Comprehensive Guide to Basic Linux Commands and Shell Scripting
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This guide covers fundamental Linux commands and utilities necessary for the final exam preparation. Learn about file management using commands like `ls`, `cat`, `echo`, and understand file permissions with `chmod`. Dive into shell scripting concepts, including executing commands, managing foreground and background jobs, and utilizing conditionals and loops for control flow. Additionally, this guide touches on advanced text-processing tools such as Perl, AWK, and SED for efficient data handling. Perfect for students preparing for exams on basic utilities and scripting.
Comprehensive Guide to Basic Linux Commands and Shell Scripting
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Final Exam • May 17th • 3:00~4:00pm • S-3-143 • Same types of questions as in mid-term
Basic Utilities • ls, cat, echo • ls -l • e.g., regular file or directory, permissions, file size • ls -a • cat file1 file2 file3 … • echo “hello world” > file • echo $VAR, echo hello* • echo –n “hello world”
Basic Utilities • more, less, head, tail • head -5 file • tail -f file • pwd, cd • cp, mv, rm, mkdir, rmdir • ‘-i’ option • ‘-r’ option: • rm –r dir1, cp –r dir1 dir2 • rmdiran empty directory
Basic Utilities • sort, wc, grep, |(pipe) • wc -l file, wc –c file • grep “keyword” file • grep -v • sort linux | head -5 • command1 | command2 • command1 > temp • command2 < temp • rm temp
Filename and Path • Hierarchical tree structure • Absolute path name • ls /home/it244/it244 • Relative path name • cd /home/it244 • ls it244 • ‘.’ ‘..’ ‘~’ • ‘*’ ‘?’ • echo hello*, echo ./.?a*
Access Permissions • r, w, x (chmod) • chmoda+rw file • chmod a=rx file • chmod 644 file • Execute permission of a directory • A directory is a table of files (filename:meta) • Executable means searchable
Access Permissions • Operations on book/book1 • ls book/book1 • book ( x ) • cat book/book1 • book1 ( r ), book ( x ) • rm book/book1 • book ( xw ), book1 ( w )
Links • Hard link and soft link • Hard link cannot refer to a directory • Soft link can point to a nonexistent file $ ln -s hello hello.sl $ rm hello $ echo “new” > hello $ cat hello.sl new
Execute Commands • Foreground and background jobs • command &, CTRL+Z • difference between a suspended job and a background job, PID • jobs, ps, kill, bg, fg • Group commands • (cmd1; cmd2; cmd3)
Redirection • Standard input, output, error output • cat file1 file2 > out 2>err • cat file1 file2 &> out • cat file1 file2 > out 2>&1 • Redirected output will not be displayed on the screen.
Variables • System variables • $PATH, $PWD, $HOME • User defined variables • VAR=hello • export VAR=hello • declare; declare -x
Variables • New shell for executing a script $cat script1 echo $VAR VAR=new echo $VAR $VAR=old;./script1;echo $VAR $VAR=new ./script1;echo $VAR $export VAR=old;./script1;echo $VAR
Expansion/ Quote • Types • echo hello*; echo ~ • echo $VAR • echo $(pwd) • echo $((1+1)) • VAR=hello* • echo $VAR • Single quote suspends all expansions
Control Flow if..then..fi, if..then..else..fi, while test-command do commands done for loop-index in argument-list do commands done
Control Flow • Conditions • test $# -eq 0 • [ -f “$1” ] • [ condition1 –a condition2 ], -o • [[ && ]], (( || )) • Exit status $? • if [ read <file $line ] …
Expressions • Arithmetic • a=5;b=3;echo $((a%b)) • echo $((a+++3)) • Logic • cat abc && ls • echo $?
Control Flow • Boolean operators
Control Flow • Boolean operators • Short circuiting $((a=2, b=2)) $((a++ || b--)) $echo $a $b $((--a>2 || b++)) $echo $a $b
Control Flow • Boolean operators • Short circuiting (($#==2)) || echo "Usage:require two arguments” mkdir $1 && cp file1 $1
Variables • Arguments • $1-$n, $0, $#, $@ • set set a 123 hello echo $1 $2 $3 set $(date) • shift echo $@ shift echo $@ • $$, $?
File I/O • exec exec 3 < infile exec 4 > outfile • read • read -p “Enter sth: ” input1 input2 • read <&3 line, read –u3 line • Determine the end of a file • $line has no value • ‘read’ returns false • Write to a file • echo “abc” >&4
Expressions • Arithmetic • a=5;b=3;echo $((a%b)) • echo $((a+++3)) • Logic • cat abc && ls • echo $?
Run a Program • Perl • perl simple.pl • perl –e ‘print “hi\n”’ • AWK • gawk -f simple file • gawk ‘/chevy/ {print}’ file • SED • sed -f simple file • sed ‘/line/ p’ file Stand-alone scripts #!/usr/bin/perl #!/usr/bin/gawk -f #!/bin/sed -f
Patterns • AWK • awk ‘/chevy/ {print}’ cars • gawk ‘$1 ~ /h/’ cars • gawk ‘$5 > 2000 && $5<6000’ cars • SED • sed -n ‘3,6 p’ lines • sed–n ‘s/line/sentence/w temp’ lines
File I/O in perl • Read open ($infile, '<', “filename”); while ($line=<$infile>){ print $line; } $line=<> • Write $filename="./log"; open($outfile,'>',$filename); print $outfile "Hello world!\n";
Questions If we execute the following sequence of commands in order, what are the values of variables a, b and the output AFTER executing each line? $((a=1, b=2)) $echo $((a-- + b++))a=_0_, b=_3_, output=_3_ $echo $((a++ + ++b)) a=_1_, b=_4_, output=_4_
Questions • Read the following shell script and explain its function. $ cat script1 #!/bin/bash count=0 for i in * then ((count++)) fi echo $count
Questions • Read the following shell script and answer the questions $cat script1 #!/bin/bash exec 3<$1 exec 4<$2 exec 5>$3 while read line <&3 do echo “$line” >&5 done while read line <&4 do echo “$line” >&5 done • Q1. The script requires 3 filenames as arguments, e.g., • ./script1 file1 file2 file3 • Which files are input files and which are output files? • Q2. Inside the while-loop, why variable “$line” has to • be enclosed by double quotes?
