Embed Size (px)
Citation preview
Simple commands
There are three types of commands:
•shell functions,•built-in commands, and•normal programs.
The command is searched for (by name) in that order.
Command list
A list is a sequence of one or more simple commands or pipelines, separated by ‘ ; ’ ‘&’ ‘&&’ ‘ || ’ . With ‘&’ sign , command is executed in background.
Examples:
$ sync ; sync ; sync ; shutdown now
Conditional Execution
• With ‘&&’ , command2 is executed only if the preceding pipeline (or command1) returns a zero exit status.
command1 && command2
• With ‘ || ’, command2 is executed only if the preceding pipeline (or command1) returns a nonzero exit status.
command1 || command2
The command groups
• Executes list in a sub-shell. (list)
• List is simply executed ( in current shell ).
{list;}
The command groups(cont.)
Examples:$ pwd
/usr/home/rlwu
$ ( cd wu; pwd )
/usr/home/rlwu/wu
$ pwd
/usr/home/rlwu
$ { echo -n "hello"; echo " world"; } > greeting
Standard devices
• stdin file descriptor : 0• stdout file descriptor : 1• stderr file descriptor : 2
Input/Output redirection• command < word
Use file word as standard input (file descriptor 0).
• command > wordUse file word as standard output (file descriptor 1). If the file does not exist it is created; otherwise, it is truncated to zero length.
• command >>word Use file word as standard output. If the file exists output is appended to it (by first seeking to the EOF); otherwise, the file is created.
Input/Output redirection(cont.)
• command <&digitUse the file associated with file descriptor digit as standard input (file descriptor 0).
• command >&digitUse the file associated with file descriptor digit as standard output (file descriptor 1).
Input/Output redirection(cont.)
If any of the above is preceded by a digit, the file descriptor which will be associated with the file is that specified by the digit (instead of the default 0 or 1). For example :
command 2>&1associates file descriptor 2 with the file currently associated with file descriptor 1.
Examples:$ ( cc a.c 2>&1 ) > log.a$ command1 2>&1 | command2
here-document
The following redirection is often called a ''here-document''.
command << delimiter here-doc-text ... delimiter
All the text on successive lines up to the delimiter is saved away and made available to the command on standard input. If the operator is ''<<-'' instead of ''<<'', then leading tabs in the here-doc-text are stripped.
here-document(cont.)
Examples:
% cat << eof
? This is the first line.
? This is the second line.
? eof
This is the first line.
This is the second line.
Error Redirection
Any command that produces error messages to stderr can have those messages redirected to another file.
•Overwrite 2>•Append 2>>
Examples:
$ cc a.c 2> another_file
$ cc a.c 2>> another_file
Pipeline
• A pipeline is a sequence of one or more commands separated by `|' .
• The standard output of each command but the last is connected by a pipe to the standard input of the next command.
• The exit status of a pipeline is the exit status of its last command.
command1 | command2 | command3
Comments
A word beginning with # and all the following characters up to a NEWLINE are ignored. If the first line in a shell script is #!/bin/xshell the shell “/bin/xshell” will be invoked to execute the shell script.
Examples:
#!/bin/sh
# This is a comment line
echo Hi!
The if Construction
Syntax:
if list A
then list B
[ elif list C
then list D ] ...
[ else list E ]
fi
The if Construction
Examples:
# multi-line
if [ -d wu ]
then ls –l wu
fi
#
# single-line
if [ -d wu ] ; then ls –l wu; fi
The case Construction
Used for multi-way branchingSyntax:
case word inpattern [ | pattern ] ) list A
;;pattern [ | pattern ]) list B;;pattern [ | pattern ]) list N;;esac
Pattern Matching* Matches any string, including the null string.? Matches any single character.[...] Matches any one of the enclosed characters. A
pair of characters separated by `-' matches any character lexically between the pair, inclusive. If the first character following the opening [ is a ! any character not enclosed is matched.
Examples:
abc)a??)a*)ab | cd)a* | b?[123])a[!de]*)[!a-z]*)
The while ConstructionSyntax:
whilelist A
dolist B
done
A while command repeatedly executes List A and, if the return value of the last command in list A is zero, executes list B ; otherwise the loop terminates.
The while Construction(cont.)
Examples:#!/bin/sh# This is a comment line.trap "echo bye; exit" 2while read A ; [ $A != q -a $A != Q ]doecho $Adoneexit
The until Construction
Syntax:until
list Ado
list Bdone
A until command repeatedly executes List A and, if the return value of the last command in list A is not zero, executes list B ; otherwise the loop terminates.
The for ConstructionSyntax:
for var [ in word list …]
do
list A
done
In which var is any shell variable, and word list is a space delimited list of strings. Each time a for command is executed, var is set to the next word taken from the in word list. Execution ends when there are no more words in the word list.
If in word is omitted, then the for command executes the do list once for each positional parameter that is set. Execution ends when there are no more words in the list.
The for Construction(cont.)
Examples:
#!/bin/sh
#
for AA in $*
do
echo $AA
done
exit
The trap Command
Syntax:trap 'cmds' signo [signo...]
Description:Perform cmds upon receipt of signal .
The trap Command(cont.)
Example:$ cat myprog#!/bin/sh# shell script named : “myprog”trap 'echo bye; exit' 2whiletruedoecho hellodone$ myproghellohellohello^Cbye
Define a function
Syntax:name ( )
{
list;
}
Define a function Examples:
#!/bin/sh#trap "echo bye; exit" 2fn1 ( ){
for AA in $*do
echo $AAdone
}fn1 tom bill johnexit
Special Commands
. filename Read and execute commands from filename and return.
break [ n ] Exit from the enclosing for or while loop, if any. If n is specified break n levels.
continue [ n ] Resume the next iteration of the enclosing for or while loop. If n is specified resume at the n'th enclosing loop.
Special Commands(cont.)
exit [ n ] Exit a shell with the exit status specified by n. If n is omitted the exit status is that of the last command executed (an EOF will also cause the shell to exit.)
export [ name ... ] The given names are marked for automatic export to the environment of subsequently-executed commands. If no arguments are given, variable names that have been marked for export during the current shell's execution are listed.
Setting Shell Variables
Syntax:
name=value
*No blanks are allowed in the expression.
Examples:
$ my_name_is=bob
$ MESSAGE=/usr/man/man1/ls.1
$ var5="hello mom"
Show variables
envReports the names and values of all environment variables.
setReports the names and values of all shell variables.
Parameter Substitution $parameter
or
${parameter}The braces are required only when parameter is followed by a letter, digit, or underscore that is not to be interpreted as part of its name.
Examples:$ echo $my_name_isbob$ echo $MESSY/usr/man/man1/ls.1$ more $MESSY<contents of /usr/man/man1/ls.1>
Parameter Substitution (cont.)
${parameter:-word}If parameter is set and is non-null, substitute its value; otherwise substitute word.
${parameter:=word}If parameter is not set or is null set it to word; the value of the parameter is substituted. Argument parameters may not be assigned to in this way.
Parameter Substitution (cont.)
${parameter:?word}If parameter is set and is non-null, substitute its value; otherwise, print word and exit from the shell.
${parameter:+word} If parameter is set and is non-null, substitute word; otherwise substitute nothing.
If the colon (:) is omitted from the above expressions, the shell only checks whether parameter is set or not.
Parameter Substitution Examples:
$ AA="Jonn"$ CC=""$ echo ${BB:-Tom}Tom$ echo ${CC:-Tom}Tom$ echo ${BB-Tom}Tom$ echo ${CC-Tom}
* < null >$ echo ${AA:-Tom}John$ echo ${BB:=Tom}Tom$ echo $BBTom
Command Substitution
The shell executes commands from the string between two grave accents (``) and the standard output from these commands may be used as all or part of a word.
Examples:$ pwd
/usr/home/rlwu
$ AA = `pwd`
$ echo $AA
/usr/home/rlwu
Quoting
• A character may be quoted (made to stand for itself) by a backslash (\) or a pair of quote marks (' ' or " ").
• All characters enclosed between a pair of single quotes (' '), except a single quote, are quoted by the shell.
Examples:$ aa=1122$ echo '$aa'$aa$ echo ' "$aa" '"$aa"
Quoting(Cont.)
• Inside a pair of double quotes (" "), parameter and command substitution occurs. The backslash inside double quotes remains literal unless it precedes the following characters, which it serves to quote:
$ ` " \ NEWLINE
Examples:$ echo " '$aa' is the content of parameter \$aa"'1122' is the content of parameter $aa$ echo "`pwd` is the current directory"/usr/home/rlwu is the current directory
Quoting(continued)
•A backslash preceding a newline is treated as a line continuation.
Examples:$ echo \$ \" \' \\ \`$ " ' \ `$ echo "This is the first sentence.\> This is the second sentence.\> "This is the first sentence.This is the second sentence.
Quoting(cont.)
If $* is within a pair of double quotes, the arguments are substituted and quoted, separated by quoted spaces ("$1 $2 ..."); however, if $@ is within a pair of double quotes, the arguments parameters are substituted and quoted, separated by unquoted spaces ("$1" "$2" ... ).
“$*” “$1 $2 …”
“$@” “$1” “$2” …
Special variables set by the shell
# The number of arguments in decimal.
- Flags supplied to the shell on invocation or by the set command.
? The decimal value returned by the last synchronously executed command.
$ The process number of this shell.
! The process number of the last background command invoked.
Special variables set by the shell
Examples:$ echo $$7828$ echo $!
$ echo $?0$ echo $-ims$ echo $#0
The shell variables used by the shell
HOME The default argument (home directory)for the cd command.
PATH The search path for commands.CDPATH The search path for the cd command.PS1 Primary prompt string, by default `$ '.PS2 Secondary prompt string, by default `>'.IFS Internal field separators, normally
SPACE, TAB, and NEWLINE.SHELL When the shell is invoked, it scans the
environment for this name.
Positional parameters
Positional parameters are used to refer the arguments list.
$0 the shell script name$1 the first argument$* or
$@ all the arguments, starting with$1, are substituted (separated by SPACE characters).
$# The number of arguments in decimal.
The read Command
Syntax:read variable [variable...]
Examples:#!/bin/sh# Shell Program#echo enter a sentenceread Aecho $A
Return statusThe shell variable ? holds the return status of the last
command executed.zero - true or no errornon-zero - false or error
Examples:$ ls$ echo $?0$ cpUsage: cp f1 f20cp [-r] f1 . . . fn d1$ echo $?1$ echo $?0
The test Command
Syntax:
test expression
or
[ expression ]
We can test three types of things:
1. Files
2. Integers
3. Strings
The test Command - File Tests
Syntax:test -option filenameor[ -option filename ]
File test Options:r readablew writablex executabled directoryf plain files size greater than 0
File Tests
Examples:
$ test -f new.jersey
$ echo $?
0
$ [ -d new.jersey ]
$ echo $?
1
The test Command - String Tests
Syntax:test string1 = string2test string1 != string2test -z s1 True if the length of string s1 is zero.test -n s1 True if the length of the string s1 is
non-zero.test s1 True if s1 is not the null string. or [ string1 = string2 ][ string1 != string2 ][ -z s1 ][ -n s1 ]
The test Command - String Tests
Examples:
$ X=abc
$ [ "$X" = "abc" ]
$ echo $?
0
$ [ "$X" != "abc" ]
$ echo $?
1
The test Command - Numeric Tests
Syntax:test number relation number or[ number relation number ]
Relations:-lt Less than-le Less than or equal to-gt Greater than-ge Greater than or equal to-eq Equal to-ne Not equal to
The test Command - Numeric Tests
Examples:
$ x=3
$ [ $x -lt 7 ]
$ echo $?
0
The test Command - Other Operators
-o OR
-a AND
! NOT
\( \) GROUPING*
Examples:$ test a != b
$ test ! -d name
$ test -f name1 -o -f name2
$ test \( -f name1 -o -f name2 \) -a -d name3
* Note: the ( ) must be escaped with a backslash.
