1、command &>filename redirects both the stdout and the stderr of command to
filename.
1 if cmp a b &> /dev/null # Suppress output.
2 then echo "Files a and b are identical."
3 else echo "Files a and b differ."
4 fi
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2、|
pipe. Passes the output of previous command to the input of the next one,
or to the shell. This is a method of chaining commands together.
12 word=Linux
13 letter_sequence=inu
14 if echo "$word" | grep -q "$letter_sequence"
15 # The "-q" option to grep suppresses output.
16 then
17 echo "$letter_sequence found in $word"
18 else
19 echo "$letter_sequence not found in $word"
20 fi
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3、这个嵌套结构有点怪,还有(())和[[]]的有什么区别?
它的结构是：
if ()
if()
then()
else()
endif
then()
endif
总觉得不如这样：
if ()
then
if()
then()
else()
endif
()
endif
· An if/then construct can contain nested comparisons and tests.
1 if echo "Next *if* is part of the comparison for the first *if*."
2
3 if [[ $comparison = "integer" ]]
4 then (( a < b ))
5 else
6 [[ $a < $b ]]
7 fi
8
9 then
10 echo '$a is less than $b'
11 fi
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4、example 7-1
bash里面的测试命令真是多呀？test [] [[]] (()) let ... ，快昏了。
[0]和[1]和[-1]的测试结果都是true，不懂？
[ -n "$xyz" ] 里面的-n什么作用？(
[]就是test命令
在man test中查到如下：
[-n] STRING
the length of STRING is nonzero
)
#!/bin/bash
# Tip:
# If you're unsure of how a certain condition would evaluate,
#+ test it in an if-test.
echo
echo "Testing \"0\""
if [ 0 ] # zero
then
echo "0 is true."
else
echo "0 is false."
fi # 0 is true.
echo
echo "Testing \"1\""
if [ 1 ] # one
then
echo "1 is true."
else
echo "1 is false."
fi # 1 is true.
echo
echo "Testing \"-1\""
if [ -1 ] # minus one
then
echo "-1 is true."
else
echo "-1 is false."
fi # -1 is true.
echo
echo "Testing \"NULL\""
if [ ] # NULL (empty condition)
then
echo "NULL is true."
else
echo "NULL is false."
fi # NULL is false.
echo
echo "Testing \"xyz\""
if [ xyz ] # string
then
echo "Random string is true."
else
echo "Random string is false."
fi # Random string is true.
echo
echo "Testing \"\$xyz\""
if [ $xyz ] # Tests if $xyz is null, but...
# it's only an uninitialized variable.
then
echo "Uninitialized variable is true."
else
echo "Uninitialized variable is false."
fi # Uninitialized variable is false.
echo
echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ] # More pedantically correct.
then
echo "Uninitialized variable is true."
else
echo "Uninitialized variable is false."
fi # Uninitialized variable is false.
echo
xyz= # Initialized, but set to null value.
echo "Testing \"-n \$xyz\""
if [ -n "$xyz" ]
then
echo "Null variable is true."
else
echo "Null variable is false."
fi # Null variable is false.
echo
# When is "false" true?
echo "Testing \"false\""
if [ "false" ] # It seems that "false" is just a string.
then
echo "\"false\" is true." #+ and it tests true.
else
echo "\"false\" is false."
fi # "false" is true.
echo
echo "Testing \"\$false\"" # Again, uninitialized variable.
if [ "$false" ]
then
echo "\"\$false\" is true."
else
echo "\"\$false\" is false."
fi # "$false" is false.
# Now, we get the expected result.
# What would happen if we tested the uninitialized variable "$true"?
echo
exit 0
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5、[[]]比[]更加有用？
The [[ ]] construct is the more versatile Bash version of [ ]. This is the
extended test command, adopted from ksh88.
...
Tip Using the [[ ... ]] test construct, rather than [ ... ] can prevent many
logic errors in scripts. For example, the &&, ||, <, and > operators work
within a [[ ]] test, despite giving an error within a [ ] construct.
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6、
Following an if, neither the test command nor the test brackets ( [ ] or
[[ ]] ) are strictly necessary.
例如：
[mdx@localhost abs-exercises]$ if dir; then echo "dir success"; else echo
"fail" ; fi
3 ch4-2.txt ch7-0-1.sh ctrl-m.sh ex2-2.sh ex4-5.sh ex5-2.sh ex7-1.sh
3]] ch5-2.txt ch7-1.txt env.txt ex2-3.sh ex4-6.sh ex6-1.sh plan2.txt
b]] ch6.txt ctrl-h.sh ex2-1.sh ex4-2.sh ex4-7.sh ex7-0.sh plan.txt
dir success
[mdx@localhost abs-exercises]$
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[mdx@localhost abs-exercises]$ if Dir; then echo "dir success"; else echo
"fail" ; fi
bash: Dir: command not found
fail
[mdx@localhost abs-exercises]$
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7、(( 1 / 0 )) 2>/dev/null不懂？
29 (( 1 / 0 )) 2>/dev/null # Illegal division by 0.
30 # ^^^^^^^^^^^
31 echo "Exit status of \"(( 1 / 0 ))\" is $?." # 1
32
33 # What effect does the "2>/dev/null" have?
34 # What would happen if it were removed?
35 # Try removing it, then rerunning the script.
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8、英语疑问:
1)The (( ... )) and let ... constructs also return an exit status of 0 if the
arithmetic expressions they evaluate expand to a non-zero value.
2)This is in marked contrast to using the test and [ ] constructs previously discussed.