Homework & Coursework Questions

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1. The problem statement, all variables and given/known data:
Hello all so i just started learning linux and i need to make script which compares 2 files by size and shows on the screen the bigger one but i can't figure out how to make if statement work.






3. The attempts at a solution (include all code and scripts):

Code:

#!/bin/bash
echo "type 1st file"
read a
echo "type 2nd file"
read b
if [ stat -c %s $a > stat -c %s $b]
then
echo "$a"
else
echo "$b"
fin

4. Complete Name of School (University), City (State), Country, Name of Professor, and Course Number (Link to Course):
Lithuania , Vilnius , Vilniaus Collegue 1st course M.Liogys

Two tips:
Also take a look at: http://en.tldp.org/LDP/abs/html/refcards.html#AEN22313

Code:

if [ `stat -c %s $a` -gt `stat -c %s $b` ]
then
    echo "$a"
else
    echo "$b"
fi

The more modern syntax would be something like this:

Code:

if (( $(stat -c %s file1) >  $(stat -c %s file2) ))
then
    echo "$a"
else
    echo "$b"
fi

Thanks to you both but can u explain me what

Code:

` `

exactly does ? Because im confused.

` ... ` or $( ... ) denote command substitution - the specified command is executed, and the resulting output used in place of the backquoted string.

In the above cases, stat is run, and the output stored in the variable size (in bartus' example) or used directly in a comparison (in fpmurphy's).

First: the "backquotes" should not be used any more. They are only there for compatibility issues but their usage is strongly discouraged. instead of `command` use

Code:

$(command)

.

Second: command substitution. Suppose you run a command "somecommand" and it produces some output - any output.

Code:

user@host:/some/where> somecommand
this is some output

You can use this output inside your own commands by using the command substitution device ("$( .... )"). The shell will first run the denoted command, replace the command substitution with its output and only then execute the rest of the line. In the example above, consider:

Code:

user@host:/some/where> echo "== $(somecommand) =="

The first step is to run "somecommand" and replace it with its output:

Code:

echo "== this is some output =="

Then, the resulting command is executed, simply displaying the original output decorated with surrounding equal-signs.

This example is not doing something impressive, but in fpmurphy's case it dealt with a possibly varying command output. This line:

Code:

if (( $(stat -c %s file1) >  $(stat -c %s file2) ))

boils down to:

Code:

if (( 5 > 4 ))

or something such, depending on what "stat -c %s <filename>" would yield.

Btw.: you might notice a slight flaw in your programs logic. What would happen if the two files are of absolutely equal size? The solution is left to the reader, but a hint may be in order: "if"-statements can be nested.

I hope this helps.

bakunin

First time i see constantly used the ( for if statements, i'm used to [..

Regard 'if statements'...
[[ condition ]] && echo succeeded || echo failed

Example:
[[ -z $XDG_DESKTOP_DIR ]] && echo "Your desktop is: $XDG_DESKTOP_DIR" || echo "$HOME/.config/user-dirs.dirs not loaded yet."

Which is the same as: (note the '\', that tells the line isnt dont yet, there must be no trailing spaces!)

Code:

[[ -z $XDG_DESKTOP_DIR ]] && \
    echo "Your desktop is: $XDG_DESKTOP_DIR" || \
    echo "$HOME/.config/user-dirs.dirs not loaded yet."

Which i prefer for 'short code' like setting a variable.

The && is appending the following command if previous command returns 0/true.
The || indicates what is done if the previous condition/command failed.

Examples:

Code:

[[ ! 0 -eq $UID ]] && echo "You're not root, and we abort" && exit
[[ 0 -eq $UID ]] || echo "You're not root, but we continue anyway"
[[ 0 -eq $UID ]] && echo "Must not be root!" && exit || echo "You may pass $USER"

hth