CSc 352Shell Scripts
Saumya DebrayDept. of Computer Science
The University of Arizona, [email protected]
What is a shell script?
• A shell script is a list of commands to be run by a shell– basically a program, but using shell commands instead of C
or Java statements
• Why?– automate repetitious tasks
• e.g.: testing a program on a large set of test inputs
– package up commonly executed command sequences– create our own commands
2
Creating and executing a shell script
1. Create a file, say “foo”, containing the commands to be executed by the script
2. Execute the script:– invoke the appropriate shell with foo as argument, e.g.:
• bash foo• csh foo
– chmod a+x foo ./foo
3
A simple example
• a shell script to print “hello world!”
4
the “echo” command, as executed from the command line
contents of the shell script file “hello_world.script”
this script behaves the same for a variety of different shells
with appropriate permissions, the script can be executed directly
Syntax issues across shells
Different shells have slightly different syntax rules– the same script may not work on all shells:
5
script contents
different behaviors
Which shell runs my script?
6
my current shell is csh
the script executes as expected when csh is invoked explicitly
if not invoked explicitly, the default shell /bin/sh is used
Solution: specify shell in the script
7
#!pathToShell on first line specifies which shell to use
executes as expected when invoked in csh
executes as expected when invoked in bash
Specifying the shell interpreter
• ‘#!’ at the beginning of a file indicates that it is a script
• The rest of the first line specifies (absolute) path to the interpreter
• What if the interpreter is at a different location on a different system?– use /usr/bin/env to improve portability:
8
#!/bin/csh
#!/usr/bin/env csh
Variables
• Variables different than in C or Java:– don’t have to be declared in advance– they are untyped: the same variable can hold an integer
value or a string
• Syntax for using variables (bash):– Defining the value of a variable X:
X=value– Using the variable X:
$X or ${X}
9
Example
10
variable definitions
variable uses
undefined variable evaluates to empty string
Guarding against uninitialized variables
11
-u : “exit if using an unitialized variable”
-u :• works if we specify /bin/bash• but not with /usr/bin/env
Guarding against uninitialized variables
12
Bash scripts: whitespace
13
without whitespace the command works as expected
with whitespace • weird error message• variable value not set as expected
Different kinds of quotes
14
back quotes: evaluates to the output of the enclosed commands
single quotes: variables not expanded
double quotes: variables inside the quotes are expanded
A simple scripting problem
• Problem:– Iterate over the files in a given directory:– for each file, express its size in human-readable form (i.e.,
using KB and MB where appropriate):• give 2 decimal places for MB sizes; 1 decimal place for KB sizes• functionality similar to using “ls –lh”
• Requires:– list files in a directory [valid directory? if-then-else]– iterate over these files– obtain the size of each file– perform arithmetic [KB/MB decision if-then-else]
15
The test command
• Used to guide control flow in conditionals and loops• Syntax:
test operand1 operator operand2
test operand– often abbreviated to:
[ operand1 operator operand2 ]
[ operand ]– see “man test” for a list of operators
• Returns a nonzero status if the test is true; zero o/w
16
Example: test
17
Example: if-then-else
18
if-then-else command
Example: if-then-else
19
need a ‘;’ between the test expression and ‘then’
Example: if-then-else
20
Alternative formulation of test
21
For Loops
• Syntax 1:
for X in word ; docmds
done
Convenient for iterating over lists
• Syntax 2:
for (( e1; e2; e3 )) ; do cmds
done
Convenient for iteration involving arithmetic expressions
22
For Loops: Example 1
23
For Loops: Example 2
24
Iterating over lists: pitfalls
25
print out the first couple of lines of a file listing
!
Iterating over lists
26
use quotes to prevent splitting at whitespace
Note: the loop iterates only once – not once per file!
Arithmetic
27
arithmetic evaluation doesn’t happen automatically; must be specified explicitly
get file size (see “man stat” for other file properties)
Arithmetic
28
arithmetic evaluation:$(( expr ))
integer arithmetic only!
Arithmetic
29
get bc to do the floating point math for us
Command-line arguments
• Arguments at known positions, e.g., 1st, 2nd, ..., can be accessed using $1, $2, …
• $0 = the name of the script file
• Total no. of arguments: $#
• However, we can’t get to the ith argument this way– $i value of variable i
30
Example
31
Iterating over the argument list
32
Getting at the entire list of arguments
33
Iterating over the argument list
34
Placement of quotes matters:
all of the arguments are seen as a single word
Iterating over the argument list
35
Placement of quotes matters:
Arguments with whitespace
• $@ is similar to $*, but each argument is a quoted string– no interpretation or expansion– allows us to properly access arguments that may contain
whitespace– needs to be quoted “…”
36
Arguments with whitespace
37
Debugging
38
Debugging: -v
39
prints shell input lines as they are read
Debugging: -v
40
these are the lines from the script that are being executed
Note that variable values etc. are not being shown
Debugging: -x
41
prints out command traces before they are executed
Debugging: -x
42
variable values, command arguments are printed out
Debugging: summary
• Debugging can be done by observing what commends get executed– “set –v”
• prints out shell commands as they are read in• does not print out runtime variable values
– “set –x”• prints out a trace of the command before it is actually executed• prints out variable and argument values
• Debugging can be turned off using “set +v”, “set +x”– useful for localizing which parts of the script are observed
43
String manipulation
• shell scripts often involve string manipulation– e.g., to pattern match on file names, create new files
based on existing files, etc.
• bash gives extensive facilities for string matching and manipulation– see “man expr”
44
A sample problem
Write a bash script to convert Unix make files to “mymake” make files
45
unix make@target a : b.o c.o d.o@cmd gcc *.o –o a
@target b.o : b.c b.h@cmd gcc –Wall –c b.c
a : b.o c.o d.ogcc *.o –o a
b.o : b.c b.hgcc –Wall –c b.c
mymake
Attempt 1
• Ignore definitions– assume make file does not have stuff like “CC = gcc”
• Assume – user has added whitespace where necessary
• e.g.: on either side of ‘ : ’ in rules
– all dependencies have been made explicit
• Issues:– need to process a line at a time
• lines may contain whitespace
– need to check whether a line starts with a ‘\t’
46
Attempt 1
47
Attempt 1
48
IFS = “Internal Field Separator”used by bash to split the input into words
the new definition of IFS means that the input is now split at newlines
identify lines that begin with a tab
Attempt 1
49
Attempt 2
• relax assumption about whitespace around :– want:
• input: “a.o: a.c b.h c.h”• output: “@target a.o : a.c b.h c.h”
• Issue:– replace ‘:’ by ‘ : ’
• bash offers two replacement operators:– ${str/substr/replacement}
• replaces the first occurrence of substr in str with replacement
– ${str//substr/replacement}• replaces all occurrences of substr in str with replacement
50
Attempt 2
51
Attempt 2
52
Attempt 2
53
Only the first ‘:’ had whitespace inserted
Attempt 2
54
“replace all occurrences”
Attempt 2
55
Attempt 3
• Relax assumption about definitions– allow “CC=gcc”, “CFLAGS= –Wall –g –c”– assume no whitespace around ‘=‘
• Issues:– identify definitions, i.e., lines that contain a ‘=‘– extract LHS, RHS of the definition– replace every occurrence of LHS by RHS
56
Attempt 3
57
if the line contains a ‘=‘ …
figure out where it occurs
LHS = substring from position 0 to position of ‘=‘
RHS = substring after position of ‘=‘ to end
substitute RHS for every occurrence of LHS
Attempt 3
58
Attempt 3
59
Oops!
Attempt 3
60
Attempt 3
61
