Lecture 4. Log into Linux Reminder: Homework 1 due today, 4:30pm Homework 2 out, due next Tuesday Project 1 out, due next Thursday Questions?

Transcription

1 Lecture 4 Log into Linux Reminder: Homework 1 due today, 4:30pm Homework 2 out, due next Tuesday Project 1 out, due next Thursday Questions? Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 1

2 Outline More BASH programming Command Substitution Arithmetic Substitution Additional Useful Commands Grouping Exercises Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 2

3 Command Substitution Command substitution allows the standard output of a command to replace the command name. There are two forms: `COMMAND` or $ (COMMAND). rm `cat filelist` # rm files in a list txtfiles=$(ls *.txt) # capture ls display userpass=$(grep i '^ar63:' /etc/passwd) passwd=$(cat /etc/passwd) # capture file Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 3

4 Arithmetic Substitution Arithmetic evaluation uses either let expr or ((expr)). Substitution uses $((expr)). b=12 let a=$b*3 # a is 36 ((a = $b * 3)) # alternative to let val=$(($b**2)) # val is 144 count=0 while [ $count le 10 ] do # do stuff here count=$(($count + 1)) # or ((count++)) done Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 4

5 HERE Documents HERE documents are a special form of input redirection that read input from the script. cat <<EOTEXT Hi there! How are you? EOTEXT # Display multiline message # Example with variable substitution sftp $user@csserver <<SFTPINPUT cd $remotedir get $file bye SFTPINPUT Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 5

6 Functions and Aliases Functions only have local scope by default, but can be exported. error() { echo $* > /dev/stderr ; } You can create aliases for commands too: alias ls='ls color=tty' # alias for ls alias ll='ls l color=tty' alias rm='rm i' # force rm to prompt alias # display aliases \ls # run unaliased ls Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 6

7 BASH Startup Bash shells can be interactive or non-interactive a login or non-login shell. A login shell reads and executes commands from /etc/profile and then one of (in order) either ~/.bash_profile, ~/.bash_login, or ~/.profile. A non-login interactive shell reads commands only from ~/.bashrc. Set exported environment variables in ~/.bash_profile. Define functions and aliases in ~/.bashrc. Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 7

8 Useful Commands find will locate files and directories that satisfy certain criteria. It recurses through directories. Syntax: find <dir> <matching criteria> <actions> Lots of matching criteria, e.g. mtime n, type c, user u, name <regex>. (See man page.) All must match unless use o (OR) explicitly. Some built-in actions, e.g. print, ls. General exec <cmd> {} \; (where optional '{}' represents the current match). Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 8

9 Useful Commands find examples: find. name '*.txt' # all *.txt in current dir find. mtime 1 # modified in last day # Use grep to search for strings # print is required here find. exec grep iq "dh27" {} \; print # Change perms on all files find. type f exec chmod 644 {} \; Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 9

10 Useful Commands grep search for files containing regular expressions (similar to the wildcards, see BLP pages 66-67). xargs takes filenames from standard input and feeds the names to a command. ls xargs p l gzip expr is an all-purpose expression evaluator a=$(expr 5 + 3) # arithmetic b=$(expr length $filename) # string length Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 10

11 Useful Commands exec replaces the current process with a new one. It is often used at the end of a script. You can also use it from within a script to redirect subsequent standard output exec vi exec > output.txt echo hello there # "become" vi # sent to output.txt bc is an arbitrary precision calculator. # get pi to 40 places pi=$(echo 'scale=40; 4*a(1)' bc l) Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 11

12 Useful Commands export makes a shell variable available in subshells. export myhome=/home/hwang read will read a line of input from standard input. read line read # user input is $line # read line and throw away printf can give fancier output than echo. printf %8.4f\t\t%s\n $val $myhome Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 12

13 Grouping Use the following code to read lines of input from a file: while read line do # Do something with $line done < inputfile > outputfile Input (and output) is redirected for all commands inside the body of the while-loop. Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 13

14 Grouping You can also redirect input and/or output by using braces to group commands: { read x; read y; } < input Use space between the command group and the braces. The final semicolon is required. Alternatively use this form: { read x read y } < input Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 14

15 Grouping You can use ( ) for grouping too. With ( ), the command group is executed in a subshell (another bash process), so if you set or change any variables those changes will not be seen in the parent process. myvar=abc { myvar=xyz; } echo $myvar (myvar=mno) echo $myvar # Will display xyz # Will display xyz Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 15

16 In-class Exercises Write a shell script named lastfirst that takes a single filename argument and writes the file to standard output in complete reverse order (last line, last character first). Hint: Try man -k reverse to find relevant utilities. Write appropriate error messages if an incorrect number of arguments is used or if the input file does not exist. Exit with appropriate status. Read from standard input if no arguments are given. Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 16

17 In-class Exercises Write a shell script named sortusers1 that displays every other username (from /etc/passwd) in alphabetical order. Display the username only. Write sortusers2 that displays every other username sorted by user id. Display the username only. Write a shell script named revargs that displays its arguments in reverse order (eval may be useful here). Tuesday, September 7 CS 375 UNIX System Programming - Lecture 4 17