Linux Server Shell programming learning notes

Shell is a system user interface that provides an interface for users to interact with the kernel. It receives user-input commands and sends them to the kernel for execution. In fact, Shell is a command interpreter that explains the commands entered by the user and sends them to the kernel.

Getting started with Shell script programming is what we will introduce in this article. We can use any text editor, such as gedit, kedit, emacs, and vi to write shell scripts, it must start with the following line (the first line of the file must be placed ):

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#! /Bin/sh

... Note: it is best to use "! /Bin/bash "instead of"! /Bin/sh ", if you use tc shell to change to tcsh, others are similar.

Symbol #! The program used to tell the system to execute the scripts. This example uses/bin/sh. After editing and saving, if you want to execute the shell script, you must first make it executable:

After chmod + x filename is in the directory where the shell script is located, enter./filename to execute the shell script.

Some special symbols in Shell

A []

Shell departure Function

If you have written complicated shell scripts, you may find that the same code may be used in several places. In this case, it is much easier to use functions. The functions are roughly as follows:

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Functionname () { # Inside the body $1 is the first argument given to the function #$2 the second... Body }

You need to declare the function at the beginning of each script.

The following is a shell script named xtitlebar, which can change the terminal window name. Here we use a function named help, which is used twice in the shell script:

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#! /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 "33] 0; $107"

# It is a good programming habit to help other users (and themselves) use and understand scripts.

Command Line Parameters

We have seen $ * and $1, $2... $9 and other special variables include the parameters you input from the command line. So far, we have only learned some simple command line syntax (such as some mandatory parameters and the-h option for viewing help ). However, when writing more complex programs, you may find that you need more custom options. The common practice is to add a minus sign before all optional parameters, followed by a parameter value (such as a file name ).

There are many ways to analyze input parameters, but the example using the case expression below is undoubtedly a good method.

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#! /Bin/sh Help () { Cat <HELP This is a generic command line parser demo. Usage example: extends parser-l hello-f ---somefile1 somefile2 HELP Exit 0 } While [-n "$1"]; do Case $1 in -H) help; shift 1; # function help is called -F) opt_f = 1; shift 1; # variable opt_f is set -L) opt_l = $2; shift 2; #-l takes an argument-> shift by 2 --) Shift; break; # end of options -*) Echo "error: no such option $1.-h for help"; exit 1 ;; *) Break ;; Esac Done Echo "opt_f is $ opt_f" Echo "opt_l is $ opt_l" Echo "first arg is $1" Echo "2nd arg is $2"

You can run the script like this:

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Extends parser-l hello-f ---somefile1 somefile2

The returned results are as follows:

Opt_f is 1
Opt_l is hello
First arg is-somefile1
2nd arg is somefile2

How does this shell script work? The script first loops through all input command line parameters and compares the input parameters with the case expression. If the input parameters match, a variable is set and the parameter is removed. According to the Convention of the unix system, the first input should be the parameter containing the minus sign.

Shell script example

General programming steps

Now let's discuss the general steps for writing a script. Any excellent script should have help and input parameters. Write a framework script (framework. sh) that contains the framework structure required by most scripts. It is a very good idea. In this way, when writing a new script, you can first execute the following command:

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Cp framework. sh myscript

Then insert your own function.

Let's take a look at the two examples below.

Binary to decimal conversion

The script b2d converts the binary number (such as 1101) to the corresponding decimal number. This is also an example of a mathematical operation using the expr command:

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#! /Bin/sh # Vim: set sw = 4 ts = 4 et: Help () { Cat <HELP B2d -- convert binary to decimal USAGE: b2d [-h] binarynum OPTIONS:-h help text EXAMPLE: b2d 111010 Will return 58 HELP Exit 0 } Error () { # Print an error and exit Echo "$1" Exit 1 } Lastchar () { # Return the last character of a string in $ rval If [-z "$1"]; then # Empty string Rval = "" Return Fi # Wc puts some space behind the output this is why we need sed: Numofchar = 'echo-n "$1" | sed's // G' | wc-C' # Now cut out the last char Rval = 'echo-n "$1" | cut-B $ numofchar' } Chop () { # Remove the last character in string and return it in $ rval If [-z "$1"]; then # Empty string Rval = "" Return Fi # Wc puts some space behind the output this is why we need sed: Numofchar = 'echo-n "$1" | wc-c | sed's // g'' If ["$ numofchar" = "1"]; then # Only one char in string Rval = "" Return Fi Numofcharminus1 = 'expr $ numofchar "-" 1' # Now cut all but the last char: Rval = 'echo-n "$1" | cut-B-$ numofcharminus1' # The original rval = 'echo-n "$1" | cut-B 0-$ {numofcharminus1} 'encountered an error during running. # The reason is that cut starts counting from 1. It should be cut-B 1-$ {numofcharminus1} } While [-n "$1"]; do Case $1 in -H) help; shift 1; # function help is called --) Shift; break; # end of options -*) Error "error: no such option $1.-h for help ";; *) Break ;; Esac Done # The main program Sum = 0 Weight = 1 # One arg must be given: [-Z "$1"] & help Binnum = "$1" Binnumorig = "$1" While [-n "$ binnum"]; do Lastchar "$ binnum" If ["$ rval" = "1"]; then Sum = 'expr "$ weight" "+" "$ sum "' Fi # Remove the last position in $ binnum Chop "$ binnum" Binnum = "$ rval" Weight = 'expr "$ weight" "*" 2' Done Echo "binary $ binnumorig is decimal $ sum"

# The shell script uses decimal and binary weights (, 16,...). For example, binary "10" can be converted to decimal:

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0*1 + 1*2 = 2

To obtain a single binary number, we use the lastchar function. This function uses wc-c to calculate the number of characters, and then uses the cut command to retrieve the last character. The Chop function removes the last character.

Reprinting objects cyclically

You may want to do this all the time: Save all sent emails to a file. However, after a few months, the file may become so large that the access speed of the file will be slow. The following shell script rotatefile can solve this problem. This script can rename the email storage file (assuming outmail) to outmail.1, and the original outmail.1 is changed to outmail.2 and so on...

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#! /Bin/sh # Vim: set sw = 4 ts = 4 et: Ver = "0.1" Help () { Cat <HELP Rotatefile -- rotate the file name USAGE: rotatefile [-h] filename OPTIONS:-h help text EXAMPLE: rotatefile out This will e. g rename out.2 to out.3, out.1 to out.2, out to out.1 [BR] And create an empty out-file The max number is 10 Version $ ver HELP Exit 0 } Error () { Echo "$1" Exit 1 } While [-n "$1"]; do Case $1 in -H) help; shift 1 ;; --) Break ;; -*) Echo "error: no such option $1.-h for help"; exit 1 ;; *) Break ;; Esac Done # Input check: If [-z "$1"]; then Error "ERROR: you must specify a file, use-h for help" Fi Filen = "$1" # Rename any. 1,. 2 etc file: For n in 9 8 7 6 5 4 3 2 1; do If [-f "$ filen. $ n"]; then P = 'expr $ n + 1' Echo "mv $ filen. $ n $ filen. $ p" Mv $ filen. $ n $ filen. $ p Fi Done # Rename the original file: If [-f "$ filen"]; then Echo "mv $ filen $ filen.1" Mv $ filen $ filen.1 Fi Echo touch $ filen Touch $ filen

How does this shell script work? After detecting that a file name is provided by the user, perform a loop of 9 to 1. Rename file name. 9 to file name. 10, and rename file name. 8 to file name. 9 ...... And so on. After the loop ends, name the original file as file name. 1 and create an empty file (touch $ filen) with the same name as the original file)

Script debugging

The simplest debugging method is to use the echo command. You can use echo to print variable values wherever possible, which is why most shell Programmers spend 80% of their time debugging. The benefit of Shell scripts is that it does not take much time to insert an echo command without recompilation.

Shell also has a real debugging mode. If the script "strangescript" has an error, run the following command to debug it:

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Sh-x strangescript7

The preceding command executes the script and displays the values of all variables.

In the shell script, there is also a mode for checking only the syntax without executing the script. The command is as follows:

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Sh-n your_script

This command returns all syntax errors.

We hope that you can now write your own shell scripts and enjoy it!