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Lecture 4 Log into Linux Reminder: Homework 1 due today, 4:30pm Homework 2 out, due next Tuesday Project 1 out, due next Thursday Questions? Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 1 Outline More

SLIDE 1

Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 1

Lecture 4

 Log into Linux  Reminder: Homework 1 due today, 4:30pm  Homework 2 out, due next Tuesday  Project 1 out, due next Thursday  Questions?

SLIDE 2

Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 2

Outline

 More BASH programming  Command Substitution  Arithmetic Substitution  Additional Useful Commands  Grouping  Exercises

SLIDE 3

Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 3

Command Substitution

 Command substitution allows the standard

utput of a command to replace the command
name. There are two forms: `COMMAND` or $

(COMMAND).

rm `cat filelist` # rm files in a list txtfiles=$(ls *.txt) # capture ls display userpass=$(grep -i '^ar63:' /etc/passwd) passwd=$(cat /etc/passwd) # capture file

SLIDE 4

Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 4

Arithmetic Substitution

 Arithmetic evaluation uses either let expr or

((expr)). Substitution uses $((expr)).

b=12 let a=$b3 # a is 36 ((a = $b 3)) # alternative to let val=$(($b**2)) # val is 144 count=0 while [ $count -le 10 ] do # do stuff here count=$(($count + 1)) # or ((count++)) done

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 5

HERE Documents

 HERE documents are a special form of input

redirection that read input from the script.

cat <<EOTEXT # Display multiline message Hi there! How are you? EOTEXT # Example with variable substitution sftp $user@csserver <<SFTPINPUT cd $remotedir get $file bye SFTPINPUT

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 6

Functions and Aliases

 Functions only have local scope by default, but

can be exported.

error() { echo “$*” > /dev/stderr ; }

 You can create aliases for commands too:

alias ls='ls --color=tty' # alias for ls alias ll='ls -l --color=tty' alias rm='rm -i' # force rm to prompt alias # display aliases \ls # run unaliased ls

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 7

BASH Startup

 Bash shells can be

 interactive or non-interactive  a login or non-login shell.

 A login shell reads and executes commands

from /etc/profile and then one of (in order) either ~/.bash_profile, ~/.bash_login, or ~/.profile.

 A non-login interactive shell reads commands

nly from ~/.bashrc. Set exported environment

variables in ~/.bash_profile. Define functions and aliases in ~/.bashrc.

SLIDE 8

Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 8

Useful Commands

 find will locate files and directories that satisfy

certain criteria. It recurses through directories.

 Syntax: find <dir> <matching criteria> <actions>  Lots of matching criteria, e.g. -mtime n, -type c,

user u, -name <regex>. (See man page.) All

must match unless use -o (OR) explicitly.

 Some built-in actions, e.g. -print, -ls. General

exec <cmd> {} \; (where optional '{}'

represents the current match).

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 9

Useful Commands

 find examples:

find . -name '.txt' # all .txt in current dir find . -mtime -1 # modified in last day # Use grep to search for strings # print is required here find . -exec grep -iq "dh27" {} \; -print # Change perms on all files find . -type f -exec chmod 644 {} \;

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 10

Useful Commands

 grep search for files containing regular

expressions (similar to the wildcards, see BLP pages 66-67).

 xargs takes filenames from standard input and

feeds the names to a command.

ls | xargs -p -l gzip

 expr is an all-purpose expression evaluator

a=$(expr 5 + 3) # arithmetic b=$(expr length $filename) # string length

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 11

Useful Commands

 exec replaces the current process with a new

ne. It is often used at the end of a script. You

can also use it from within a script to redirect subsequent standard output

exec vi # "become" vi exec > output.txt echo hello there # sent to output.txt

 bc is an arbitrary precision calculator.

# get pi to 40 places pi=$(echo 'scale=40; 4*a(1)' | bc -l)

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 12

Useful Commands

 export makes a shell variable available in

subshells.

export myhome=/home/hwang

 read will read a line of input from standard

input.

read line # user input is $line read # read line and throw away

 printf can give fancier output than echo.

printf “%8.4f\t\t%s\n” $val $myhome

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 13

Grouping

 Use the following code to read lines of input

from a file:

while read line do # Do something with $line done < inputfile > outputfile

 Input (and output) is redirected for all

commands inside the body of the while-loop.

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 14

Grouping

 You can also redirect input and/or output by

using braces to group commands:

{ read x; read y; } < input

 Use space between the command group and

the braces. The final semicolon is required. Alternatively use this form:

{ read x read y } < input

SLIDE 15

Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 15

Grouping

 You can use ( ) for grouping too. With ( ), the

command group is executed in a subshell (another bash process), so if you set or change any variables those changes will not be seen in the parent process.

myvar=abc { myvar=xyz; } echo $myvar # Will display xyz (myvar=mno) echo $myvar # Will display xyz

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 16

In-class Exercises

 Write a shell script named lastfirst that takes a

single filename argument and writes the file to standard output in complete reverse order (last line, last character first).

 Hint: Try “man -k reverse” to find relevant utilities.  Write appropriate error messages if an incorrect number of

arguments is used or if the input file does not exist.

 Exit with appropriate status.  Read from standard input if no arguments are given.

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Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 17

In-class Exercises

 Write a shell script named sortusers1 that

displays every other username (from /etc/passwd) in alphabetical order. Display the username only.

 Write sortusers2 that displays every other

username sorted by user id. Display the username only.

 Write a shell script named revargs that

Lecture 4

Outline

Command Substitution

(COMMAND).

rm `cat filelist` # rm files in a list txtfiles=$(ls *.txt) # capture ls display userpass=$(grep -i '^ar63:' /etc/passwd) passwd=$(cat /etc/passwd) # capture file

Arithmetic Substitution

((expr)). Substitution uses $((expr)).

b=12 let a=$b*3 # a is 36 ((a = $b * 3)) # alternative to let val=$(($b**2)) # val is 144 count=0 while [ $count -le 10 ] do # do stuff here count=$(($count + 1)) # or ((count++)) done

HERE Documents

redirection that read input from the script.

cat <<EOTEXT # Display multiline message Hi there! How are you? EOTEXT # Example with variable substitution sftp $user@csserver <<SFTPINPUT cd $remotedir get $file bye SFTPINPUT

Functions and Aliases

can be exported.

error() { echo “$*” > /dev/stderr ; }

alias ls='ls --color=tty' # alias for ls alias ll='ls -l --color=tty' alias rm='rm -i' # force rm to prompt alias # display aliases \ls # run unaliased ls

BASH Startup

from /etc/profile and then one of (in order) either ~/.bash_profile, ~/.bash_login, or ~/.profile.

variables in ~/.bash_profile. Define functions and aliases in ~/.bashrc.

Useful Commands

certain criteria. It recurses through directories.

must match unless use -o (OR) explicitly.

represents the current match).

Useful Commands

find . -name '*.txt' # all *.txt in current dir find . -mtime -1 # modified in last day # Use grep to search for strings # print is required here find . -exec grep -iq "dh27" {} \; -print # Change perms on all files find . -type f -exec chmod 644 {} \;

Useful Commands

expressions (similar to the wildcards, see BLP pages 66-67).

feeds the names to a command.

ls | xargs -p -l gzip

a=$(expr 5 + 3) # arithmetic b=$(expr length $filename) # string length

Useful Commands

can also use it from within a script to redirect subsequent standard output

exec vi # "become" vi exec > output.txt echo hello there # sent to output.txt

# get pi to 40 places pi=$(echo 'scale=40; 4*a(1)' | bc -l)

Useful Commands

subshells.

export myhome=/home/hwang

input.

read line # user input is $line read # read line and throw away

printf “%8.4f\t\t%s\n” $val $myhome

Grouping

from a file:

while read line do # Do something with $line done < inputfile > outputfile

commands inside the body of the while-loop.

Grouping

using braces to group commands:

{ read x; read y; } < input

the braces. The final semicolon is required. Alternatively use this form:

{ read x read y } < input

Grouping

command group is executed in a subshell (another bash process), so if you set or change any variables those changes will not be seen in the parent process.

myvar=abc { myvar=xyz; } echo $myvar # Will display xyz (myvar=mno) echo $myvar # Will display xyz

In-class Exercises

single filename argument and writes the file to standard output in complete reverse order (last line, last character first).

arguments is used or if the input file does not exist.

In-class Exercises

displays every other username (from /etc/passwd) in alphabetical order. Display the username only.

username sorted by user id. Display the username only.

displays its arguments in reverse order (eval may be useful here).

b=12 let a=$b3 # a is 36 ((a = $b 3)) # alternative to let val=$(($b**2)) # val is 144 count=0 while [ $count -le 10 ] do # do stuff here count=$(($count + 1)) # or ((count++)) done

find . -name '.txt' # all .txt in current dir find . -mtime -1 # modified in last day # Use grep to search for strings # print is required here find . -exec grep -iq "dh27" {} \; -print # Change perms on all files find . -type f -exec chmod 644 {} \;