Shell / command interpreter

Shell/command interpreter

● interactive user interface with an operating system● command line interface (CLI)● understands and executes the commands a user

enters● outer layer of an operating system (in contrast to

the kernel)● invokes another program, shows the system

settings, allows file system modification etc.● many shells available on a typical

Linux/Unix/MacOs systems: sh, bash, zsh, ksh

BASH

● Bourne Again SHell - enhanced version of the original Bourne shell program, sh (written by Steve Bourne)

● on local system: -started after a successful login (if specified in/etc/passwd) -terminal emulators (xterm, gnome-terminal, konsole, ... - depends on window manager)

● on remote system : -ssh user@remotehost (Unix, Linux, MacOS,...) -ssh client program e.g. PuTTY (Windows)

mailto:user@remotehost

BASH● interactive shell

vanilla:etc root$ ls -al vanilla:etc root$ mkdir backup vanilla:etc root$ mv *.ini backup

● non-interactive shell vanilla:etc root$ . script.sh

script.sh

if [ -d backup ]; then mv *.ini backup else if [ -e backup ]; then echo “Sorry, I can't create \ directory \"backup\" in directory `pwd`” else mkdir backup mv *.ini backup fifi

Some linux commands● grep - search the file or standard input for lines containing a match to the given

pattern

● find – find files

● cat - read files sequentially, writing them to the standard output

● wc - print the number of newlines, words, and bytes in files

● ls - list directory contents

● mkdir - create the directories

● rm - remove files and directories (-r)

● cp - copy the contents of the source file to the target file

● mv - rename/move the file

● man - show command manual (man bash, man ls,...)

● whoami - display effective user id (also: id -un or echo "$USER")

● who - display a list of all users currently logged on

Redirection● there are always three default files open:

stdin - file descriptor 0 (generally keyboard) stdout - file descriptor 1 (generally screen) stderr - file descriptor 2 (generally screen)

● redirect the standard output to a file ls -alR / > diretorytree.txt (recursively list all directories in long format starting from /)

● redirect stderr to file cat * 2>errors.txt (writes all errors to the specified file)

● redirect and append stdout to file cat file2.txt>>all.txt

● redirect and append stderr to file cat privatestuff/* 2>>errors.txt

Redirection● redirect both stdout and stderr to file

cp -vrn privatestuff backup &>copy.txt

● redirects stderr to stdoutcp -vrn privatestuff backup > copy.txt 2>&1

● redirect stdin from file wc -l <file.txt writes number of lines in file.txt, the same as: $ wc -l twinkle, twinkle

little star

● ^D 2

● redirect stdin and stdout wc < ala.txt >lines.txt

Pipeline● a set of chained processes, so that the output of each process (stdout)

feeds directly as input (stdin) to the next one

command1 > filecommand2 < file > file2 command1|command2|command3command3 < file2

cat *.txt >file1.tmp

sort <file1.tmp >file2.tmp cat *.txt|sort|uniq

uniq <file2.tmp

Variables● no data types

● can contain a number, a character, a string of characters

● no need to declare a variable, assigning a value to its reference will create it

● value of the variable is retrieved by putting the '$' before name● dir=”/usr/local/bin”

file=”php”echo running $dir/$file...$dir/$file

● capturing a commands output to a variable var=`command` or var=$(command)

● name=`whoami`

● dir=`pwd`

● echo $name is in $dir directory

●

Environment vs local variables● global variables or environment variables are available in all shells

– use command:$ env

SHELL=/bin/bash

USER=ania

ORACLE_HOME=/Users/oracle/oracle/product/10.2.0/db_1

PATH=/usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/X11/bin: /Users/oracle/oracle/product/10.2.0/db_1/bin:/Users/oracle/ oracle/product/10.2.0/db_1/bin

PWD=/Users/ania

LANG=pl_PL.UTF-8

HOME=/Users/ania …

● local variables are only available in the current shell - use the set command to display a list of all variables (including environment ones)

Variables● making variable a global one

$ export VAR=”value” $ ORACLE_HOME=/home/oracle/db_10; export ORACLE_HOME

● removing variable $ unset VAR

● reading variable from stdin $ read VAR

● reserved BASH variables

HOME - current user's home directory IFS - list of characters that separate fields PATH - colon-separated list of directories in which the shell looks for commandsPS1 the primary prompt stringUID - numeric real user ID of the current userRANDOM generates random integer between 0 and 32767 PWD the current working directory OSTYPE - string describing the operating system HOSTNAME - name of the current hostBASH - full pathname used to execute the current instance of Bash and many more ...

If … then ... else ... fi

● if condition; then

● command1 command2 … else command3 command4 …

● fi

● conditions test -f file.txt [ -f file.txt ] ((5==5&&6==6)) true ((5<9)) … … …

test

● man test

● [ -f file.txt ] true if file exists and is a regular file

● [ -d file.txt ] true if file exists and is a directory

● [ -e file.txt ] true if file exists (regardless of type)

● [ -z string ] true if the length of string is zero

● [ s1 = s2 ] true if the strings s1 and s2 are identical

● [ s1 != s2 ] true if the strings s1 and s2 are not identical.

● [ s1 < s2 ] true if string s1 comes before s2 based on the ASCII value of their characters

●

test

● [ n1 -eq n2 ] true if the integers n1 and n2 are algebraically equal

● [ n1 -ne n2 ] true if the integers n1 and n2 are not algebraically equal

● [ n1 -ge n2 ] true if the integer n1 is algebraically greater than or equal to the integer n2

● [ n1 -lt n2 ] true if the integer n1 is algebraically less than the integer n2

● [ ! expression ] true if expression is false

● [ expression1 -a expression2 ] true if both expression1 and expression2 are true.

● [ expression1 -o expression2 ] true if either expression1 or expression2 are true.

no_backup=0;

if test -e backup

then

echo removing directory backup

rm -r backup 2>/dev/null

no_backup=$?

fi

if test $no_backup -eq 0 ; then

echo creating directory backup

mkdir backup

echo copying files

cp -v *.ini *.conf backup >summary.txt

cat summary.txt

echo Files copied:

cat summary.txt| wc -l

else

echo Directory \"backup\" can\'t be removed

fi

Special parameters $? expands to the exit status of the most recently executed foreground command

$@ expands to the positional parameters. when the expansion occurs within double quotes, each parameter expands to a separate word (similar to $*, but $* is no longer recommended)

$# expands to the number of positional parameters in decimal.

$$ Expands to the process ID of the shell.

$0 Expands to the name of the shell or shell script.

● Parameters $1, $2, $3, …$#$script.sh 5 12 3Script name : script.sh 5123

● command shift

script.sh

echo Script name: $0if [ $# -eq 3 ]; thenecho $1echo $2echo $3else echo To few parametersfi

While loop

● while condition; do command1 command2 …done

● show script agruments

while [ $# -gt 0 ];

do

echo $1

shift

done

Arithmetic evaluation● expression evaluation

((expression))((i=i+3)) or i=$((i+3))

● i=$[i + 3]

● i=`expr $i + 3`

● zm=0

while [ $1 ]; do

zm=$[zm+$1]

shift

done

echo Sum: $zm

sum=0

first=true

while read line; do

if ($first); then

first=false

max=$line

min=$line

fi

if [ $line -gt $max ]; then

max=$line

fi

if [ $line -lt $min ]; then

min=$line

fi

((sum=sum+line))

done

echo Min: $min Max: $max Sum: $sum

Script, which finds sum,minimum and maximum from the numbers

on the standard input

For loop (1)● for variable_name [in list] ; do

command1command2…done

● for (( expr1 ; expr2 ; expr3 )) ; do command1command2… done

i=1;

for param ; do

echo Parameter $i: $param

i=$((i+1))

done

for user in Mike Ann Maxdologged=`who|grep $user`if [ "$logged" ];then echo $user logged inelse echo $user is not logged infidone

For loop (2)● for filedir in * ; do

if [ -d "$filedir" ]; then

echo Directory: $filedir

elif [ -f "$filedir" ];then

echo File $filedir

fi

done

● for ((a=1;a<=$#;a++)); do

echo $a : ${!a}

done

a: 1 2 3 … $#${!a}: $1 $2 $3 ….${$#}

For loop (3)● for filedir in `ls` ; do

if [ -d "$filedir" ]; then echo Directory: $filedir elif [ -f "$filedir" ];then echo File: $filedir else echo Problem: $filedir fidone

● IFS=$'\n'for filedir in `ls -1` ; do if [ -d "$filedir" ]; then echo Directory: $filedir elif [ -f "$filedir" ];then echo File: $filedir else echo Problem: $filedirfi done

Wrong solution for a filenames with spaces:File file1.txtFile file2.txtProblem: fileProblem: withProblem: spaces.txt

Good solution for a filenames with spaces:File file1.txtFile file2.txtFile: file with spaces.txt

Quotes, double quotes● used where arguments contains spaces● rm “big file” = rm 'big file'

● mkdir "max ' has ' a dog" creates max ' has ' a dog

mkdir 'max “ has “ a dog' creates max “ has “ a dog ● var=1000

echo $var 1000 echo '$var' $varecho “$var” 1000

● echo * file1.txt file2.txt file3.txt …..

echo '*' *

echo “*” *

● escapingecho “\$var” $var

Functions● syntax:

function_name (){ command1 command2 ...}

● the exit status of a function is the exit status of the last command executed in the body

● executed in the context of the current shell - no new process is created to interpret them

● the arguments to the function become the positional parameters during its execution

● variables are shared between the function and its caller - variables local to the function may be declared with the local builtin command

● if the return command is executed in a function, the function completes and execution resumes with the next command after the function call

function function_name { command1 command2 ...}

max_in ()

{

(

max=0;

maxline="";

while read line; do

newmax=`echo $line|wc -m `

if [ $newmax -gt $max ] ; then

max=$newmax

maxline=$line

fi

done

echo $max ":" $maxline

)<$1

}

for file

do

if [ -f $file ]

then

max_in $file

fi

done

Functions

Script finds the longest line in every file provided as a parameter

Factorial● factorial() {

if [ $1 -eq 0 ]; then

echo 1

else #or:

echo $[$1*$(factorial $[$1-1])] #echo $[$1*`factorial $[$1-1]`]; fi

}

● factorial2(){

if [ $1 -eq 0 ]; then

result=1

else

factorial2 $[$1-1]

result=$[result*$1]

fi

}

factorial 6

factorial2 6echo $result

Shell / command interpreter

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