection
 On command line, one can redirect these three files
 To redirect standard output to a disk file:
 command [ [ - ] option (s) ]
[ option argument (s) ] [
command argument (s) ] > FILENAME
 Execute the command, sending its standard output to specified
file
 Existing content of the file is deleted
 E.g.: ls –lt > InfoFilelist.txt
 To append standard output to a file: use >> instead of >
 grep “tax reform” *.txt > output
 grep “fuel efficiency” *.txt >> output
44
Input/Output Redirection (cont’d)
 To redirect standard error to a file
$ command [ [ - ] option (s) ] [ option argument (s) ] [
command argument (s) ] 2> ERRORMSGS
 Examples:
[zhang@storm ~]$ ls abc
ls: cannot access abc: No such file or directory
[zhang@storm ~]$ ls abc 2> error
[zhang@storm ~]$ more error
ls: cannot access abc: No such file or directory
45
User > and 2> together
 To split error messages from normal output
[zhang@storm ~]$ ls research.tex abc
ls: cannot access abc: No such file or directory
research.tex
[zhang@storm ~]$ ls research.tex abc 2> error > output
[zhang@storm ~]$ cat error
ls: cannot access abc: No such file or directory
[zhang@storm ~]$ cat output
research.tex
 This is useful for running a command that might take
long time to finish, or generates very long output …
46
More on redirection
 To redirect both output and error to same file:
 ./a.out > dd 2> dd : does not work. Error output is not
captured.
 sort file.txt > dd 2>&1
 2>&1: redirect error output to same place as standard output
 grep numOfStudents 2>dd >&2
 >&2: redirect standard output to same place as error output
 To discard output, redirect it to /dev/null
 /dev/null: a special virtual file, “a black hole”
./a.out > /dev/null 2>&1
 I don’t want to see the output or error message, nor do I want
them saved to a file …
47
Input/Output Redirection (cont’d)
 To read standard input from a file, instead of keyboard
$ command [ [ - ] option (s) ] [ option argument (s) ] [
command argument (s) ] < FILENAME
 Examples
 mail zhang –s “Question” < proj1.cpp
 ./a.out < values.txt
//a.out is your program that reads integers from standard input
and calculate the sum
48
Command line
 Short options (-) and long options (--)
 in POSIX, use two dashes (– –) to signify end of options, i.e.,
remaining arguments on command line that look like options
are treated as arguments (for example, as filenames).
 To delete a file named “-l”, rm -- -l
 Semicolons separate multiple commands on same line. The
shell executes them sequentially.
 ampersand (&), tell shell to run preceding command in
background, which simply means that shell doesn’t wait for
command to finish before continuing to next command.
49
Pipe: getting rid of temporary file
 Pipe: an inter-process communication mechanism provided
by kernel
 Has a reading end and a writing end
 Any data write to writing end can be read back from reading
end
 Read/write pipe is no different from read/write files, i.e., any
prog. that reads from standard input can read from pipe,
similarly for the standard output
Writing end
50
Reading end
Command Pipeline
 Shell set things up
 create a pipe, “start” two programs simultaneously, with the first
program’s output redirected to writing end of pipe, second
program’s input redirected to reading end of pipe
 individual program/command knows nothing about redirection
and pipe
51
Outline
 Shell: interactive mode and batch mode
 Unix commands: syntax and some basic useful commands
 File systems and commands, emacs or vi
 Users and groups
 Environment variables and configuration files
 Standard input/output, redirection, pipeline
 grep command basics
 Shell scripting: control structures (branch and loop)
52
Filter programs
 Filter: program that takes input, transforms input,
produces output.
 default: input=stdin, output=stdout
 e.g.: grep, sed, awk
 Typical use:
$ program pattern_action filenames
program scans files (if no file is specified, scan standard input),
looking for lines matching pattern, performing action on matching
lines, printing each transformed line.
53
grep/egrep/fgrep commands
 grep comes from ed (Unix text editor) search command
“global regular expression print” or g/re/p
 so useful that it was written as a standalone utility
 two other variants
 grep - pattern matching using Basic Regular Expression
 fgrep – file (fast, fixed-string) grep, does not use regular expressions,
only matches fixed strings but can get search strings from a file
 egrep - extended grep, uses a Extended Regular Expression (more
powerful, but does not support backreferencing)
54
grep syntax
 Syntax
grep [-hilnv] [-e expression] [filename], or
grep [-hilnv] expression [filename]
 Options










55
-E
use extended regular expression (replace egrep)
-F
match using fixed string (replace fgrep)
-h
do not display filenames
-i
Ignore case
-l
List only filenames containing matching lines
-n
Precede each matching line with its line number
-v
Negate matches
-x
Match whole line only (fgrep only)
-e expression Specify expression as option
-f filename
Take regular expression (egrep) or
a list of strings (fgrep) from filename
What Is a Regular Expression?
 A regular expression (regex) describes a set of
possible input strings, i.e., a pattern
 e.g., ls –l | grep ^d ## list only directories
 e.g., grep MAX_INT *.h ## where is MAX_INT defined
 Regular expressions are endemic to Unix
 vi, ed,
 grep, egrep, fgrep; sed
 emacs, awk, tcl, perl, Python
 more, less, page, pg
 Libraries for matching regular expressions: GNU C
Library, and POSIX.2 interface (link)
56
Meta characters
^ (Caret)
match expression at start of a line, as in ^d.
$ (Dollar)
match expression at end of a line, as in A$.
\ (Back slash) turn off special meaning of next character, as in \^.
[ ] (Brackets) match any one of the enclosed characters, as in
[aeiou], use hyphen "-" for a range, as in [0-9].
[^ ]
match any one character except those enclosed in [ ],
as in [^0-9].
. (Period)
match a single character of any value, except end of
line.
*(Asterisk) match zero or more of preceding character or
expression.
57
Protect Metacharacters from Shell
 Some regex metachars have special meaning for shell:
globbing and variable reference
$grep e* .bash_profile ## suppose there are files email.txt, e_trace.txt
# under current dir
Actual command executed is:
grep email.txt e_trace.txt .bash_profile
$grep $PATH file
## $PATH will be replaced by value of PATH…
 Solution: single quote regexs so shell won’t interpret special
characters
grep ′e*′ .bash_profile
 double quotes differs from single quotes: allows for variable
substitution whereas single quotes do not.
58
Escaping Special Characters
 \ (backslash): match special character literally, i.e., escape it
 E.g., to match character sequence 'a*b*‘
 'a*b*' : ## match zero or more ‘a’s followed by zero or more
## ‘b’s, not what we want
 'a\*b\*' ## asterisks are treated as regular characters
 Hyphen when used as first char in pattern needs to be escaped
 ls –l | grep '\-rwxrwxrwx'
# list all regular files that are readable, writable and executable to all
 To look for reference to shell variable PATH in a file
grep '\$SHELL' file.txt
59
Regex special char: Period (.)
 Period . in regex matches any character.
 grep ′o. ′ file.txt
regular expression
o .
For me to poop on.
match 1
match 2
 How to list files with filename of 5 characters ?
 ls | grep ′….. ′ ## actually list files with filename 5 or more chars
long? Why?
 How to list normal files that are executable by owners?
 ls –l | grep ′\-..x ′
60
This is one line of text
o.*o
x
xyz
\m
^
$
.
[xy^$x]
[^xy^$z]
[a-z]
r*
r1r2
\(r\)
\n
\{n,m\}
r+
r?
r1|r2
(r1|r2)r3
(r1|r2)*
61 {n,m}
Ordinary characters match themselves
(NEWLINES and metacharacters excluded)
Ordinary strings match themselves
Matches literal character m
Start of line
End of line
Any single character
Any of x, y, ^, $, or z
Any one character other than x, y, ^, $, or z
Any single character in given range
zero or more occurrences of regex r
Matches r1 followed by r2
Tagged regular expression, matches r
Set to what matched the nth tagged expression
(n = 1-9)
Repetition
One or more occurrences of r
Zero or one occurrences of r
Either r1 or r2
Either r1r3 or r2r3
Zero or more occurrences of r1|r2, e.g., r1, r1r1,
r2r1, r1r1r2r1,…)
Repetition
input line
regular expression
fgrep, grep, egrep
grep, egrep
grep
egrep
Quick
Reference
Examples
 Interesting examples of grep commands
 To search lines that have no digit character:
 grep -v '^[0-9]*$' filename
 Look for users with uid=0 (root permission)
 grep '^[^:]*:[^:]*:0:' /etc/passwd
 To search users without passwords:
 grep ‘^[^:]*::’ /etc/passwd
 To search for binary numbers
 To search for telephone numbers
 To match time of day, e.g., 12:14 am, 9:02pm, …
62
Outline
 Shell: interactive mode and batch mode
 Unix commands: syntax and some basic useful commands
 File systems and commands, emacs or vi
 Users and groups
 Environment variables and configuration files
 Standard input/output, redirection, pipeline
 grep command basics
 Shell scripting: control structures (branch and loop)
63
Control Structures & Conditions
 Control structures in bash
 if … then … fi
 if … then … else … fi
 if … then …elif … else … fi
 for … in … do … done
 while … do … done
 until … do … done
 case … in … esac
 break, continue
 Conditions (tests): used in if structures, while, until
structures, similar to boolean expression in C/C++
64
 Dots shown in red are to be replaced with conditions
Conditions in shell
 Exit status of a command, script or shell function, e.g.,
if diff file1 file2 >& /dev/null ## if file1 and file2 are the same
…
 test command: used to perform a variety of test, e.g., test file
attributes, compares strings and numbers.
if test –e tmp.o ## if there is file named test.o
…
 Compound condition: combine above using ! (negation), && (and), ||
(or)
if !grep pattern myfile > /dev/null
…
65
Exit status command/script/function
 Exit Status: every command (built-in, external, or shell function)
returns a small integer value when it exits, to the program invoked it.
 Convention: command/program returns a zero when it succeeds and
some other status when it fails
 How to return value from shell script?
 exit command, syntax
exit [exit-value]
 Return an exit status from a shell script to its caller
 If exit-value is not given, exit status of last command executed will
be returned.
 If this is what you want, do so explicitly using
exit $?
66
 ? A special variable stores exit status of previous command.
67
test command
 Used to perform a variety of test in shell scripts, e.g., test file
attributes, compares strings and numbers.
 Provide no regular output, used exclusively for its exit status
 Syntax:
test expression
[ expression ]
Note: space between [, ] and expression …
68
69
70
Numerical tests work on integers only.
71
Test status of file: file conditionals
 File conditionals: unary expressions examining status of a file
if test –e /etc/.bashrc ## same as if [ -e /etc/.bashrc ]
## do something if /etc/.bashrc exists
then
## do something else if it doesn’t
fi
 More testing
 -d file: true if the file is a directory
 -e file: true if the file exists
 -f file: true if the file is a regular file.
 -s file: true if the file has nonzero size
72
numerical comparison: example
echo -n "Enter your age: "
read age
if [ $age -lt 18 ]
then
echo "You need to be 18 or older to apply for account"
else
echo "Choose your preferred account name"
fi
73
Numeric Comparison: check
arguments
 Often in shell script, need to check # of command line
arguments:
#! /bin/bash
## this script …
## check arguments …
if [ $# -lt 2 ] ## less than 2 command lines arguments are specified …
then
echo “usage: $0 file1 file2”
exit 1 ## 0 means succeed, otherwise failed
fi
74
…
string comparison
 Exercise #1:
 Write a script that read from standard input a string, and check
if it’s the same as your secret password “secret”; if yes, print out
“welcome!”; print out “Go away” if not.
75
Logical NOT, AND, and OR
if !grep pattern myfile > /dev/null ## Negation
then
#
pattern not here
fi
if grep pattern1 file && grep pattern2 file
then
##contain both pattern
Fi
76
if grep pattern1 file || grep pattern2 file
then
## contain at least one pattern
fi
Roadmap
 Shell variables
 Shell Arithmetic
 Control Structures
 Test, condition, logic operations
 Selection: if statements
 Loop: while, until, for loops; break,
continue
 Case structure
77
Selection in shell
 Implementing branch: change flow of control based on
conditions
78
if condition
then
commands
fi
if condition
then
commands1
else
commands2
fi
if condition1
then
commands1
elif condition2
commands2
else
commands3
fi
case … in …
esac
Can be repeated…
if control structure
 Single-line Syntax
if TEST-COMMANDS; then CONSEQUENT-COMMANDS; fi
 Multi-line Syntax
if TEST-COMMANDS
then
CONSEQUENT-COMMANDS
fi
 Recall: command line terminates with NEWLINE, ;, &.
79
[zhang@storm ~]$ cat checkps
#!/bin/bash
echo -n "Enter your password: "
read password
if [ $password == "secret" ]
then
echo "Welcome!"
else
echo "Go away!"
fi
[zhang@storm ~]$ checkps
Enter your password: secret
Welcome!
[zhang@storm ~]$ checkps
Enter your password: guess
Go away!
[zhang@storm ~]$
Important note: When spanning multiple lines, do not
need the ;
80
if … then … elif … then …else
if [[ "$op" == "+" ]]
then
result=$(($x + $y))
echo $x $op $y = $result
elif [[ "$op" == "-" ]]
then
result=$(($x - $y))
echo $x $op $y = $result
81
elif [[ "$op" == "*" ]]
then
result=$(($x * $y))
echo $x \* $y = $result
elif [[ "$op" == "/" ]]
then
result=$(($x / $y))
echo $x $op $y = $result
else
echo "Unknow operator $op"
fi
82
if… statements can be nested
#!/bin/bash
# This script will test if we're in a leap year or not.
year=`date +%Y`
if [ $[$year % 400] -eq 0 ]; then
echo "This is a leap year. February has 29 days."
elif [ $[$year % 4] -eq 0 ]; then
if [ $[$year % 100] -ne 0 ];
then echo "This is a leap year, February has 29 days."
else echo "This is not a leap year. February has 28 days."
fi
else echo "This is not a leap year. February has 28 days."
fi
Roadmap
 Shell variables
 Shell Arithmetic
 Control Structures
 Test, condition, logic operations
 Selection: if statements
 Loop: while, until, for loop; break, continue
 case structure
83
Loop structure: while loop
 Multi-line Syntax:
while condition
do
commands
done
 Single-line Syntax (useful in interactive mode)
while condition; do commands; done
 Note: condition and commands terminated with ;
84
while loop
declare –i i=1 ## an integer variable I
while [ $i -le 10 ]
do
echo "loop $i"
i=i+1 ## can use this since i is integer
done
If i is not declared as integer …
i=$(($i+1))
i=$[$i+1]
85
[zhang@storm ~]$ cat checkps
#!/bin/bash
echo -n "Enter your password: "
read password
if [ $password == "secret" ]
then
echo "Welcome!"
else
echo "Go away!"
fi
• How to modify this to allow user to try until the
password matches?
86
[zhang@storm ~]$ cat checkps
#!/bin/bash
while test $password != "secret" ## as long as password is not same as “secret”
do
echo -n "Enter your password: "
read password
done
echo "Welcome!”
• What if we give the user at most 3 tries?
1. use a variable to keep track number of tries …
2. modify condition …
87
until loop
 Tests for a condition, and keeps looping as long as that
condition is false (opposite of while loop).
until condition
do
command(s)...
done
 e.g.:
88
$until [ $passwd == "secret" ] ; do echo -n "Try again: "; read passwd; done
Try again: guess
Try again: password
Try again: secret
$
Wait for a user to log in
# wait for a user to log in, check every 30 seconds
printf “Enter username:”
read user
until who | grep “$user” > /dev/null
do
sleep 30
done
Can you convert it to while loop?
89
For loops
 For loop: iterates over a list of objects, executing loop body
90
for each individual object in the list
for variable in a_list_of_objects
do
## do something on $variable
commands ..
done
e.g., for filename in lab1.cpp lab2.cpp lab3.cpp
do
indent $filename
done
For loop: example
 Save student account name in a file, all.txt
for student in `cat all.txt`
do
echo “Hello, $student” > greeting_$student
grep $student student_rec.txt >>greeting_$student
write $student < greeting_$student
rm greeting_$student
done
How to avoid using temporary file, greeting_$student ?
91
Using for loop
 Use for loop to print out 2’s power
 Command seq: print out a sequence of number
#!/bin/bash
# print out 2’s powers
for a in `seq 1 10`
do
echo 2^$a=$((2**a))
done
Note: ** is the exponent operator
92
case construct: branching
 case construct is analogus to switch in C/C++.
93
case "$variable" in
shellpattern1 )
command...
;;
shellpattern2)
command …
;;
shell pattern n)
command...
;;
esac
• Quoting variables is not mandatory
• Each pattern can contain shell wildcard
(*,?,[a-z]), ends with a )
• Each condition block ends with ;;
• If a condition tests true, then associated
commands execute and the case block
terminates.
• entire case block ends with an esac
94
#!/bin/bash
OPT=$1 # option
Lazy evaluation of && and ||
FILE=$2 # filename
# test -e and -E command line args matching
case $OPT in
-e|-E)
test if string is null
echo "Editing $2 file..."
# make sure filename is passed else an error displayed
[ -z $FILE ] && { echo "File name missing"; exit 1; } || vi $FILE ;;
-c|-C)
echo "Displaying $2 file...“
[ -z $FILE ] && { echo "File name missing"; exit 1; } || cat $FILE ;;
-d|-D)
echo "Today is $(date)" ;;
*)
echo "Bad argument!"
echo "Usage: $0 -ecd filename"
echo " -e file : Edit file."
echo " -c file : Display file."
echo " -d : Display current date and time." ;;
esac
Case example
95
case $1 in
-f)
## case for –f option
;;
-d | --directory)
## -f or –directory option
;;
*)
echo $1: unknown option >&2
exit 1;
esac