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What this switch of "if" does?

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# 1  
Old 12-22-2008
What this switch of "if" does?
Hi All

I have a script which is....

if [ "file1.log" -nt "file2.txt" ]
then
touch file2.txt
else
kill -9 `ps -ef | grep -i /opt/xxx/bin/yyy | grep -v grep | awk '{print $2}'`
fi

What does it do? specially what is this logical checking?
if [ "file1.log" -nt "file2.txt" ]
I am not able to understand the "-nt" option...

Thanks in advance

Pritam
# 2  
Old 12-22-2008
Quote:
Originally Posted by pritam1980
Hi All

I have a script which is....

if [ "file1.log" -nt "file2.txt" ]
then
touch file2.txt
else
kill -9 `ps -ef | grep -i /opt/xxx/bin/yyy | grep -v grep | awk '{print $2}'`
fi

What does it do? specially what is this logical checking?
if [ "file1.log" -nt "file2.txt" ]
I am not able to understand the "-nt" option...

Thanks in advance

Pritam


Hello Pritam,


[Chapter 13] 13.3 Understanding Shell Scripts




"Read it is good for the mind"
# 3  
Old 12-22-2008
-nt Test if file1 is newer than file 2. The modification date is used for this and the next comparison.
# 4  
Old 12-22-2008
Thanks a lot for helping me to understand.
# 5  
Old 12-22-2008
You can "test" a lot of conditions with the shell built-in command "test".

If you have a look at the man/help page, it should tell you all of the available comparisons/tests.

Code:
man test

or
Code:
help test

Here is what I got from typing in "help test" at a Bash shell prompt:

Code:
$ help test
test: test [expr]
    Exits with a status of 0 (true) or 1 (false) depending on
    the evaluation of EXPR.  Expressions may be unary or binary.  Unary
    expressions are often used to examine the status of a file.  There
    are string operators as well, and numeric comparison operators.

    File operators:

        -a FILE        True if file exists.
        -b FILE        True if file is block special.
        -c FILE        True if file is character special.
        -d FILE        True if file is a directory.
        -e FILE        True if file exists.
        -f FILE        True if file exists and is a regular file.
        -g FILE        True if file is set-group-id.
        -h FILE        True if file is a symbolic link.
        -L FILE        True if file is a symbolic link.
        -k FILE        True if file has its `sticky' bit set.
        -p FILE        True if file is a named pipe.
        -r FILE        True if file is readable by you.
        -s FILE        True if file exists and is not empty.
        -S FILE        True if file is a socket.
        -t FD          True if FD is opened on a terminal.
        -u FILE        True if the file is set-user-id.
        -w FILE        True if the file is writable by you.
        -x FILE        True if the file is executable by you.
        -O FILE        True if the file is effectively owned by you.
        -G FILE        True if the file is effectively owned by your group.
        -N FILE        True if the file has been modified since it was last read.

      FILE1 -nt FILE2  True if file1 is newer than file2 (according to
                       modification date).

      FILE1 -ot FILE2  True if file1 is older than file2.

      FILE1 -ef FILE2  True if file1 is a hard link to file2.

    String operators:

        -z STRING      True if string is empty.

        -n STRING
        STRING         True if string is not empty.

        STRING1 = STRING2
                       True if the strings are equal.
        STRING1 != STRING2
                       True if the strings are not equal.
        STRING1 < STRING2
                       True if STRING1 sorts before STRING2 lexicographically.
        STRING1 > STRING2
                       True if STRING1 sorts after STRING2 lexicographically.

    Other operators:

        -o OPTION      True if the shell option OPTION is enabled.
        ! EXPR         True if expr is false.
        EXPR1 -a EXPR2 True if both expr1 AND expr2 are true.
        EXPR1 -o EXPR2 True if either expr1 OR expr2 is true.

        arg1 OP arg2   Arithmetic tests.  OP is one of -eq, -ne,
                       -lt, -le, -gt, or -ge.

    Arithmetic binary operators return true if ARG1 is equal, not-equal,
    less-than, less-than-or-equal, greater-than, or greater-than-or-equal
    than ARG2.

Hope this helps for future questions on comparisons and tests using the shell.
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