if Statementsif ... then Constructtesttest operatorstest Operators for Comparing Numberstest String OperatorstestAre there any questions before I begin?
$ history 5
540 cat output.txt
541 echo "Go Red Sox" > output.txt
542 cat output.txt
543 echo foo
544 history 5
$ !543
echo foo
foo
$ alias ll="ls -l" $ ll total 2 lrwxrwxrwx 1 it244gh man 34 Sep 6 21:09 it244 -> /courses/it244/s19/ghoffman/it244gh drwxr-xr-x 2 it244gh ugrad 512 Oct 27 09:16 work
$ touch foo{1,2,3,4,5}.txt $ ls foo1.txt foo2.txt foo3.txt foo4.txt foo5.txt
$ touch {a,ab,abc}.txt $ ls abc.txt ab.txt a.txt
$ echo ~ /home/ghoffman
$ echo ~it244gh /home/it244gh
$ echo $SHELL /bin/bash $ echo $? 0
$ echo 5 + 4 5 + 4
$ echo $(( 5 + 4 )) 9
$ echo $a $b 5 3$ echo $(($a * $b)) 15
$ echo $(( a * b))
15
$(COMMAND)
$ today=$(date) $ echo $today Tue Oct 25 17:00:07 EDT 2011
$ args.sh $PATH 1 args: [/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin:/usr/games:/usr/local/games:/snap/bin:/usr/lib/oracle/12.1/client64/bin:/home/ghoffman/bin/shell:/home/ghoffman/bin/python/umb:/home/ghoffman/bin:/home/ghoffman/bin/shell:/home/ghoffman/bin/python/umb] $ IFS=: $ args.sh $PATH 15 args: [/usr/local/sbin] [/usr/local/bin] [/usr/sbin] [/usr/bin] [/sbin] [/bin] [/usr/games] [/usr/local/games] [/snap/bin] [/usr/lib/oracle/12.1/client64/bin] [/home/ghoffman/bin/shell] [/home/ghoffman/bin/python/umb] [/home/ghoffman/bin] [/home/ghoffman/bin/shell] [/home/ghoffman/bin/python/umb]
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if Statementsif statementif statement, a certain segment of code is executed ...if statements more complicatedif ... then Constructif ... then construction
if TEST_COMMAND
then
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COMMAND_2
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fi is if spelled backwardsif ...then and fi are run
if statement
$ cat foo_in_dir.sh
#! /bin/bash
# prompts for a directory and creates foo.txt there
read -p 'Directory: ' dir
if cd $dir
then
touch foo.txt
fi
if statement then and fithen on the same line as ifthen
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then and fiif statement might seem strangecd to go to that directorytestif statement testtest takes a
boolean expression
as an argument
test is a
built-intest is a kludgetest operatorstest can be used to check for a number of different conditionstest is much more complicatedtest has five different categories of operators
test Operators for Comparing Numbers| Operator | Result |
|---|---|
| -eq | True if the numbers are equal |
| -ne | True if the numbers are not equal |
| -ge | True if the first number is greater than, or equal to, the second |
| -gt | True if the first number is greater than the second, the second |
| -le | True if the first number is less than, or equal to, the second, the second |
| -lt | True if the first number is less than the second, the second |
echo ? to see
the exit status
$a=1; b=2 $ test $a -gt $b; echo $? # is a greater than b? 1 $ test $a -ge $b; echo $? # is a greater than or equal to b? 1 $ test $a -lt $b; echo $? # is a less than b? 0 $ test $a -le $b; echo $? # is a less than or equal to b? 0 $ test $a -eq $b; echo $? # is a equal to b? 1 $ test $a -ne $b; echo $? # is a not equal to b? 0
test String Operatorstest uses different operators with strings| Operator | Result |
|---|---|
| = | True if two strings are the same |
| != | True if two strings are NOT the same |
| -n | True if the string has a length greater than 0 |
| -z | True if the string has a length of 0 |
$ s1=foo; s2=bar $ test $s1 = $s2; echo $? 1 $ test $s1 != $s2; echo $? 0 $ test -n $s2; echo $? 0 $ test -n ""; echo $? 1 $ test -z ""; echo $? 0
$ test "foo" = "FOO"; echo $? 1 $ test "foo" != "FOO"; echo $? 0
test that can be used on files and directories| Operator | Result |
|---|---|
| -d | True if the argument is the name of a directory |
| -e | True if the argument is the name of a file or directory |
| -f | True if the argument is the name of an ordinary file (i.e. not a directory) |
| -s | True if the argument is the name of a file or directory that is not empty |
| -r | True if the argument is the name of a file or directory that is readable |
| -w | True if the argument is the name of a file or directory that is writable |
| -x | True if the argument is the name of a file or directory that is executable |
$ ls -ld dir1 # the directory I will use for testing drwxr-xr-x 4 glenn staff 128 Aug 9 11:24 dir1 $ test -d dir1; echo $? # is it a directory? 0 $ test -e dir1; echo $? # is it a file or a directory? 0 $ test -f dir1; echo $? # is it an ordinary file? 1 $ test -s dir1; echo $? # does the directory have things in it? 0 $ test -r dir1; echo $? # do I have read permission? 0 $ test -w dir1; echo $? # do I have write permission? 0 $ test -x dir1; echo $? # do I have execute permission? 0 $ ls -l foo1.txt # the file I will use for testing -rw-r--r-- 1 glenn staff 46 Aug 9 11:24 foo1.txt $ test -e foo1.txt ; echo $? # is it a file or a directory? 0 $ test -f foo1.txt ; echo $? # is it an ordinary file? 0 $ test -s foo1.txt ; echo $? # does the file have something in it? 0 $ test -r foo1.txt ; echo $? # do I have read permission? 0 $ test -w foo1.txt ; echo $? # do I have write permission? 0 $ test -x foo1.txt ; echo $? # do I have execute permission? 1
test expressions| Operator | Condition Tested |
|---|---|
| -a | Logical AND, meaning both expressions must be true |
| -o | Logical OR, meaning either of the two expressions must be true |
| ! | Logical NOT, meaning true becomes false and false becomes tru |
$ test 1 -lt 5 -a 5 -lt 10; echo $? # is 1 less than 5 AND 5 less than 10 0 $ test 5 -lt 1 -a 5 -lt 10; echo $? # is 5 less than 1 AND 5 less than 10 1 $ test 5 -lt 1 -o 5 -lt 10; echo $? # is 5 less than 1 OR 5 less than 10 0 $ test 5 -lt 1; echo $? # is 5 less than one 1 $ test ! 5 -lt 1; echo $? # is 5 NOT less than 1 0
testtest much it will be hard to remember them