Shell Common commands and Process Control

Three types of commands can be used in shell scripts:

>>> Unix Commands <

Although any Unix command can be used in shell scripts, some more common commands are used. These commands are usually used for file and text operations.

Common command syntax and functions

Echo "some text": print the text on the screen

Ls: file list

WC-l file WC-W file WC-C file: calculates the number of file lines, the number of words in the file, and the number of characters in the file.

CP sourcefile destfile: file copy

MV oldname newname: rename a file or move a file

RM file: delete an object

Grep 'pattern' file: searches for strings in a file, for example, grep 'searchstring' file.txt.

Cut-B colnum file: specify the content range of the file to be displayed, and output them to the standard output device, for example: output 5th to 9th characters in each line cut-b5-9 file.txt do not confuse with Cat command, this is two completely different commands

Cat file.txt: output file content to the standard output device (screen)

File somefile: get the file type

Read var: prompt the user to input and assign the input value to the variable.

Sort file.txt: sorts the rows in the file.txt file.

Uniq: Delete the columns in a text file, for example, sort file.txt | uniq

Expr: perform mathematical operations example: Add 2 and 3 expr 2 "+" 3

Find: search for a file. For example, search for find.-Name filename-print based on the file name.

Tee: outputs data to the standard output device (screen) and files such as: somecommand | tee OUTFILE

Basename file: returns a file name that does not contain a path, for example, basename/bin/tux.

Dirname file: the path of the returned file. For example, dirname/bin/tux will return/bin.

Head file: prints the first few lines of a text file.

Tail file: number of rows at the end of a text file

Sed: SED is a basic search replacement program. You can read text from standard input (such as command pipeline) and

The result is output to the standard output (screen ). This command uses a regular expression (see references) for search. Do not confuse with wildcards in shell. For example, replace linuxfocus with linuxfocus: CAT text. File | SED's/linuxfocus/'> newtext. fileawk: awk to extract fields from text files. By default, the field delimiter is a space. You can use-F to specify other separators.

Cat file.txt | awk-F, '{print $1 "," $3}', which is used here as a field delimiter and prints both the First and Third fields. If the file contains the following content: Adam Bor, 34, indiakerry Miller, 22, and USA, the output result is Adam Bor, indiakerry Miller, USA.

2) concept: pipelines, redirection, and backtick

These are not system commands, but they are really important.

The pipeline (|) uses the output of a command as the input of another command.

Grep "hello" file.txt | WC-l

Search for a row containing "hello" in file.txt and calculate the number of rows.

Here, the grep command output serves as the WC command input. Of course, you can use multiple commands.

Redirection: output the command result to a file instead of a standard output (screen ).

> Write the file and overwrite the old file

> Add it to the end of the file to retain the content of the old file.

Backlash

You can use a backslash to output a command as a command line parameter of another command.

Command:

Find.-mtime-1-type F-print

Used to search for files modified in the past 24 hours (-mtime-2 indicates the past 48 hours. If you want to pack all the searched files, you can use the following script:

#! /Bin/sh

# The ticks are backticks (') not normal quotes ('):

Tar-zcvf lastmod.tar.gz 'Find.-mtime-1-type F-print'

>>> Process Control <

1>. If

If the expression "if" is true, the part after then is executed:

If...; then

....

Elif...; then

....

Else

....

Fi

In most cases, you can use test commands to test the conditions. For example, you can compare strings, determine whether a file exists, and whether the file is readable...

"[]" Is usually used to represent a conditional test. Note that spaces are important. Make sure that the square brackets have spaces.

[-F "somefile"]: determines whether it is a file.

[-X "/bin/ls"]: determines whether/bin/ls exists and has the executable permission.

[-N "$ Var"]: determines whether the $ var variable has a value.

["$ A" = "$ B"]: determines whether $ A and $ B are equal.

Run man test to view all types of test expressions that can be compared and judged.

Directly execute the following script:

#! /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 logon shell. We compared it with/bin/bash.

Shortcut Operators

If you are familiar with the C language, you may like the following expressions:

[-F "/etc/shadow"] & Echo "this computer uses shadow passwors"

Here & is a shortcut operator. If the expression on the left is true, execute the Statement on the right.

You can also think of it as a logical operation. In the above example, if the/etc/shadow file exists, print "this computer uses shadow passwors ". Similarly, operations (|) are also available in shell programming. 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 first checks whether mailfolder is readable. If it is readable, the "from" line in the file is printed. If it is not readable or the operation takes effect, print the error message and exit the script. There is a problem here, that is, we must have two commands:

-Print the error message.

-Exit the program.

We use curly braces to put the two commands together as one command in the form of an anonymous function. General functions will be mentioned below.

We can use the if expression to do anything without the sum or operator, but it is much more convenient to use the sum or operator.

2>. Case

Case: The expression can be used to match a given string, not a number.

Case... in

...) Do something here ;;

Esac

Let's look at an example. The file command can identify the file type of a given file, for example:

File lf.gz

This will return:

Lf.gz: gzip compressed data, deflated, original filename,

Last modified: Mon Aug 27 23:09:18 2001, OS: Unix

We use this to write a script named smartzip, which can automatically decompress Bzip2, Gzip, and zip compressed files:

#! /Bin/sh

FTYPE = 'file "$1 "'

Case "$ FTYPE" in

"$1: ZIP Archive "*)

Unzip "$1 ";;

"$1: gzip COMPRESSED "*)

Gunzip "$1 ";;

"$1: Bzip2 COMPRESSED "*)

Bunzip2 "$1 ";;

*) Echo "File $1 can not be uncompressed with smartzip ";;

Case

You may notice that we use a special variable $1 here. The variable contains the first parameter value passed to the program.

That is, when we run:

Smartzip articles.zip

$1 is the string articles.zip

3>. Select

The Select expression is a bash extension application, especially for interactive use. You can select from a group of different values.

Select VaR in...; do

Break

Done

... Now $ VaR can be used ....

The following is an example:

#! /Bin/sh

Echo "What is your favorite OS? "

Select VaR in "Linux" "GNU Hurd" "Free BSD" "other"; do

Break

Done

Echo "you have selected $ Var"

The following is the result of running the script:

What is your favorite OS?

1) Linux

2) GNU Hurd

3) Free BSD

4) Other

#? 1

You have selected Linux

4>. Loop

Loop expression:

While...; do

....

Done

While-loop will run until the expression test is true. Will run while the expression that we test for is true.

The keyword "break" is used to jump out of the loop. The keyword "continue" is used to directly jump to the next loop without executing the remaining part.

The for-loop expression is used to view a string list (strings are separated by spaces) and then assigned to a variable:

For VaR in...; do

....

Done

In the following example, ABC is printed to the screen:

#! /Bin/sh

For var in a B C; do

Echo "Var is $ Var"

Done

The following is a more useful script showrpm. Its function is to print statistics of some 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

The second special variable $ * is displayed, which contains all input command line parameter values.

If you run showrpm OpenSSH. RPM w3m. RPM webgrep. rpm

$ * Contains three strings: OpenSSH. rpm, w3m. rpm, and webgrep. rpm.

5>. quotation marks

Before passing any parameters to a program, the program extends the wildcards and variables. Here, the extension means that the program will replace the wildcard (for example, *) with the appropriate file name, and replace the variable with the variable value. To prevent this type of replacement, you can use quotation marks: Let's take a look at an example. Suppose there are some files in the current directory, two JPG files, mail.jpg and tux.jpg.

1.2 compile shell scripts

# Ch #! /Bin/sh mod + x filename

Cho *. jpg is slow. Why ?. /Filename to execute your script.

This will print the result of "mail.jpg tux.jpg.

Quotation marks (single quotation marks and double quotation marks) will prevent such wildcard extension:

#! /Bin/sh

Echo "*. jpg"

Echo '*. jpg'

This will print "*. jpg" twice.

Single quotes are stricter. It can prevent any variable extension. Double quotation marks can prevent wildcard extension but allow variable extension.

#! /Bin/sh

Echo $ Shell

Echo "$ shell"

Echo '$ shell'

The running result is:

/Bin/bash

/Bin/bash

$ Shell

Finally, there is also a method to prevent this extension, that is, to use the Escape Character -- reverse oblique ROD:

Echo *. jpg

Echo $ Shell

This will output:

*. Jpg

$ Shell

6>. Here documents

To pass several lines of text to a command, here documents is a good method. It is very useful to write a helpful text for each script. If we have the here documents, we do not need to use the echo function to output a row. A "Here document" starts with <, followed by a string, which must also appear at the end of the here document. The following is an example. In this example, we rename multiple files and print the help using here documents:

#! /Bin/sh

# We have less than 3 arguments. Print the help text:

If [$ #-lt 3]; then

Cat <

Ren -- renames a number of files using sed Regular Expressions

Usage: Ren 'regexp' 'replace '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

# 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 a complex example. Let's discuss it in detail. The first if expression determines whether the number of input command line parameters is smaller than 3 (special variable $ # indicates the number of parameters included ). If the number of input parameters is less than three, the help text is passed to the cat command and then printed on the screen by the cat command. Print the help text and exit the program. If the input parameter is equal to or greater than three, we assign the first parameter to the variable old, and the second parameter to the variable new. Next, we use the shift command to delete the first and second parameters from the parameter list, so that the original third parameter becomes the first parameter in the parameter list $. Then we start the loop. The command line parameter list is assigned to the variable $ file one by one. Then we determine whether the file exists. If yes, we use the SED command to search for and replace the file to generate a new file name. Then, assign the command result in the backslash to newfile.

Shell Common commands and Process Control