Script Course

File Operators Tests

Escape Sequence

Test Statements

Commands used with pipe

• -d File True if file is a directory
• -e File True if file exists
• -f File True if file exists and is a regular file
• -r File True if file is readable by you
• -s File True if file exist and is not empty
• -w File True if the file is writable by you
• -x File True if the file is executable by you
• -z String True if string is empty
• -n String true if string is not empty

Syntax

[condition to test for]

Example

[ -e /etc/passwd ]
-e True if file exists

Exit Status return codes

• Every command returns an exit status
• range from 0 - 255
• 0 = success
• Other than 0 = error condition
• use for error checking

$? Contains the return code of the previously executed command

ls /not/here
echo "$?"

-

HOST = "google.com"
ping -c 1 $HOST
if [ "$?" -eq "0" ]
then 
echo "$HOST reachable."
else
echo "$HOST unreachable"
fi

' And and || or

• && = And : if first fails the other won't be executed

mkdir /tmp/bak && cp test.txt /tmp/bak/

• || OR - if one fails the other will be processed

cp test.txt /tmp/bak || cp test.txt /tmp

Redirection

redirect to non error using status code 1 to filename goodoutput

ls /etc 1>goodoutput
ls /etc > 2>badoutput

Redirect both at the same time

ls /etc >goodoutput 2>badoutput

tee command, which takes information from Standard Input and sends that information to a file, as well as to Standard Output.

cat prologue|tr a A|sort|pr –d|tee newfile|less

Chaining multiple commands

cp test.txt /temp; cp test.text /tmp

Bash Functions

#!/bin/bash
function hello(){
for NAME in $@
do
 echo "Hello $NAME"
done
}

Call Script

hello Jason Dan Ryan

Those three names will be processed individually

SED Command

The sed command is typically used to search for a certain string of text, and replaces that text string with another text string using the syntax s/search/replace/. For example, the following output demonstrates how sed can be used to search for the string “the” and replace it with the string “THE”

cat filename.text | sed /s/the/THE

Notice from the preceding output that sed only searched for and replaced the first occur- rence of the string “the” in each line. To have sed globally replace all occurrences of the string “the” in each line, simply append a g to the search-and-replace expression:

cat prologue | sed s/the/THE/g

You can also force sed to perform a search-and-replace on certain lines only. To replace the string “the” with “THE” globally on lines 5 to 8 only, you can use the following command:

cat prologue | sed 5,8s/the/THE/g

You can also use sed to remove unwanted lines of text. To delete all the lines that contain the word “the,” you can use the following command:

 cat prologue | sed /the/d

AWK Command

Like sed, the awk command searches for patterns of text and performs some action on the text found. However, the awk command treats each line of text as a record in a database, and each word in a line as a database field. For example, the line “Hello, how are you?” has four fields: “Hello,” “how,” “are,” and “you?”. These fields can be referenced in the awk command using $1, $2, $3, and $4. For example, to display only the first and fourth words only on lines of the prologue file that contains the word “the,” you can use the fol- lowing command:

 cat prologue | awk ’/the/ {print $1, $4}’

Bash Environment Variables

By default, the awk command uses space or tab characters as delimiters for each field in a line. Most configuration files on Linux systems, however, are delimited using colon ( : ) char- acters. To change the delimiter that awk uses, you can specify the –F option to the com- mand. For example, the following example lists the last 10 lines of the colon-delimited file / etc/passwd and views only the 6th and 7th fields for lines that contain the word “bob” in the last 10 lines of the file

 tail /etc/passwd | awk –F : ’/bob/ {print $6, $7}’
# prints lines 1 and 7 with search term 'system' \t adds the tab between fields
cat /etc/passwd | awk -F : '/system/ {print $1, "\t"$7'

ls -l | grep file\w* | awk -F: ' {print "

" $2"

"}'

List Environment Variables

Many environment variables are set by default in the BASH shell. To see a list of these vari- ables and their current values, you can use the set command,

set | less

Setting User Variables

• They can contain alphanumeric characters (0–9, A–Z, a–z), the dash (-) character, or the underscore (_) character.
• They must not start with a number.
• They are typically capitalized to follow convention (e.g., HOME, PATH

MYVAR = "hello"
echo $MYVAR

Use the export command to make a variable available to all sub-shells

export MYVAR
#env command is used to view user variables
env |grep MYVAR

Environment Files

Recall that variables are stored in memory. When a user exits the BASH shell, all variables stored in memory are destroyed along with the shell itself. To ensure that variables are acces- sible to a shell at all times, you must place variables in a file that is executed each time a user logs in and starts a BASH shell. These files are called environment files. Common BASH shell environment files and the order in which they are typically executed are as follows:

• /etc/profile
• /etc/bashrc
• ~/.bashrc
• ~/.bash_profile
• ~/.bash_login
• ~/.profile

If Statements

echo –e "\nThe people logged into the system include:" 
who 
echo –e "\nWould you like to see the contents of /?(y/n) --> \c" 
read ANSWER 
if [ $ANSWER = "y" –o $ANSWER = "Y" ] 
then 
echo –e "\nThe contents of the / directory are:" 
ls –F / 
fi

Select Case

 #!/bin/bash
echo –e "What would you like to see? 
Todays date (d) 
Currently logged in users (u) 
The contents of the / directory (r) 
Enter your choice(d/u/r)--> \c" 
read ANSWER 
case $ANSWER in d | D ) echo -e "\nToday’s date is: \c" 
date ;; 
u | U ) echo –e "\nThe people logged into the system include:" 
who
;;

r | R ) echo –e "\nThe contents of the / directory are:" ls –F /
;; *) echo –e "Invalid choice! \a"
;;
esac