1. The Bourne Shell 吳 瑞 麟 rlwu@mail.ncku.edu.tw National Cheng Kung University, Computer and Network Center

2. 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.

3. 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

4. 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

5. The command groups Executes list in a sub-shell. (list) List is simply executed ( in current shell ). {list;}

6. 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

7. Standard devices stdinfile descriptor : 0 stdoutfile descriptor : 1 stderrfile descriptor : 2

8. Input/Output redirection command < word Use file word as standard input (file descriptor 0). command > word Use 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.

9. Input/Output redirection (cont.) command <&digit Use the file associated with file descriptor digit as standard input (file descriptor 0). command >&digit Use the file associated with file descriptor digit as standard output (file descriptor 1).

10. 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>&1 associates file descriptor 2 with the file currently associated with file descriptor 1. Examples: $ ( cc a.c 2>&1 ) > log.a $ command1 2>&1 | command2

11. 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.

12. 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.

13. Error Redirection Any command that produces error messages to stderr can have those messages redirected to another file. Overwrite2> Append 2>> Examples: $ cc a.c 2> another_file $ cc a.c 2>> another_file

14. 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

15. 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!

16. The if Construction Syntax: iflist A thenlist B [ eliflist C then list D ]... [ elselist E ] fi

17. The if Construction Examples: # multi-line if [ -d wu ] then ls –l wu fi # # single-line if [ -d wu ] ; then ls –l wu; fi

18. The case Construction Used for multi-way branching Syntax: case word in pattern [ | pattern ] )list A ;; pattern [ | pattern ])list B ;; pattern [ | pattern ])list N ;; esac

19. 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]*)

20. The while Construction Syntax: while list A do list 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.

21. The while Construction (cont.) Examples: #!/bin/sh # This is a comment line. trap "echo bye; exit" 2 while read A ; [ $A != q -a $A != Q ] do echo $A done exit

22. The until Construction Syntax: until list A do list B done 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.

23. The for Construction Syntax: 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.

24. The for Construction (cont.) Examples: #!/bin/sh # for AA in $* do echo $AA done exit

25. The trap Command Syntax: trap 'cmds' signo [signo...] Description: Perform cmds upon receipt of signal.

26. The trap Command (cont.) Example: $ cat myprog #!/bin/sh # shell script named : “myprog” trap 'echo bye; exit' 2 while true do echo hello done $ myprog hello ^Cbye

27. Define a function Syntax: name ( ) { list; }

28. Define a function Examples: #!/bin/sh # trap "echo bye; exit" 2 fn1 ( ) { for AA in $* do echo $AA done } fn1 tom bill john exit

29. 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.

30. 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.

31. 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"

32. Show variables env Reports the names and values of all environment variables. set Reports the names and values of all shell variables.

33. 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_is bob $ echo $MESSY /usr/man/man1/ls.1 $ more $MESSY

34. 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.

35. 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.

36. Parameter Substitution Examples: $ AA="Jonn" $ CC="" $ echo ${BB:-Tom} Tom $ echo ${CC:-Tom} Tom $ echo ${BB-Tom} Tom $ echo ${CC-Tom} * $ echo ${AA:-Tom} John $ echo ${BB:=Tom} Tom $ echo $BB Tom

37. 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

38. 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"

39. 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

40. 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.

41. 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” …

42. 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.

43. Special variables set by the shell Examples: $ echo $$ 7828 $ echo $! $ echo $? 0 $ echo $- ims $ echo $# 0

44. The shell variables used by the shell HOMEThe 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.

45. Positional parameters Positional parameters are used to refer the arguments list. $0the shell script name $1the first argument $* or $@all the arguments, starting with$1, are substituted (separated by SPACE characters). $#The number of arguments in decimal.

46. The read Command Syntax: read variable [variable...] Examples: #!/bin/sh # Shell Program # echo enter a sentence read A echo $A

47. Return status The shell variable ? holds the return status of the last command executed. zero - true or no error non-zero - false or error Examples: $ ls $ echo $? 0 $ cp Usage:cp f1 f20 cp [-r] f1... fn d1 $ echo $? 1 $ echo $? 0

48. The test Command Syntax: test expression or [ expression ] We can test three types of things: 1.Files 2.Integers 3.Strings

49. The test Command - File Tests Syntax: test -option filename or [ -option filename ] File test Options: rreadable wwritable xexecutable ddirectory fplain file ssize greater than 0

50. File Tests Examples: $ test -f new.jersey $ echo $? 0 $ [ -d new.jersey ] $ echo $? 1

51. The test Command - String Tests Syntax: test string1 = string2 test string1 != string2 test -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 ]

52. The test Command - String Tests Examples: $ X=abc $ [ "$X" = "abc" ] $ echo $? 0 $ [ "$X" != "abc" ] $ echo $? 1

53. The test Command - Numeric Tests Syntax: test number relation number or [ number relation number ] Relations: -ltLess than -leLess than or equal to -gtGreater than -geGreater than or equal to -eqEqual to -neNot equal to

54. The test Command - Numeric Tests Examples: $ x=3 $ [ $x -lt 7 ] $ echo $? 0

55. 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.