Why Bourne Shell? A Bourne Shell Script. The UNIX Shell. Ken Wong Washington University. The Bourne Shell (CSE 422S)

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1 The Bourne Shell (CSE 422S) Ken Wong Washington University The UNIX Shell A shell is a command line interpreter» Translates commands typed at a terminal (or in a file)» Input is from the terminal (stdin) or file» User s /etc/passwd entry indicates the interactive shell Some Shells» Bourne Shell (sh): What I use for shell scripts» C-Shell, TC-Shell (csh, tcsh): Typical interactive shell» Bash Shell (bash, GNU Bourne-Again Shell ): Default Linux interactive shell Shell Script (Shell Program)» A file containing shell commands» Used heavily in system administration n Scripts in /etc/rc*.d/ are executed during system booting 2 -Ken Wong, June Ken Wong, June 2002 Why Bourne Shell? sh(1) (i.e., `man sh`) Csh Difficulties» File descriptor manipulation» Lack of command language orthogonality» Can only trap SIGINT Bourne Shell» Ubiquitous» Used in system-level scripts (e.g., /etc/rc*.d/)» But lacks many interactive features found in other shells» The Bash shell (bsh) is a popular alternative ATutorial on Scripting: 4 -Ken Wong, June 2002 A Bourne Shell Script 01 #!/bin/sh which interpreter 02 # Usage: t_proc.sh 03 usage ( ) { function definition 04 echo Usage: t_proc.sh ; exit 1; 05 } global variable value 06 doit ( ) { 07 echo ========== t_proc $C 1000 ; 08 t_proc $C 1000; 09 } 10 while true the command /bin/true; exit code = 0 11 do 12 echo n Timing menu end of case 13 read cmd; 14 case $cmd in 15 f fo for fork) echo do_fork: ; C= fork ; doit; ;; 16 e ex exe exec) echo do_execve: ; C= execve ; doit; ;; 17 s sh) echo do_sh: ; C= sh ; doit; ;; 18 q qu qui quit) break; ;; 19 *) echo *** Invalid Command $cmd; ;; 20 esac 21 done pattern matching Only 1 data type: string!!!

2 Overview Of sh sh is a macro processor that provides: backslash» Parameter Substitution: $Name (e.g., $X)» Command Substitution: `Command... ` (e.g., `pwd`)» File Name Generation: *? [...] (e.g., F="*") Before command execution, sh does substitutions:» Parameter Substitution; Command Substitution; Field Separator Interpretation n Field separators are defined by $IFS and by default include the space, tab and newline characters» File Name Generation A newline in a list acts like ; Characters with special meaning» ; & ( ) < > newline space tab Example 1 (Simple File Backup) Features: Copy a list of files to the directory./old/ Example mkdir OLD; # make OLD directory cp x.c y.c OLD; # copy files x.c, y.c to OLD Alternative # use file name generation mkdir OLD; cp [xy].c OLD; Shell Script #!/bin/sh # usage: bkup1 mkdir OLD; cp [xy].c OLD; 5 -Ken Wong, June Ken Wong, June 2002 As an argument to a shell command Make the file executable Then use the script Running The Script sh bkup1 chmod 755 bkup1 chmod ugo+x bkup1 chmod a+x bkup1./bkup #./ not needed if. is in PATH File Name Generation Used in file name generation * : Matches any string including the null string? : Matches any single character [...] : Matches any one of the enclosed characters» [a-z]: Matches any one of the characters between 'a' and 'z Examples» ls y*.c # File names beginning with y and ending with.c» ls y??.c # Same but restricted to 5-character names» ls y[ab][gh].c # yag.c, yah.c, ybg.c or ybh.c» echo y*.c 7 -Ken Wong, June Ken Wong, June 2002

3 9 -Ken Wong, June 2002 Input Output < File Use File as standard input (stdin)» Note: stdin is file descriptor (fd) 0 > File Use File as standard output (stdout, fd 1) << Word Read from the current file until end of file or an input line containing Word alone >> File Append stdout to File Digit1 >& Digit2 fd Digit2 = fd Digit1 (duplicate)» (e.g., wc foo.c > wc.out 2>&1) Command1 Command2 Create a pipeline» Connect stdout of Command1 to stdin of Command2 10 -Ken Wong, June 2002 A Better Script Purpose» Copy files to./old with a suffix of 0,..., 9 appended to the name. Usage: bkup File1 File2... Example: bkup x.c y.c z.c» Copy files x.c, y.c, and z.c to./old directory.» Append the single-digit suffix to the base filename, smallest digit first Method (Pseudo-Code) for each file F listed on the command line for each suffix S from 1 to 9 if file OLD/$F.$S does not exist Copy file $F to./old/$f.$s; break; 11 -Ken Wong, June 2002 A Backup Script 01 #!/bin/sh 02 # Usage: bkup2 File1 File FILES=$*; # all command-line args $1, $2, for F in $FILES 05 do 06 for S in do 08 D=OLD/$F.$S; 09 if test! -f $D; then # $D is not a regular file 10 echo "Copying $F to $D"; 11 cp $F $D; 12 break; 13 fi 14 done 15 done 12 -Ken Wong, June 2002 Parameter/Command Substitution Example 1» (Parameter Substitution) Example 2» (Command Substitution) WL= ; FL=`ls *.txt`; F=foo; # F is a variable cp $F.3 $F.4; cp $F.2 $F.3; cp $F.1 $F.2; cp $F.0 $F.1; # null string # evaluate command and assign # (can I use FL= *.txt ;)??? for F in $FL do W=`wc $F`; WL=$WL $W; done

4 Example 1 Example 2a After Substitution F=foo; # F is a variable cp foo.3 foo.4; cp foo.2 foo.3; cp foo.1 foo.2; cp foo.0 foo.1; WL= ; FL= foo.txt goo.txt ; # assuming only foo.txt and goo.txt for F in $FL do W=`wc $F`; WL=$WL $W; done Parameter Substitution ${Parameters} indicates parameter substitution» The result of substitution is the value of the parameter» e.g., $X X=foo echo $XX # output is null string echo ${X}X # output is foox Parameters» A Name: A sequence of letters, digits or underscores starting with a letter» A Number: $1 is the first command line argument, $2 is the second, etc.» Special Parameters: #? - $! 13 -Ken Wong, June Ken Wong, June Ken Wong, June 2002 Special Parameters $* Equivalent to "$1 $2..." $@ Equivalent to "$1" "$2",... $# Number of positional parameters $$ Process number of this shell $? Value returned by last executed command in decimal $! Process number of last backgrounded command $- Options supplied to shell on invocation or by set Environment Variables: HOME, PATH, IFS, etc.» `export` will list automatically exportable names 16 -Ken Wong, June 2002 Quoting Quoting is indicated by \, '...', or "..."» Double quotes allow parameter and command substitution Strict Quoting» Single Character: Backslash (\)» Multiple Characters: Single Quotes ('...') Loose Quoting» Double Quotes (")» Use backslash (\) to quote \ ' " $ What if $F is the null string??» if test z $F then... fi» The Solution n if test z $F then... fi # loose quote

5 Debugging Use echo for displaying Debug control structure first» Put echo commands where blocks would normally go» Systematically replace echo commands with actual statements set -v: Lines are displayed as they are read set -x: Each command is displayed AFTER parameter substitution set -: Turns off both -v and -x flags Syntax Summary (1) CommandList ::= AndOr CommandList ; CommandList & CommandList ; AndOr CommandList & AndOr AndOr ::= PipeLine AndOr && PipeLine AndOr PipeLine PipeLine ::= Command Pipeline Command Command ::= SimpleCommand ( CommandList ) { CommandList } for Name do CommandList done for Name [in Word]... do CommandList done while CommandList do CommandList done until CommandList do CommandList done case Word in CasePart... esac if CommandList then CommandList ElsePart fi 17 -Ken Wong, June Ken Wong, June 2002 Syntax Summary (2) SimpleCommand ::= Item SimpleCommand Item Item ::= Word InputOutput Name = Value Word ::= A sequence of non-blank characters InputOutput ::= > Filename < Filename >> Word << Word Name ::= a sequence of letters, digits or underscores starting with a letter 19 -Ken Wong, June 2002