Linux

Seamless directory navigation in bash

Jul 31st

Posted by liz in Bash

1 comment

It is time (and brain) consuming to remember and type the directory path if you require to move across directories frequently. You can save some time and energy with three in-built commands supported by bash. They are pushd, popd and dirs.

User can use pushd command in the place of cd comand. When you use pushd command to change directory, bash remembers and pushes the directory name to stack before mobing to specified directory. Later user can use popd to move back to the last remembered directory.

This mechanism operates in the form of stack. That means you can use as many pushd commands to change directories and use More >

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How to extract files from RPM

Jul 26th

Posted by Neo in Linux

No comments

All the files present in a RPM file can be extracted using rpm2cpio and cpio utilities. The rpm2cpio command is used to convert RPM to cpio compatible archive and cpio command can be used to extract files from the archive.

The following example shows how extract files from ElectricFence RPM.

[liz@techpulp ~]# rpm2cpio ElectricFence-2.2.2-23.rpm | cpio -idv

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Identify processes that have open sockets

Jul 26th

Posted by Neo in Linux

1 comment

The lsof can be used to find the list of processes that have active sockets open.

Let us see the list of current TCP connections using netstat command.

[liz@techpulp ~]# netstat -nt
Active Internet connections (w/o servers)
Proto Recv-Q Send-Q Local Address               Foreign Address             State
tcp        0      0 192.168.20.24:40816         74.125.19.83:80             ESTABLISHED
tcp        0      0 192.168.20.24:43059         74.125.19.83:80             ESTABLISHED
[liz@techpulp ~]#

Now let us find which process has these TCP sockets open using lsof command.

[liz@techpulp ~]# /usr/sbin/lsof -i
COMMAND    PID   USER   FD   TYPE DEVICE SIZE NODE NAME
firefox-b 3288    liz   13u  IPv4 157814       TCP
                          liz.techpulp.com:43058->cf-in-f83.google.com:http (ESTABLISHED)
firefox-b 3288    liz   40u  IPv4 158205       TCP
                          liz.techpulp.com:43059->cf-in-f83.google.com:http (ESTABLISHED)
[liz@techpulp ~]#

So Firefox More >

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Linux file compression tools/utilities

Jul 26th

Posted by Neo in Linux

1 comment

Linux has various types of compression and decompression tools like tar, gzip, bzip2, zip and compress. This article describes practical usage of these utilities.

tar and untar

tar command operates on one or more files or directories and creates an archive. An archive created by tar command is called a tarball. Generally such archives will have .tar as file extension. tar is more of a archiving utility than a compression utility. Typically it is used along with file compression utilities like gzip and bzip2. See various ways of creating a tarball using -cvf option in the following examples.

[liz@techpulp ~]# tar -cvf mydir.tar mydir/
[liz@techpulp ~]# tar -cvf myfiles.tar file1.txt More >
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Strip unused/dead functions from Linux application

Jul 26th

Posted by Neo in C/C++

1 comment

ELF executable can be opimized for size by removing unused or dead functions. The options -ffunction-sections -Wl,–gc-sections can be used to do that.

Let us practically verify it using the following C file which has a function named dead_one which is not used anywhere in the code.

bash# cat test.c

#include <stdio.h>

int dead_one()
{
	int i;
	i = 10*30*40;
	return i;
}

int main(int argc, char *argv[])
{
	printf("Wake up, Neo\n");
}

Now let us compile the above C file with and without -ffunction-sections -Wl,–gc-sections options. Let us produce binary named test.1 without the options and a binary named test.2 with the options.

bash# cc test.c -o test.1 More >
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Stack of a Running Process in Unix/Linux

Jul 26th

Posted by Neo in C/C++

1 comment

The command pstack can be used to view the stack trace of a running process.

Let us select a running process.

[neo@techpulp ~]# ps -e | grep konqueror
22937 pts/5 00:00:17 konqueror

Run pstack command with PID of the process.

[neo@techpulp ~]# pstack 22937
22937: konqueror
(No symbols found)
0x001fb402: ???? (89c6ed8, 2, bfa39b08, 3812df0, 89c6ed8, 3c21362)
0x0325074b: ???? (89c6ed8, 8a00908)
0x03250656: ???? (89c6ed8, 3c372ec)
0x03237a59: ???? (bfa39c20, bfa39d48, bfa39de8, bfa39d1c, 0, bfa39fb4) + 4c0
0x03bb1d0d: ???? (1, bfa3a094, bfa3a09c, 0, bfa3a094, 1) + 40
0x00248de6: ???? (80485ec, 1, bfa3a094, 8048600, 8048650, 21ff2d) + 405c5f78
[neo@techpulp ~]#

There is an alternate way to use gdb and attach More >

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ELF Binary Optimization for size

Jul 26th

Posted by Nick in C/C++

1 comment

The following steps help in reducing the size of a Linux application ELF binary. These operations can be used for application binary as well as shared library.

Use compiler optimization flags

        -Os (optimize for size)
        -O2

While linking, try to use following options. But do not use ‘-s’ option for kernel modules.

# ld -r -s --discard-local --discard-all testApp.o -o testApp
# ld -r -s --discard-local --discard-all testLib1.o testLib2.o -o testLib.so
# ld -r --discard-local --discard-all testMod1.o testMod2.o -o testMod

Strip the binary including unnecessary sections

# strip --remove-section=.note --remove-section=.comment  testApp
# strip --remove-section=.note --remove-section=.comment  testLib.so

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Find how long a command takes to complete

Jul 25th

Posted by liz in Linux

No comments

Some times it is required to know how long a command is taking to execute, especially in case of huge scripts. The command “time” can be used to determine the time taken by a command or a script. The following example shows how it can be used.

[liz@techpulp ~]# time sleep 3

real    0m3.016s
user    0m0.000s
sys     0m0.008s
[liz@techpulp ~]#

In the above example we attempted to determine time taken by the command “sleep 3” using time command. After the command finishes, time prints the information on standard error. This information contains “real” which is actual time taken by the command from the invocation More >

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