Note:
Well,
Bash is intepreted line by line as it runs and depends on calling a lot of external progs (depending on what you want to do).
You often have to use temp files as intermediate storage for result sets.
It (shell) was originally designed to talk to the system and automate cmd sequences (shell files).
Perl is more like C, it's largely self contained with a huge library of free code and it's compiled , so it runs much faster, eg about 80-90% speed of C, but easier to program (eg variable sizes are dynamic).
Syntax is similar to C.
It (shell) was originally designed to talk to the system and automate cmd sequences (shell files).
Perl is more like C, it's largely self contained with a huge library of free code and it's compiled , so it runs much faster, eg about 80-90% speed of C, but easier to program (eg variable sizes are dynamic).
Syntax is similar to C.
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.
Simple Stuff:
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.
|
First, a convention. I'll list things for you to type in this format:
$ date
I will list the computer's reply like this:
Tue Dec 23 10:52:51 PST 2003
To put this another way, enter this example:
# whoami
root
root
Where are you?
As you may be aware, a Linux filesystem is in the form of a large tree with many branches called "subdirectories". When you issue a shell command,
$ pwd
/path/path/path
/path/path/path
You can decide where you are in the tree. Type this example:
$ cd ~
$ pwd
/home/username
$ pwd
/home/username
Listing Files:
Directories contain files, and you can list them in a simple, compact format:
$ ls
filename filename filename ...
Or you can list them in more detail:
$ ls -la
(detailed list, one file per line)
And, very important, to find out what a command's options are, use the "man" (manual) command:
$ man ls
(manual page for "ls")
filename filename filename ...
(detailed list, one file per line)
(manual page for "ls")
To find files by name:
$ find . -name '*.jpg'
(list of files with .jpg suffix in current and all child directories)
To create a text diagram of the directory tree:
$ tree -d .
(diagram of the directory tree from the current directory)
(list of files with .jpg suffix in current and all child directories)
(diagram of the directory tree from the current directory)
Examining Files:
There are a number of things you can do to find out more about the files in the list. Here are just a few:
The "file" command tries to identify files by examining their contents:
$ file tux_small.png
tux_small.png: PNG image data, 128 x 151, 8-bit/color RGB, non-interlaced
tux_small.png: PNG image data, 128 x 151, 8-bit/color RGB, non-interlaced
The next example uses the obscurely named "cat" command. It prints the contents of a file. Unfortunately if the file's contents are not readable, they get printed anyway.
(prints the entire contents of a file named "zipcodes.txt")
If a file is too long to be viewed on one page, you can say:
(prints file one screenful at a time)
You can also use "grep" to print only those parts of a file you are interested in:
(prints only those lines that have the character string "10001" in them)
The "grep" command is very useful, unfortunately it has a difficult-to-remember name. Be sure to:
Pipelines and Redirection:
Enter this command:
$ echo "cherry apple peach"
cherry apple peach
cherry apple peach
$ echo "cherry apple peach" | tr " " "\n"
cherry
apple
peach
cherry
apple
peach
$ echo "cherry apple peach" | tr " " "\n" | sort
apple
cherry
peach
apple
cherry
peach
$ echo "cherry apple peach" | tr " " "\n" | sort -r
peach
cherry
apple
peach
cherry
apple
Remember: A pipeline ("|") takes the output of one command and makes it the input to another command.
- Normally the output from commands is printed on the screen. But using the symbol ">", you can redirect the output to a file:$ date > RightNow.txt
$ cat RightNow.txt
Tue Dec 23 14:43:33 PST 2003 - The above example used ">" to replace the content of any existing file having the name "RightNow.txt". To append new data to an existing file, use ">>" instead:$ date >> RightNow.txt
$ cat RightNow.txt
Tue Dec 23 14:43:33 PST 2003
Tue Dec 23 14:46:10 PST 2003 - Remember: Use ">" to overwrite any existing file, use ">>" to append to any existing file. In both cases, if no file exists, one is created.
Shell Script Basics:
To be executable, a shell script file must meet some conditions:
- The file must have a special first line that names an appropriate command processor. For this tutorial, the following will work in most cases:#!/bin/bash
- If this example doesn't work, you will need to find out where your Bash shell executable is located and substitute that location in the above example. Here is one way to find out:$ whereis bash
- The file must be made executable by changing its permission bits. An example:$ chmod +x (shell script filename)
- One normally executes a shell script this way:$ ./scriptname.sh
1. Run your choice of editor and type the following lines:
$vi myscript.sh
#!/bin/bash
echo "Hello, world."
$ ./myscript.sh
Tests and Branching
Bash shell scripts can perform, and act on, various kinds of tests. This will be just the most basic introduction
Here is an example of a test and branch:
$ ./myscript.sh
Yes.
We created a test (the part of the script between "[" and "]") which tested whether a particular element existed ("-e"). Because the symbol "." in this context means the current directory, the test succeeded. Try replacing the "." with something that is not present in the current directory, example "xyz". See how the outcome changes.
It is important to realize that "[" is an alias for the command "test". The script could have been written as:
Tests and Branching
Bash shell scripts can perform, and act on, various kinds of tests. This will be just the most basic introduction
Here is an example of a test and branch:
if [ -e . ]
then
echo "Yes."
else
echo "No."
fi
$ ./myscript.sh
Yes.
We created a test (the part of the script between "[" and "]") which tested whether a particular element existed ("-e"). Because the symbol "." in this context means the current directory, the test succeeded. Try replacing the "." with something that is not present in the current directory, example "xyz". See how the outcome changes.
It is important to realize that "[" is an alias for the command "test". The script could have been written as:
if test -e .
then
echo "Yes."
else
echo "No."
fi
$man test
test - check file types and compare values
Otherwise, EXPRESSION is true or false and sets exit status.
It is one of:
( EXPRESSION )
EXPRESSION is true
! EXPRESSION
EXPRESSION is false
EXPRESSION1 -a EXPRESSION2
both EXPRESSION1 and EXPRESSION2 are true
EXPRESSION1 -o EXPRESSION2
either EXPRESSION1 or EXPRESSION2 is true
-n STRING
the length of STRING is nonzero
-z STRING
the length of STRING is zero
STRING1 = STRING2
the strings are equal
STRING1 != STRING2
the strings are not equal
INTEGER1 -eq INTEGER2
INTEGER1 is equal to INTEGER2
INTEGER1 -ge INTEGER2
INTEGER1 is greater than or equal to INTEGER2
INTEGER1 -gt INTEGER2
INTEGER1 is greater than INTEGER2
INTEGER1 -le INTEGER2
INTEGER1 is less than or equal to INTEGER2
INTEGER1 -lt INTEGER2
INTEGER1 is less than INTEGER2
INTEGER1 -ne INTEGER2
INTEGER1 is not equal to INTEGER2
FILE1 -ef FILE2
FILE1 and FILE2 have the same device and inode numbers
FILE1 -nt FILE2
FILE1 is newer (modification date) than FILE2
FILE1 -ot FILE2
FILE1 is older than FILE2
-b FILE
FILE exists and is block special
-c FILE
FILE exists and is character special
-d FILE
FILE exists and is a directory
-e FILE
FILE exists
-f FILE
FILE exists and is a regular file
-g FILE
FILE exists and is set-group-ID
-G FILE
FILE exists and is owned by the effective group ID
-h FILE
FILE exists and is a symbolic link (same as -L)
-k FILE
FILE exists and has its sticky bit set
-L FILE
FILE exists and is a symbolic link (same as -h)
-O FILE
FILE exists and is owned by the effective user ID
-p FILE
FILE exists and is a named pipe
-r FILE
FILE exists and read permission is granted
-s FILE
FILE exists and has a size greater than zero
-S FILE
FILE exists and is a socket
-t FD file descriptor FD is opened on a terminal
-u FILE
FILE exists and its set-user-ID bit is set
-w FILE
FILE exists and write permission is granted
-x FILE
FILE exists and execute (or search) permission is granted
Note:when a test is conducted or a command returns a result value, the numerical value for "true" is 0, and "false" is 1.
$ echo $(( 0 && 0 ))
0
$ echo $(( 1 && 0 ))
0
$ echo $(( 0 && 1 ))
0
$ echo $(( 1 && 1 ))
1
A couple of rules about logical operators used as branches:
$ true && echo "Yes."
Yes.
Note:when a test is conducted or a command returns a result value, the numerical value for "true" is 0, and "false" is 1.
$ echo $(( 0 && 0 ))
0
$ echo $(( 1 && 0 ))
0
$ echo $(( 0 && 1 ))
0
$ echo $(( 1 && 1 ))
1
A couple of rules about logical operators used as branches:
- If you write "test && command", the command will only be executed if the test succeeds.
- If you write "test || command", the command will only be executed if the test fails.
$ true && echo "Yes."
Yes.
$ false || echo "Yes."
Yes.
Yes.
========================================================================
Loops and Repetition:
1. Here are some examples of loop operators:
Loops and Repetition:
1. Here are some examples of loop operators:
for fn in *; #do
echo "$fn"
done
