Comments
Comments in shell programming start with # and go until the end of the line. We really recommend you to use comments. If you have comments and you don't use a certain script for some time you will still know immediately what it is doing and how it works.
Variables
As in other programming languages you can't live without variables. In shell programming all variables have the datatype string and you do not need to declare them. To assign a value to a variable you write:
varname=value
To get the value back you just put a dollar sign in front of the variable:
#!/bin/sh
# assign a value:
a="hello world"
# now print the content of "a":
echo "A is:"
echo $a
Type this lines into your text editor and save it e.g. as first. Then make the script executable by typing chmod +x first in the shell and then start it by typing ./first
The script will just print:
A is:
hello world
Sometimes it is possible to confuse variable names with the rest of the text:
num=2
echo "this is the $numnd"
This will not print "this is the 2nd" but "this is the " because the shell searches for a variable called numnd which has no value. To tell the shell that we mean the variable num we have to use curly braces:
num=2
echo "this is the ${num}nd"
This prints what you want: this is the 2nd
There are a number of variables that are always automatically set. We will discuss them further down when we use them the first time.
If you need to handle mathematical expressions then you need to use programs such as expr (see table below).
Besides the normal shell variables that are only valid within the shell program there are also environment variables. A variable preceeded by the keyword export is an environment variable. We will not talk about them here any further since they are normally only used in login scripts.
Shell commands and control structures
There are three categories of commands which can be used in shell scripts:
1)Unix commands:
Although a shell script can make use of any unix commands here are a number of commands which are more often used than others. These commands can generally be described as commands for file and text manipulation.
Command syntax Purpose
echo "some text" write some text on your screen
ls list files
wc -l file
wc -w file
wc -c file count lines in file or
count words in file or
count number of characters
cp sourcefile destfile copy sourcefile to destfile
mv oldname newname rename or move file
rm file delete a file
grep 'pattern' file search for strings in a file
Example: grep 'searchstring' file.txt
cut -b colnum file get data out of fixed width columns of text
Example: get character positions 5 to 9
cut -b5-9 file.txt
Do not confuse this command with "cat" which is something totally different
cat file.txt write file.txt to stdout (your screen)
file somefile describe what type of file somefile is
read var prompt the user for input and write it into a variable (var)
sort file.txt sort lines in file.txt
uniq remove duplicate lines, used in combination with sort since uniq removes only duplicated consecutive lines
Example: sort file.txt | uniq
expr do math in the shell
Example: add 2 and 3
expr 2 "+" 3
find search for files
Example: search by name:
find . -name filename -print
This command has many different possibilities and options. It is unfortunately too much to explain it all in this article.
tee write data to stdout (your screen) and to a file
Normally used like this:
somecommand | tee outfile
It writes the output of somecommand to the screen and to the file outfile
basename file return just the file name of a given name and strip the directory path
Example: basename /bin/tux
returns just tux
dirname file return just the directory name of a given name and strip the actual file name
Example: dirname /bin/tux
returns just /bin
head file print some lines from the beginning of a file
tail file print some lines from the end of a file
sed sed is basically a find and replace program. It reads text from standard input (e.g from a pipe) and writes the result to stdout (normally the screen). The search pattern is a regular expression (see references). This search pattern should not be confused with shell wildcard syntax. To replace the string linuxfocus with linuxFocus in a text file use:
cat text.file | sed 's/linuxfocus/linuxFocus/' > newtext.file
This replaces the first occurance of the string linuxfocus in each line with linuxFocus. If there are lines where linuxfocus appears several times and you want to replace all use:
cat text.file | sed 's/linuxfocus/linuxFocus/g' > newtext.file
awk Most of the time awk is used to extract fields from a text line. The default field separator is space. To specify a different one use the option -F.
cat file.txt | awk -F, '{print $1 "," $3 }'
Here we use the comma (,) as field separator and print the first and third ($1 $3) columns. If file.txt has lines like:
Adam Bor, 34, India
Kerry Miller, 22, USA
then this will produce:
Adam Bor, India
Kerry Miller, USA
There is much more you can do with awk but this is a very common use.
2) Concepts: Pipes, redirection and backtick
They are not really commands but they are very important concepts.
pipes (|) send the output (stdout) of one program to the input (stdin) of another program.
grep "hello" file.txt | wc -l
finds the lines with the string hello in file.txt and then counts the lines.
The output of the grep command is used as input for the wc command. You can concatinate as many commands as you like in that way (within reasonable limits).
redirection: writes the output of a command to a file or appends data to a file
> writes output to a file and overwrites the old file in case it exists
>> appends data to a file (or creates a new one if it doesn't exist already but it never overwrites anything).
Backtick
The output of a command can be used as command line arguments (not stdin as above, command line arguments are any strings that you specify behind the command such as file names and options) for another command. You can as well use it to assign the output of a command to a variable.
The command
find . -mtime -1 -type f -print
finds all files that have been modified within the last 24 hours (-mtime -2 would be 48 hours). If you want to pack all these files into a tar archive (file.tar) the syntax for tar would be:
tar xvf file.tar infile1 infile2 ...
Instead of typing it all in you can combine the two commands (find and tar) using backticks. Tar will then pack all the files that find has printed:
#!/bin/sh
# The ticks are backticks (`) not normal quotes ('):
tar -zcvf lastmod.tar.gz `find . -mtime -1 -type f -print`
3) Control structures
The "if" statement tests if the condition is true (exit status is 0, success). If it is the "then" part gets executed:
if ....; then
....
elif ....; then
....
else
....
fi
Most of the time a very special command called test is used inside if-statements. It can be used to compare strings or test if a file exists, is readable etc...
The "test" command is written as square brackets " [ ] ". Note that space is significant here: Make sure that you always have space around the brackets. Examples:
[ -f "somefile" ] : Test if somefile is a file.
[ -x "/bin/ls" ] : Test if /bin/ls exists and is executable.
[ -n "$var" ] : Test if the variable $var contains something
[ "$a" = "$b" ] : Test if the variables "$a" and "$b" are equal
Run the command "man test" and you get a long list of all kinds of test operators for comparisons and files.
Using this in a shell script is straight forward:
#!/bin/sh
if [ "$SHELL" = "/bin/bash" ]; then
echo "your login shell is the bash (bourne again shell)"
else
echo "your login shell is not bash but $SHELL"
fi
The variable $SHELL contains the name of the login shell and this is what we are testing here by comparing it against the string "/bin/bash"
Shortcut operators
People familiar with C will welcome the following expression:
[ -f "/etc/shadow" ] && echo "This computer uses shadow passwors"
The && can be used as a short if-statement. The right side gets executed if the left is true. You can read this as AND. Thus the example is: "The file /etc/shadow exists AND the command echo is executed". The OR operator (||) is available as well. Here is an example:
#!/bin/sh
mailfolder=/var/spool/mail/james
[ -r "$mailfolder" ] || { echo "Can not read $mailfolder" ; exit 1; }
echo "$mailfolder has mail from:"
grep "^From " $mailfolder
The script tests first if it can read a given mailfolder. If yes then it prints the "From" lines in the folder. If it cannot read the file $mailfolder then the OR operator takes effect. In plain English you read this code as "Mailfolder readable or exit program". The problem here is that you must have exactly one command behind the OR but we need two:
-print an error message
-exit the program
To handle them as one command we can group them together in an anonymous function using curly braces. Functions in general are explained further down.
You can do everything without the ANDs and ORs using just if-statements but sometimes the shortcuts AND and OR are just more convenient.
The case statement can be used to match (using shell wildcards such as * and ?) a given string against a number of possibilities.
case ... in
...) do something here;;
esac
Let's look at an example. The command file can test what kind of filetype a given file is:
file lf.gz
returns:
lf.gz: gzip compressed data, deflated, original filename,
last modified: Mon Aug 27 23:09:18 2001, os: Unix
We use this now to write a script called smartzip that can uncompress bzip2, gzip and zip compressed files automatically :
#!/bin/sh
ftype=`file "$1"`
case "$ftype" in
"$1: Zip archive"*)
unzip "$1" ;;
"$1: gzip compressed"*)
gunzip "$1" ;;
"$1: bzip2 compressed"*)
bunzip2 "$1" ;;
*) error "File $1 can not be uncompressed with smartzip";;
esac
Here you notice that we use a new special variable called $1. This variable contains the first argument given to a program. Say we run
smartzip articles.zip
then $1 will contain the string articles.zip
The select statement is a bash specific extension and is very good for interactive use. The user can select a choice from a list of different values:
select var in ... ; do
break
done
.... now $var can be used ....
Here is an example:
#!/bin/sh
echo "What is your favourite OS?"
select var in "linux" "Gnu Hurd" "Free BSD" "Other"; do
break
done
echo "You have selected $var"
Here is what the script does:
What is your favourite OS?
1) linux
2) Gnu Hurd
3) Free BSD
4) Other
#? 1
You have selected linux
In the shell you have the following loop statements available:
while ...; do
....
done
The while-loop will run while the expression that we test for is true. The keyword "break" can be used to leave the loop at any point in time. With the keyword "continue" the loop continues with the next iteration and skips the rest of the loop body.
The for-loop takes a list of strings (strings separated by space) and assigns them to a variable:
for var in ....; do
....
done
The following will e.g. print the letters A to C on the screen:
#!/bin/sh
for var in A B C ; do
echo "var is $var"
done
A more useful example script, called showrpm, prints a summary of the content of a number of RPM-packages:
#!/bin/sh
# list a content summary of a number of RPM packages
# USAGE: showrpm rpmfile1 rpmfile2 ...
# EXAMPLE: showrpm /cdrom/RedHat/RPMS/*.rpm
for rpmpackage in $*; do
if [ -r "$rpmpackage" ];then
echo "=============== $rpmpackage =============="
rpm -qi -p $rpmpackage
else
echo "ERROR: cannot read file $rpmpackage"
fi
done
Above you can see the next special variable, $* which contains all the command line arguments. If you run
showrpm openssh.rpm w3m.rpm webgrep.rpm
then $* contains the 3 strings openssh.rpm, w3m.rpm and webgrep.rpm.
The GNU bash knows until-loops as well but generally while and for loops are sufficient.
Quoting
Before passing any arguments to a program the shell tries to expand wildcards and variables. To expand means that the wildcard (e.g. *) is replaced by the appropriate file names or that a variable is replaced by its value. To change this behaviour you can use quotes: Let's say we have a number of files in the current directory. Two of them are jpg-files, mail.jpg and tux.jpg.
#!/bin/sh
echo *.jpg
This will print "mail.jpg tux.jpg".
Quotes (single and double) will prevent this wildcard expansion:
#!/bin/sh
echo "*.jpg"
echo '*.jpg'
This will print "*.jpg" twice.
Single quotes are most strict. They prevent even variable expansion. Double quotes prevent wildcard expansion but allow variable expansion:
#!/bin/sh
echo $SHELL
echo "$SHELL"
echo '$SHELL'
This will print:
/bin/bash
/bin/bash
$SHELL
Finally there is the possibility to take the special meaning of any single character away by preceeding it with a backslash:
echo *.jpg
echo $SHELL
This will print:
*.jpg
$SHELL
Here documents
Here documents are a nice way to send several lines of text to a command. It is quite useful to write a help text in a script without having to put echo in front of each line. A "Here document" starts with << followed by some string that must also appear at the end of the here document. Here is an example script, called ren, that renames multiple files and uses a here document for its help text:
#!/bin/sh
# we have less than 3 arguments. Print the help text:
if [ $# -lt 3 ] ; then
cat <<HELP
ren -- renames a number of files using sed regular expressions
USAGE: ren 'regexp' 'replacement' files...
EXAMPLE: rename all *.HTM files in *.html:
ren 'HTM$' 'html' *.HTM
HELP
exit 0
fi
OLD="$1"
NEW="$2"
# The shift command removes one argument from the list of
# command line arguments.
shift
shift
# $* contains now all the files:
for file in $*; do
if [ -f "$file" ] ; then
newfile=`echo "$file" | sed "s/${OLD}/${NEW}/g"`
if [ -f "$newfile" ]; then
echo "ERROR: $newfile exists already"
else
echo "renaming $file to $newfile ..."
mv "$file" "$newfile"
fi
fi
done
This is the most complex script so far. Let's discuss it a little bit. The first if-statement tests if we have provided at least 3 command line parameters. (The special variable $# contains the number of arguments.) If not, the help text is sent to the command cat which in turn sends it to the screen. After printing the help text we exit the program. If there are 3 or more arguments we assign the first argument to the variable OLD and the second to the variable NEW. Next we shift the command line parameters twice to get the third argument into the first position of $*. With $* we enter the for loop. Each of the arguments in $* is now assigned one by one to the variable $file. Here we first test that the file really exists and then we construct the new file name by using find and replace with sed. The backticks are used to assign the result to the variable newfile. Now we have all we need: The old file name and the new one. This is then used with the command mv to rename the files.
Functions
As soon as you have a more complex program you will find that you use the same code in several places and also find it helpful to give it some structure. A function looks like this:
functionname()
{
# inside the body $1 is the first argument given to the function
# $2 the second ...
body
}
You need to "declare" functions at the beginning of the script before you use them.
Here is a script called xtitlebar which you can use to change the name of a terminal window. If you have several of them open it is easier to find them. The script sends an escape sequence which is interpreted by the terminal and causes it to change the name in the titlebar. The script uses a function called help. As you can see the function is defined once and then used twice:
#!/bin/sh
# vim: set sw=4 ts=4 et:
help()
{
cat <<HELP
xtitlebar -- change the name of an xterm, gnome-terminal or kde konsole
USAGE: xtitlebar [-h] "string_for_titelbar"
OPTIONS: -h help text
EXAMPLE: xtitlebar "cvs"
HELP
exit 0
}
# in case of error or if -h is given we call the function help:
[ -z "$1" ] && help
[ "$1" = "-h" ] && help
# send the escape sequence to change the xterm titelbar:
echo -e "
