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2020.05.26 11:22

# [sh] 쉘스크립트 if 비교 연산

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binary comparison operator compares two variables or quantities. Note that integer and string comparison use a different set of operators.

integer comparison

-eq

is equal to

if [ "\$a" -eq "\$b" ]

-ne

is not equal to

if [ "\$a" -ne "\$b" ]

-gt

is greater than

if [ "\$a" -gt "\$b" ]

-ge

is greater than or equal to

if [ "\$a" -ge "\$b" ]

-lt

is less than

if [ "\$a" -lt "\$b" ]

-le

is less than or equal to

if [ "\$a" -le "\$b" ]

<

is less than (within double parentheses)

(("\$a" < "\$b"))

<=

is less than or equal to (within double parentheses)

(("\$a" <= "\$b"))

>

is greater than (within double parentheses)

(("\$a" > "\$b"))

>=

is greater than or equal to (within double parentheses)

(("\$a" >= "\$b"))

string comparison

=

is equal to

if [ "\$a" = "\$b" ]

 Note the whitespace framing the =. if [ "\$a"="\$b" ] is not equivalent to the above.
==

is equal to

if [ "\$a" == "\$b" ]

This is a synonym for =.

The == comparison operator behaves differently within a double-brackets test than within single brackets.

 ```[[ \$a == z* ]] # True if \$a starts with an "z" (pattern matching). [[ \$a == "z*" ]] # True if \$a is equal to z* (literal matching). [ \$a == z* ] # File globbing and word splitting take place. [ "\$a" == "z*" ] # True if \$a is equal to z* (literal matching). # Thanks, Stéphane Chazelas```

!=

is not equal to

if [ "\$a" != "\$b" ]

This operator uses pattern matching within a [[ ... ]] construct.

<

is less than, in ASCII alphabetical order

if [[ "\$a" < "\$b" ]]

if [ "\$a" \< "\$b" ]

Note that the "<" needs to be escaped within a [ ] construct.

>

is greater than, in ASCII alphabetical order

if [[ "\$a" > "\$b" ]]

if [ "\$a" \> "\$b" ]

Note that the ">" needs to be escaped within a [ ] construct.

See Example 27-11 for an application of this comparison operator.

-z

string is null, that is, has zero length

 ``` String='' # Zero-length ("null") string variable. if [ -z "\$String" ] then echo "\\$String is null." else echo "\\$String is NOT null." fi # \$String is null.```

-n

string is not null.

 The -n test requires that the string be quoted within the test brackets. Using an unquoted string with ! -z, or even just the unquoted string alone within test brackets (see Example 7-6) normally works, however, this is an unsafe practice. Always quote a tested string. [1]

Example 7-5. Arithmetic and string comparisons

 ```#!/bin/bash a=4 b=5 # Here "a" and "b" can be treated either as integers or strings. # There is some blurring between the arithmetic and string comparisons, #+ since Bash variables are not strongly typed. # Bash permits integer operations and comparisons on variables #+ whose value consists of all-integer characters. # Caution advised, however. echo if [ "\$a" -ne "\$b" ] then echo "\$a is not equal to \$b" echo "(arithmetic comparison)" fi echo if [ "\$a" != "\$b" ] then echo "\$a is not equal to \$b." echo "(string comparison)" # "4" != "5" # ASCII 52 != ASCII 53 fi # In this particular instance, both "-ne" and "!=" work. echo exit 0```

Example 7-6. Testing whether a string is null

 ```#!/bin/bash # str-test.sh: Testing null strings and unquoted strings, #+ but not strings and sealing wax, not to mention cabbages and kings . . . # Using if [ ... ] # If a string has not been initialized, it has no defined value. # This state is called "null" (not the same as zero!). if [ -n \$string1 ] # string1 has not been declared or initialized. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Wrong result. # Shows \$string1 as not null, although it was not initialized. echo # Let's try it again. if [ -n "\$string1" ] # This time, \$string1 is quoted. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Quote strings within test brackets! echo if [ \$string1 ] # This time, \$string1 stands naked. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # This works fine. # The [ ... ] test operator alone detects whether the string is null. # However it is good practice to quote it (if [ "\$string1" ]). # # As Stephane Chazelas points out, # if [ \$string1 ] has one argument, "]" # if [ "\$string1" ] has two arguments, the empty "\$string1" and "]" echo string1=initialized if [ \$string1 ] # Again, \$string1 stands unquoted. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Again, gives correct result. # Still, it is better to quote it ("\$string1"), because . . . string1="a = b" if [ \$string1 ] # Again, \$string1 stands unquoted. then echo "String \"string1\" is not null." else echo "String \"string1\" is null." fi # Not quoting "\$string1" now gives wrong result! exit 0 # Thank you, also, Florian Wisser, for the "heads-up".```

Example 7-7. zmore

 ```#!/bin/bash # zmore # View gzipped files with 'more' filter. E_NOARGS=85 E_NOTFOUND=86 E_NOTGZIP=87 if [ \$# -eq 0 ] # same effect as: if [ -z "\$1" ] # \$1 can exist, but be empty: zmore "" arg2 arg3 then echo "Usage: `basename \$0` filename" >&2 # Error message to stderr. exit \$E_NOARGS # Returns 85 as exit status of script (error code). fi filename=\$1 if [ ! -f "\$filename" ] # Quoting \$filename allows for possible spaces. then echo "File \$filename not found!" >&2 # Error message to stderr. exit \$E_NOTFOUND fi if [ \${filename##*.} != "gz" ] # Using bracket in variable substitution. then echo "File \$1 is not a gzipped file!" exit \$E_NOTGZIP fi zcat \$1 | more # Uses the 'more' filter. # May substitute 'less' if desired. exit \$? # Script returns exit status of pipe. # Actually "exit \$?" is unnecessary, as the script will, in any case, #+ return the exit status of the last command executed.```

compound comparison

-a

logical and

exp1 -a exp2 returns true if both exp1 and exp2 are true.

-o

logical or

exp1 -o exp2 returns true if either exp1 or exp2 is true.

These are similar to the Bash comparison operators && and ||, used within double brackets.

 `[[ condition1 && condition2 ]]`

The -o and -a operators work with the test command or occur within single test brackets.

 ```if [ "\$expr1" -a "\$expr2" ] then echo "Both expr1 and expr2 are true." else echo "Either expr1 or expr2 is false." fi```

But, as rihad points out:

 ```[ 1 -eq 1 ] && [ -n "`echo true 1>&2`" ] # true [ 1 -eq 2 ] && [ -n "`echo true 1>&2`" ] # (no output) # ^^^^^^^ False condition. So far, everything as expected. # However ... [ 1 -eq 2 -a -n "`echo true 1>&2`" ] # true # ^^^^^^^ False condition. So, why "true" output? # Is it because both condition clauses within brackets evaluate? [[ 1 -eq 2 && -n "`echo true 1>&2`" ]] # (no output) # No, that's not it. # Apparently && and || "short-circuit" while -a and -o do not.```

Refer to Example 8-3Example 27-17, and Example A-29 to see compound comparison operators in action.

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