Week 3 #

Introduction to Shell #

More features available from man sh

Simple Commands #

4 general cases of commands

Built in commands: cd, ls etc.
User defined functions
Aliases eg. alias pls="sudo"
Commands that refer to the program name
- eg. tr command runs the tr program

Sequential List #

Multi-line commands in a single line

$ cd projects ; sort file1 | uniq # runs cd then runs sort with pipe
# equivalent to
$ cd projects
projects$ sort file1 | uniq

Grouping:
- Explicit: {grep foo file1 ; ls ; } > file2

Exit Codes #

0 for success, non-0 for failure

$ sort file1    # assume this works
$ echo $?       # prints `?` command
0
$ cd asdgkj
-bash: cd: asdgkj: No such file or directory
$ echo $?
127             # non-zero -> fail

Logical AND OR NOT #

$ mkdir foo && cd foo               # sequential unless one fails
$ mkdir foo1 || mkdir foo2 || exit  # runs sequential command if first one fails
$ ! mkdir foo                       # trivial (true to false, v.v)

Operator Precedence:

| highest
!
&& || - same precedence
; lowest

Boolean Conditions #

String comparisons:
```
[ s1 = s2 ] # `!=`, `<`, `>` also work
```

Number comparisons:

[ num1 -eq num2 ] # `-ne`,`-gt`,`-ge`,`-lt`,`-le` also work

Logical connectives
- -a: and
- -o: or
- -e: not

Conditionals and Iteratives #

if statement:

if list1 ; then
  list2
elif list 3 ; then
  list4
else
  list5
fi # end statement

while loop:

while list1 ; do
  list2 
done # end statement

for loop:

for var in word1 word2 word3 ; do
  echo $var
  mkdir $var
done # end statement

Filename Patters #

*: any string
?: matches one character
[nav]: matches ’n’, ‘a’, ‘v’
[0-9]: matches digit
[!0-9]: matches non-digit

eg.

for i in *.sh ; do # for all files in dir with ext .sh
  echo $i          # echo its name
done

Escaping and Quoting #

\: backslash
'': single quotes
"": double quotes

Use Cases:

Spaced files: cd "Onedrive - University of Toronto"
Regex for grep: grep ’<[a-z]*>’ *.html
Operators in test commands: [ ! ’(’ aaa ’<’ abc -o aaa ’>’ abc ’)’ ]

Variables #

Declaration:

var=69 
# no spaces
# if uninit, `echo $var` returns empty string (`echo ${var}` also works)
# uninitialized var is not neccessarily an empty string by default

Double Quotes

$ f=`Stupid Example.js`
$ ls $f   # equiv to `ls Stupid ; ls Example.js`
$ ls "$f" # equiv to `ls "Stupid Example.js"`
# double quotes and variables($) are 'special'

String Testing #

Non-Empty String: [ -n string ]
Empty String: [ -z string ]

Tricky:

# suppose {V} is an empty string
[ -n $V ]   # doesnt work (interpreted as `[ -n ]`)
[ -n "$V" ] # works!

Case Matching #

eg.

case "$var" in
  *.py)                   # Case python 
    rm "$var"           
    ;;
  *.c | *.sh | myscript)  # Case C, Shell, myscript
    echo w00t "$var"
    ;;
  *)                      # else
    echo meh "$var"
esac # end statement