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Week 5 Notes

Command Line Editors

  • Working with text files in the terminal
  • Editors
    • Line Editors (Present in almost every flavour of UNIX / GNU Linux)
      • ed
      • ex (improved version of ed)
    • Terminal Editors
      • pico (Came along with the pine email application)
        • nano (Features added to pico)
      • vi (most popular and complex)
      • emacs
    • GUI Editors
      • KDE
        • kate
        • kwrite
      • GNOME
        • gedit
      • sublime
      • atom (popular among github users)
      • brackets (Popular for those writing html code)
    • IDE
      • eclipse
      • Bluefish
      • NetBeans
  • Features of text editors
    • Scrolling , view modes, current position in file
    • Navigation (char,word,line,pattern)
    • Insert, Replace, Delete
    • Cut-Copy-Paste
    • Search-Replace
    • Language-aware syntax highlighting
    • Key-maps, init scripts, macros
    • Plugins
    • Both vi and emacs editors satisfy all the above requirements
ed commands
Action Command
Show the Prompt P
Command Format [addr[,addr]]cmd[params]
Commands for location 2 . $ % + - , ; /RE/
Commands for editing f p a c d i j s m u
Execute a Shell command !command
edit a file e filename
read file contents into buffer r filename
read command output into buffer r !command
write buffer to filename w filename
quit q
Using ed
  • man ed doesn't give much info . Use info ed
  • ed test.txt shows a number indicating number of bytes read into memory
  • 1 displays the first line
  • $ displays the last line
  • ,p and %p shows the contents of the entire buffer
  • 2,3p range - 2nd to 3rd line
  • /hello/ matches and shows first occurance of the pattern
  • + and - to scroll by line
  • ;p from current position to end of buffer
  • . displays the current line
  • !date running the date command within ed
  • r !date read output of date command to buffer at current position
  • w writes the file (saves it)
  • d delete current line
  • a to append after current line. Press . and enter when done
  • s/appended/Appended/ Substitute - Search and replace from current line.
  • f shows the name of the file being edited
  • p shows the contents of the current line
  • j for joining lines . Usage 5,6j to join line 5 and 6
  • m to move a line to a particular position. Usage m1 to move current line to just below line 1. m0 to move it right to the top
  • u to undo previous change
  • To add something to every line %s/\(.*\)/PREFIX \1/
    • \1 is the back substitution
    • \(.*\) indicates any character that can be matched
    • PREFIX is the replacement string
  • 3,5s/PREFIX/prefix/ substitutes prefix for PREFIX from line 3 to 5
Commands for editing in ed / ex
Command Action
f show name of file being edited
p print the current line
a append at the current line
c change the line
d delete the current line
i insert line at the current position
j join lines
s search for regex pattern
m move current line to position
u undo latest change
Using nano
Using vi
Using emacs

Scripts

  • Software Tools Principles (Ref: Classic Shell Scripting – Arnold Robbins & Nelson H.F. Beebe)
    • Do one thing well
    • Process lines of text, not binary
    • Use regular expressions
    • Default to standard I/O
    • Don’t be chatty
    • Generate same output format accepted as input
    • Let someone else do the hard part
    • Detour to build specialized tools
#! interpreter
# comments
commands
loops
variables
case statements
functions

  • program - shell , awk , sed , python , ruby , perl

  • script

    • sourced
      • . scriptname source scriptname
      • PID same as the current shell commands are executed one after other shell environment continues
      • Used to prepare environment
    • executed
      • ./scriptname
      • Needs execution permission
      • New process gets created to run script
      • PID is not same as the shell commands are executed one after other
      • New environment lost after return
      • Used to create a new functionality

  • Script location

    • Use absolute path or relative path while executing the script
    • Keep the script in folder listed in $PATH
    • Watch out for the sequence of directories in $PATH

  • bash environment

    • Login shell 
      • /etc/profile
~/.bash_profile
~/.bash_login
~/.profile
    • Non-login shell 
      • /etc/bash.bashrc
~/.bashrc

  • Output from shell scripts

    • echo
      • simple
      • terminates with a newline if -n option not given
      • echo My home is $HOME
    • printf
      • supports format specifiers like in C
      • printf “My home is %s\n” $HOME

  • Input to shell scripts

    • read var
      • string read from command line is stored in $var

  • Shell Script arguments

    • $0 name of the shell program
    • $# number of arguments passed
    • $1 or ${1} first argument
    • ${11} eleventh argument
    • $* or $@ all arguments at once
    • “$*” all argument as a single string
    • “$@” all argument as a separate strings
    • example : ./myscript.sh -l arg2 -v arg4

  • Command substitution

    • var=`command`
    • var=$(command)
      • command is executed and the output is substituted.
      • Here, the variable var will be assigned with that output.

  • for do loop

    • for var in list
do
  commands
done
    • commands are executed once for each item in the list
    • space is the field delimiters
    • set IFS if required. If the field separator is different from space.

  • case statement

    • case var in
pattern1)
  commands
  ;;
pattern2)
  commands
  ;;
esac
    • commands are executed each pattern matched for var in the options

  • if loop

    • if condition
then
  commands
fi

    • if condition; then
  commands
fi
    • commands are executed only if condition returns true

  • Conditions

    • test expression
      • test -e file
    • [ exprn ]
      • [ -e file ]
    • [[ exprn ]]
      • [[ $ver == 5.*]]
    • (( exprn ))
      • (( $v ** 2 > 10 ))
    • command
      • wc -l file
    • pipeline
      • who|grep “joy” > /dev/null
    • For negation ! condition

  • String Comparison , Numeric, file comparison

    • expressions
      • unary
      • binary

  • test numeric comparisons

    • Table
      Comparison Description
      $n1 -eq $n2 Check if n1 is equal to n2
      $n1 -ge $n2 Check if n1 is greater than or equal to n2
      $n1 -gt $n2 Check if n1 is greater than n2
      $n1 -le $n2 Check if n1 is less than or equal to n2
      $n1 -lt $n2 Check if n1 is less than n2
      $n1 -ne $n2 Check if n1 is not equal to n2

  • test string comparisons

    • Table
      Comparison Description
      $str1 = $str2 Check if str1 is same as str2
      $str1 != $str2 Check if str1 is not same as str2
      $str1 < $str2 Check if str1 is less than str2
      $str1 > $str2 Check if str1 is greater than str2
      -n $str2 Check if str1 has length greater than zero
      -z $str2 Check if str1 has length of zero

  • Unary file comparisons

    • Table
      Comparison Description
      -e file Check if file exists
      -d file Check if file exists and is a directory
      -f file Check if file exists and is a file
      -r file Check if file exists and is readable
      -s file Check if file exists and is not empty
      -w file Check if file exists and is writable
      -x file Check if file exists and is executable
      -O file Check if file exists and is owned by current user
      -G file Check if file exists and default group is same as that of current user

  • Binary file comparisons

    • Table
      Comparison Description
      file1 -nt file2 Check if file1 is newer than file2
      file1 -ot file2 Check if file1 is older than file2

  • while do loop

    • while condition
do
  commands
done
    • commands are executed only if condition returns true

  • until do loop

    • until condition
do
  commands
done
    • commands are executed only if condition returns false

  • functions

    • definition
    • myfunc()
{
  commands
}
    • call
    • myfunc
    • commands are executed eachtime myfunc is called
    • Definitions must be before the calls

  • Demo

    • #! /bin/bash
# s1.sh is my first script
echo I am invoked as 
echo $0
echo hello world
echo the PID of the process running this script is :
echo $$
ps --forest
export myvar=MYVAR
echo $myvar
    • Source it using . s1.sh or source s1.sh. This displays the same PID as the Bash terminal
    • Executing this using ./s1.sh displays an error as there is no executable permission.
    • Provide executable permission using chmod 755 s1.sh and then run using ./s1.sh
    • Ths time the PID is different.
    • ps --forest shows all the processes that are running and the spawned processes.
    • A variable set during execution in a subshell will not be available in the parent shell. If the script is sourced the variable will be available.
    • $0 displays which ever way the script has been invoked (absolute or relative path or just the name of the script)
    • ./s1.sh -l arg2 will show -l as $1 and arg2 as $2
    • #! /bin/bash
# s1.sh modified script
echo Number of arguments
echo $#
echo First argument
echo $1
echo Second argument
echo $2
if test $1 = $2;
then
  echo The arguments are the same
fi
    • Executing the above script using ./s1.sh hello hello will say that the arguments are the same.
    • #! /bin/bash
echo use of for loop
for i in arg1 arg2 arg3
do
  echo $i
done
    • The above script just prints arg1,arg2 and arg3 on 3 lines
    • #! /bin/bash
echo use of for loop
for i in file_{1..9}
do
  echo $i
done
    • prints file_1,file_2 ... file_9
    • #! /bin/bash
echo use of for loop
for i in file_{A..D}{1..9}
do
  echo $i
done
    • prints 36 lines
    • #! /bin/bash
echo use of for loop
for i in $(ls /bin/z*)
do
  echo $i
done
    • shows each file in bin directory starting with z
    • file znew | grep "shell script" identifies whether the file passed (in bin) is a shell script
    • #! /bin/bash
echo Shell Scripts in bin directory
for i in $(ls /bin)
do
  #echo /bin/$i
  file /bin/$i | grep "shell script"
done
    • Prints the files which are shell scripts in the bin directory