Lecture 3

chmod mode file

• Changes file permissions

• Mode consists of 3 parts:

  • user type

    • u = user

    • g = group

    • o = other

  • operator

    • '+' = add this permission

    • '-' = remove this permission

    • '=' = set permission exactly

  • Permission

    • 'r' = read bit

    • 'w'= write bit

    • 'x' = execute bit

chmod otr file

• Gives others permisison to read

chmod oug=rw file

• Make everyone's permission read and write (not execute)

chmod u=r file.txt

Shell Scripts

• File that contains a series of commands that we will execute as a program

• eg. print date, current user, current directory

# Make script
$ vi firstScript.sh

#!/bin/bash
date
whoami
pwd

# Run script (. means current directory, / is before name of directory)
$ ./firstScript.sh
>> May get permission error (do ls -l to check permissions)

# if permission error:
$ chmod oug=x firstScript.sh

Variables

• name=value (notice there are no spaces)

  • assigns value to variable

  • eg. x=1 and x =1 are different in bash

• ${varname}

  • retrieves data stores in variable

  • eg. $x

  • do not use $ when assigning variables

  • Good practice: ${x}

  • eg. echo "The cost is ${x}USD"

• Command line args are stored $1, $2, $3

In bash, all variables are strings.

• Double quotes

  • allow variable expansion

• Single quotes

  • supresses variable expansion

$ vi firstScript.sh

# Add variable to script
#!/bin/bash
Command line arg: ${1}

$ ./firstScript.sh hello
>> Command line arg: hello

$ ./firstScript.sh "hello world"
>> Command line arg: hello world

if Statement Format

if [ $? -eq 0 ]; then
  ...
elif [$? -eq 1 ]; then
  ...
else
  ...
fi

Example: Check if argument is in dictionary

#!/bin/bash
egrep "^1${1}$" /usr/share/dict/words

Print ${1} if it's a word in the dictionary. Otherwise, does nothing.

Example: A 'good' password should not be in the dictionary. Answer whether a word is a good password.

First way to do it:

#!/bin/bash
egrep "^${1}$" /usr/share/dict/words > /dev/null
# Note: redirecting stdout to /dev/null suppresses output

# semi colon indicates you want more than 1 commands on 1 line
# then is a command (can be on the second line, then there is no need for ;)
if [ $? -eq 0 ]; then # need a space before and after square bracket
  echo "Bad password"
else
  echo "Maybe a good password"
fi

Second way to do it:

#!/bin/bash
usage() {
  echo "Usage: $0 password"
} # $0 stores the name of the program

# -ne is not equal
if [ $# -ne 1 ]; then
  usage # function call (looks like cmd)
  exit 1
fi

Every program returns a status code

• egrep returns 0 if a match is found, 1 if not found

• In Unix: 0 is success, non-zero is failure

• ? stores status of most recently executed command

$ echo $? 
# if most recently executed command was a failure
>> 1

$ echo $?
# if most recently executed command was a success
>> 0

Syntax for doing math

$((...))

Bash Loops

Example: Print numbers from 1 to ${1}

#!/bin/bash
x=1

# -le is less than or equal to
while [ $x -le ${1} ]; do
  echo ${x}
  x=$((x+1)) 
done

Bash For loops

#!/bin/bash
# abc is an arbitrary list
for x in abc; do
  echo ${x}
done

for file in *; do
  ...
done

# Rename all .cpp files to .cc
#!/bin/bash
for file in *.cpp; do
  # mv renames files
  mv ${file} ${file % cpp} cc
done