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This article collects all kinds of useful tips and tricks around Linux usage, mostly if not all of them about command line tools. Many have been collected over the years, so any advanced user may not find them interesting. But when getting started with the command line, they may still be helpful for new users.

Common issues

“Too many open files”

• check current limit (only valid for executing user): ulimit -a or specifically ulimit -n (-n refers to “number of files”)
• set limit with ulimit -n unlimited or ulimit -n 65535 (may result in “permission denied”)
• on Fedora: edit /etc/security/limits.conf

Common filesystem tasks

List folders by size descending recursively

    find . -type d -exec du -b {} \; | sort -nr | uniq

Latest changed files (one folder deep search)

    find . -maxdepth 2 -type f -exec stat -c %y_%n {} \; | sort -r | less

10 latest changed files (recursively)

    find . -type f -printf '%TF_%TT %p \n' | sort -r | head -n 10

Change file encoding (e. g. from UTF-8)

    recode -f ascii <file>

Networking and Web

Use kernel prerouting to direct traffic from port 80 to 8080, e. g. for fronting tomcat

    iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 80 -j DNAT --to-destination :8080

Repeatedly serve a file using netcat (call with wget http://localhost:12345/)

    #!/bin/bash

    while true
    do
      { echo -ne "HTTP/1.0 200 OK\r\nContent-Length: $(wc -c <file.txt)\r\n\r\n"; cat file.txt; } | nc -l 12345
    done

    exit 0

Apply htdigest protection to Apache directory

Required modules

• mod_authz_user
• mod_auth_digest
• mod_authn_file

Configuration

    <Location /path/to/protect>
       AuthType Digest
       AuthName "Protected Area"
       AuthDigestDomain /path/to/protect http://11.22.33.44/path/to/protect
       AuthDigestProvider file
       AuthUserFile /etc/apache2/conf/digest_pw
       Require valid-user
    </Location>

Create digest_pw file

    htdigest -c /etc/apache2/conf/digest_pw "Protected Area" username
    => will be asked for password, file created

Upload multiple files via FTP (which is an interactive tool)

Requires curlftpfs and fuse + rsync

    #!/bin/bash

    SOURCE_DIR=./dist/
    HOST=some.host.com
    USER=username
    PASS=passsword
    TARGET_DIR=/target/
    MOUNT_POINT=./mnt/
    TMP_DIR=$(mktemp -d)

    #########################################################################
    ## Abort if mount point does not exist
    if [ ! -d ${MOUNT_POINT} ];
    then
      echo "Target mount point directory does not exist, aborting."
      exit 1
    fi

    #########################################################################
    ## Mount remote file system locally
    curlftpfs -o user=${USER}:${PASS} ${HOST} ${MOUNT_POINT} || exit 1

    #########################################################################
    ## Transfer files incrementally
    if [ -d "${MOUNT_POINT}/${TARGET_DIR}" ]
    then
        rsync -rtuv --modify-window=1 --delete --no-perms --size-only --temp-dir=${TMP_DIR} ${SOURCE_DIR}/ ${MOUNT_POINT}/${TARGET_DIR}/
    else
        echo "Target remote directory does not exist, aborting."
    fi

    #########################################################################
    ## Unmount remote file system
    fusermount -u ${MOUNT_POINT}

    #########################################################################
    ## Remove temporary directory
    if [ -d "${TMP_DIR}" ];
    then
        rm -rf ${TMP_DIR}
    fi

    exit 0