Shell Programming (ch 10)

Transcription

1 Vim Commands vim filename Shell Programming (ch 10) IT244 - Introduction to Linux / Unix Instructor: Bo Sheng Add contents: i/a Back to command mode: ESC Save the file: :w Delete: x Quit: :q 1 2 The order of executing The order of executing ~it244/it244/scripts/if1 3 4 The order of executing Built-in utility test man test (-eq) ~it244/it244/scripts/chkargs The order of executing Read arguments ~it244/it244/scripts/read3args $#: the number of arguments exit 1: terminate with a status code

2 The order of executing test (-f) and arguments ~it244/it244/scripts/is_ordfile The order of executing test ([ ]) and arguments ~it244/it244/scripts/chkargs2 Other options (Table 10-1) White spaces [ $# -eq 0 ] Output error message: echo 1>&2 7 8 The order of executing..else The order of executing..else Read arguments ( shift ) ~it244/it244/scripts/shift2args ( $@ ) ~it244/it244/scripts/allargs 9 10 The order of executing..else Optional arguments ~it244/it244/scripts/out if..then..elif

3 if..then..elif Example: Compare 3 input words and find the match (word1, word2, word3) 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 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 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 for loop for loop-index in argument-list ~it244/it244/scripts/fruit ~it244/it244/scripts/dirfiles for loop A range for argument-list {1..10}, {a..z} for loop-index in range ~it244/it244/scripts/for_range ~it244/it244/scripts/for_range2 for loop Repeat operations ~it244/it244/scripts/for_print Nested loops Example ~it244/it244/scripts/for_print

4 for loop for loop-index (in while loop while test-command ~it244/it244/scripts/for_test while loop while test-command : ~it244/it244/scripts/count Arithmetic evaluation: ((a=b+2)) ~it244/it244/scripts/for_while_print until loop until test-command until loop until test-command : ~it244/it244/scripts/guessnumber ~it244/it244/scripts/guessnumber2 break and continue break: jump out of the loop continue: proceed to Example: ~it244/it244/scripts/brk

5 select select varname [in arg ] ~it244/it244/scripts/select1 select select varname [in arg ] PS3: select prompt $REPLY: index of the selected item Terminate: exit, break : ~it244/it244/scripts/select Parameters and Variables Special parameters $#: number of arguments $1-$n: arguments $0: name of the script : ~it244/it244/scripts/abc./abc ~it244/it244/scripts/abc basename: ~it244/it244/scripts/abc2 basename ~it244/it244/scripts/abc Parameters and Variables Special parameters $$: PID number echo $$ ps : ~it244/it244/scripts/id2./id2 echo $$ Parameters and Variables Special parameters $?: exit status ls; echo $? ls xyz; echo $? cat yyy; echo $? : ~it244/it244/scripts/es./es echo $? echo $? Parameters and Variables Initialize command-line arguments set arg1 arg2 ~it244/it244/scripts/set_it date ~it244/it244/scripts/dateset

6 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 for i in * set -- $(ls -l "$i") echo $i: $5 bytes :09 abc :10 abc :49 bb :49 bb2 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 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 File Descriptors Example: ~it244/it244/scripts/exec_read exec 3<"$1" while read <&3 myline echo "*** $myline ***" 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) File Descriptors Example: ~it244/it244/scripts/exec_read exec 3<"$1" while read <&3 myline echo "*** $myline ***" 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 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 *

7 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 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 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<&- trap : catch a signal Signal: a report about a condition (Table 10-5) kill -l, trap -l, man 7 signal trap [ ] [signal] is not required but highly suggested : ~it244/it244/scripts/inter trap : catch a signal trap signal: ignore the signal 2: CTRL+C, 20: CTRL+Z ~it244/it244/scripts/locktty stty (input secret/passwd) ~it244/it244/scripts/locktty2 Multiple traps ~it244/it244/scripts/cleanfile kill : abort a process kill [-signal] PID The KILL signal (9) kill -9 %

8 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)) 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 $? 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 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 $? Logical operators &&, Short-circuiting: Cond1 && Cond2: es not evaluate Cond2 if Cond1 is false Cond1 Cond2: es 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 8