Introduction to Shell Programming in Linux/Unix with Vim and Control Flow Commands
Explore the basics of shell programming in Linux/Unix, including Vim commands for editing files, control flow commands like if-then statements, and examples of executing commands with built-in utilities. Learn how to handle arguments, read input, and implement conditional logic in scripts.
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Shell Programming (ch 10) IT244 - Introduction to Linux / Unix Instructor: Bo Sheng 1
Vim Commands vim filename Add contents: Back to command mode: Save the file: Delete: Quit: i/a ESC :w x :q 2
Control Flow The order of executing commands if..then 3
Control Flow The order of executing commands if..then Example ~it244/it244/scripts/if1 4
Control Flow The order of executing commands if..then Built-in utility test man test Example (-eq) ~it244/it244/scripts/chkargs $#: the number of arguments exit 1: terminate with a status code 1 5
Control Flow The order of executing commands if..then Read arguments Example ~it244/it244/scripts/read3args 6
Control Flow The order of executing commands if..then test (-f) and arguments Example ~it244/it244/scripts/is_ordfile Other options (Table 10-1) 7
Control Flow The order of executing commands if..then test ([ ]) and arguments Example ~it244/it244/scripts/chkargs2 White spaces [ $# -eq 0 ] Output error message: echo 1>&2 8
Control Flow The order of executing commands if..then..else 9
Control Flow The order of executing commands if..then..else Read arguments Example ( shift ) ~it244/it244/scripts/shift2args Example ( $@ ) ~it244/it244/scripts/allargs 10
Control Flow The order of executing commands if..then..else Optional arguments Example ~it244/it244/scripts/out 11
Control Structure if..then..elif 12
Control Structure if..then..elif Example: Compare 3 input words and find the match (word1, word2, word3) 1 & 2 & 3, 1 & 2, 1 & 3, 2 & 3, no match 13
Control Structure if..then..elif Example: Compare 3 input words and find the match (word1, word2, word3) if word1=word2 =word3 then elif word1=word2 then elif word1=word3 then elif word2=word3 then else fi 1 & 2 & 3 1 & 2 1 & 3 2 & 2 No match 14
Control Structure if..then..elif Example: Compare 3 input words and find the match (word1, word2, word3) Combine two conditions test condition1 -a condition2 test condition1 -o condition2 ~it244/it244/scripts/if3 15
Control Structure for loop for loop-index in argument-list do commands done Example ~it244/it244/scripts/fruit ~it244/it244/scripts/dirfiles 16
Control Structure for loop A range for argument-list {1..10}, {a..z} for loop-index in range do commands done Example ~it244/it244/scripts/for_range ~it244/it244/scripts/for_range2 17
Control Structure for loop Repeat operations Example ~it244/it244/scripts/for_print Nested loops Example ~it244/it244/scripts/for_print2 18
Control Structure for loop for loop-index (in $@) do commands done Example ~it244/it244/scripts/for_test 19
Control Structure while loop while test-command do commands done 20
Control Structure while loop while test-command do commands done Example: ~it244/it244/scripts/count Arithmetic evaluation: ((a=b+2)) ~it244/it244/scripts/for_while_print 21
Control Structure until loop until test-command do commands done 22
Control Structure until loop until test-command do commands done Example: ~it244/it244/scripts/guessnumber ~it244/it244/scripts/guessnumber2 23
Control Structure break and continue break: jump out of the loop continue: proceed to done Example: ~it244/it244/scripts/brk 24
Control Structure select select varname [in arg ] do commands done Example ~it244/it244/scripts/select1 25
Control Structure select select varname [in arg ] do commands done PS3: select prompt $REPLY: index of the selected item Terminate: exit, break Example: ~it244/it244/scripts/select2 26
Parameters and Variables Special parameters $#: number of arguments $1-$n: arguments $0: name of the script Example: ~it244/it244/scripts/abc ./abc ~it244/it244/scripts/abc basename: ~it244/it244/scripts/abc2 basename ~it244/it244/scripts/abc 27
Parameters and Variables Special parameters $$: PID number echo $$ ps Example: ~it244/it244/scripts/id2 ./id2 echo $$ 28
Parameters and Variables Special parameters $?: exit status ls; echo $? ls xyz; echo $? cat yyy; echo $? Example: ~it244/it244/scripts/es ./es echo $? echo $? 29
Parameters and Variables Initialize command-line arguments set arg1 arg2 ~it244/it244/scripts/set_it date ~it244/it244/scripts/dateset 30
Some More Examples Get file sizes ~/it244/it244/scripts/filesize $ls -l -rwxr-xr-x 1 it244 libuuid -rwxr-xr-x 1 it244 libuuid -rwxr-xr-x 1 it244 libuuid -rwxr-xr-x 1 it244 libuuid 50 2010-11-04 00:09 abc 62 2010-11-04 00:10 abc2 79 2010-11-04 00:49 bb1 79 2010-11-04 00:49 bb2 for i in * do set -- $(ls -l "$i") echo $i: $5 bytes done 31
File Descriptors An index number of a open file 3 system file descriptors 0(std input), 1(std output), 2(std err) Create a file descriptor exec n > outfile exec m < infile exec n<>file exec n>>outfile 32
File Descriptors Duplicate a file descriptor exec n<&m exec n>&m Close a file descriptor exec n<&- Example: ~it244/it244/scripts/exec_read ~it244/it244/scripts/exec_write 33
File Descriptors Example: ~it244/it244/scripts/exec_read exec 3<"$1" while read <&3 myline do echo "*** $myline ***" done File descriptor is 3 ($1 is the first argument) &3 represents the input file in the program (a pointer) read <&3 myline read a line from the file and assign it to variable myline (pointer &3 moves to the next line) 34
File Descriptors Example: ~it244/it244/scripts/exec_read exec 3<"$1" while read <&3 myline do echo "*** $myline ***" done Command as a condition True: if the exit status is 0 (successfully executed) False: if the exit status > 0 (sth went wrong) read gets an error when EOF is reached 35
Built-in Commands Do not fork a new process type cat echo who if read user input read firstline echo $firstline ~it244/it244/scripts/read1 ~it244/it244/scripts/read1_no_quote type in this is * 36
Built-in Commands read user input REPLY : default variable for read ~it244/it244/scripts/read1a -p option: with prompt read -p Enter a command: cmd ~it244/it244/scripts/read2 read multiple variables ~it244/it244/scripts/read3 type in this is sth else 37
Built-in Commands read user input read from files read line1 < names echo $line1 read line2 < names echo $line2 (read line1; echo $line1; read line2; echo $line2) < names 38
Built-in Commands read user input read from files ~it244/it244/scripts/readfile ./readfile < names -u option exec 3<names read u3 line1 (read <&3 line1) echo $line1 read u3 line2 echo $line2 exec 3<&- 39
Built-in Commands trap : catch a signal Signal: a report about a condition (Table 10-5) kill -l, trap -l, man 7 signal trap [ commands ] [signal] is not required but highly suggested Example: ~it244/it244/scripts/inter 40
Built-in Commands trap : catch a signal trap signal: ignore the signal Example 2: CTRL+C, 20: CTRL+Z ~it244/it244/scripts/locktty stty (input secret/passwd) ~it244/it244/scripts/locktty2 Multiple traps ~it244/it244/scripts/cleanfile 41
Built-in Commands kill : abort a process kill [-signal] PID The KILL signal (9) kill -9 %1 42
Expressions Arithmetic evaluation a=10; b=5; let a=a*10+b; echo $a No path expansion (*) ((a=a*10+b)) let a=a+1 b=a*b ((a=a+1, b=a*b)) 43
Expressions Logic expression (( )), [[ ]] In [[]], must precede a variable name with $ >, <, -lt, -gt, && (and), || (or) ~it244/it244/scripts/age2 String comparison [[ aa < bbb ]] [[ artist = a* ]]; echo $? [[ a* = artist ]]; echo $? 44
Expressions Arithmetic operators +,-,*,/,% a=5;b=3;echo $((a%b)) var++, var--, ++var, --var echo $((a++)); echo $a echo $((--a)); echo $a echo $((a+++3)) echo $((a++---b)) ~it244/it244/scripts/printstar 45
Expressions Logical operators true, false true; echo $?; false; echo $? !, ==, != ((1==1)); echo $?; echo $((1==1)) ((1!=1)); echo $?; echo $((1!=1)) &&, || true && false; echo $? true || false; echo $? ls l testop && cat testop; echo $? ls l testop || cat testop; echo $? 46
Expressions Logical operators &&, || Short-circuiting: Cond1 && Cond2: does not evaluate Cond2 if Cond1 is false Cond1 || Cond2: does not evaluate Cond2 if Cond1 is true Example mkdir bkup && cp testop bkup rm bkup/* mkdir bkup && cp testop bkup; ls bkup mkdir bkup || echo mkdir failed a=1;b=2;echo $((a||b++));echo $b 47
