if语句有多种写法

[root@shell-yankerp sh]# [ -f file ] && echo "yes" || echo "no" 
yes
修改如下：

if [ -f file ] 
then
echo "yes"
else
echo "no"
fi

简写：

[ -f file ] && echo "yes" 
表示if条件成立时返回yes
[ -f file ]  || echo "no" 
表示if条件不成立时，返回no ​​​​

可以用if判断如下条件：

​

实际上if的条件就是test命令的参数

[omcr@lnltedmr3 ~]$ man test
TEST(1)                                                   User Commands                                                  TEST(1)

NAME
       test - check file types and compare values

SYNOPSIS
       test EXPRESSION
       test

[ EXPRESSION ]
       [ ]
       [ OPTION

DESCRIPTION
       Exit with the status determined by EXPRESSION.

--help display this help and exit

--version
              output version information and exit

An omitted EXPRESSION defaults to false.  Otherwise, EXPRESSION is true or false and sets exit status.  It is one of:

( EXPRESSION )
              EXPRESSION is true

! EXPRESSION
              EXPRESSION is false

EXPRESSION1 -a EXPRESSION2
              both EXPRESSION1 and EXPRESSION2 are true

EXPRESSION1 -o EXPRESSION2
              either EXPRESSION1 or EXPRESSION2 is true

-n STRING
              the length of STRING is nonzero

STRING equivalent to -n STRING

-z STRING
              the length of STRING is zero

STRING1 = STRING2
              the strings are equal

STRING1 != STRING2
              the strings are not equal

INTEGER1 -eq INTEGER2
              INTEGER1 is equal to INTEGER2

INTEGER1 -ge INTEGER2
              INTEGER1 is greater than or equal to INTEGER2

INTEGER1 -gt INTEGER2
              INTEGER1 is greater than INTEGER2

INTEGER1 -le INTEGER2
              INTEGER1 is less than or equal to INTEGER2

INTEGER1 -lt INTEGER2
              INTEGER1 is less than INTEGER2

INTEGER1 -ne INTEGER2
              INTEGER1 is not equal to INTEGER2

FILE1 -ef FILE2
              FILE1 and FILE2 have the same device and inode numbers

FILE1 -nt FILE2
              FILE1 is newer (modification date) than FILE2

FILE1 -ot FILE2
              FILE1 is older than FILE2

-b FILE
              FILE exists and is block special

-c FILE
              FILE exists and is character special

-d FILE
              FILE exists and is a directory

-e FILE
              FILE exists

-f FILE
              FILE exists and is a regular file

-g FILE
              FILE exists and is set-group-ID

-G FILE
              FILE exists and is owned by the effective group ID

-h FILE
              FILE exists and is a symbolic link (same as -L)

-k FILE
              FILE exists and has its sticky bit set

-L FILE
              FILE exists and is a symbolic link (same as -h)

-O FILE
              FILE exists and is owned by the effective user ID

-p FILE
              FILE exists and is a named pipe

-r FILE
              FILE exists and read permission is granted

-s FILE
              FILE exists and has a size greater than zero

-S FILE
              FILE exists and is a socket

-t FD  file descriptor FD is opened on a terminal

-u FILE
              FILE exists and its set-user-ID bit is set

-w FILE
              FILE exists and write permission is granted

-x FILE
              FILE exists and execute (or search) permission is granted

Except  for  -h  and  -L,  all FILE-related tests dereference symbolic links.  Beware that parentheses need to be escaped
       (e.g., by backslashes) for shells.  INTEGER may also be -l STRING, which evaluates to the length of STRING.

NOTE: [ honors the --help and --version options, but test does not.  test treats each of those as  it  treats  any  other
       nonempty STRING.

NOTE:  your shell may have its own version of test and/or [, which usually supersedes the version described here.  Please
       refer to your shell's documentation for details about the options it supports.

GNU coreutils online help: <http://www.gnu.org/software/coreutils/> Report test translation bugs to  <http://translation‐
       project.org/team/>

AUTHOR
       Written by Kevin Braunsdorf and Matthew Bradburn.

COPYRIGHT
       Copyright    ©    2013   Free   Software   Foundation,   Inc.    License   GPLv3+:   GNU   GPL   version   3   or   later
       <http://gnu.org/licenses/gpl.html>.
       This is free software: you are free to change and redistribute it.  There is NO WARRANTY, to the extent permitted by law.

SEE ALSO
       The full documentation for test is maintained as a Texinfo manual.  If the info and test programs are properly  installed
       at your site, the command

info coreutils 'test invocation'

should give you access to the complete manual.

GNU coreutils 8.22                                          June 2018                                                    TEST(1)