{"id":183,"date":"2018-10-21T00:00:00","date_gmt":"2018-10-20T16:00:00","guid":{"rendered":"https:\/\/yeslq.com\/201810183.html"},"modified":"2019-05-07T10:43:39","modified_gmt":"2019-05-07T02:43:39","slug":"the-linux-command-line-flow-control_21","status":"publish","type":"post","link":"https:\/\/yeslq.com\/?p=183","title":{"rendered":"The Linux Command Line—Flow Control – Part 3"},"content":{"rendered":"

\nThe Linux Command Line <\/a><\/i>by William Shotts<\/span><\/h1>\n
\n
<\/span><\/div>\n
\n<\/p>\n
\nNow that you have learned about positional parameters, it is time to cover the remaining flow control statement, for<\/a>. Like while and until, for is used to construct loops. for works like this:<\/div>\n
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for variable in words; do\n    commands\ndone\n     \n<\/pre>\n<\/div>\n
\nIn essence, for assigns a word from the list of words to the specified variable, executes the commands, and repeats this over and over until all the words have been used up. Here is an example:<\/div>\n
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#!\/bin\/bash\n\nfor i in word1 word2 word3; do\n    echo $i\ndone\n     \n<\/pre>\n<\/div>\n
\nIn this example, the variable i is assigned the string “word1”, then the statement echo $i is executed, then the variable i is assigned the string “word2”, and the statement echo $i is executed, and so on, until all the words in the list of words have been assigned.<\/div>\n
\nThe interesting thing about for is the many ways you can construct the list of words. All kinds of expansions can be used. In the next example, we will construct the list of words using command substitution:<\/div>\n
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#!\/bin\/bash\n\ncount=0\nfor i in $(cat ~\/.bash_profile); do\n    count=$((count + 1))\n    echo \"Word $count ($i) contains $(echo -n $i | wc -c) characters\"\ndone<\/pre>\n<\/div>\n
\nHere we take the file .bash_profile and count the number of words in the file and the number of characters in each word.<\/div>\n
\nSo what’s this got to do with positional parameters? Well, one of the features of for is that it can use the positional parameters as the list of words:<\/div>\n
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#!\/bin\/bash\n\nfor i in \"$@\"; do\n    echo $i\ndone<\/pre>\n<\/div>\n
\nThe shell variable “$@” contains the list of command line arguments. This technique is often used to process a list of files on the command line. Here is a another example:<\/div>\n
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#!\/bin\/bash\n\nfor filename in \"$@\"; do\n    result=\n    if [ -f \"$filename\" ]; then\n        result=\"$filename is a regular file\"\n    else\n        if [ -d \"$filename\" ]; then\n            result=\"$filename is a directory\"\n        fi\n    fi\n    if [ -w \"$filename\" ]; then\n        result=\"$result and it is writable\"\n    else\n        result=\"$result and it is not writable\"\n    fi\n    echo \"$result\"\ndone<\/pre>\n<\/div>\n
\nTry this script. Give it a list of files or a wildcard like “*” to see it work.<\/div>\n
\nHere is another example script. This one compares the files in two directories and lists which files in the first directory are missing from the second.<\/div>\n
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#!\/bin\/bash\n\n# cmp_dir - program to compare two directories\n\n# Check for required arguments\nif [ $# -ne 2 ]; then\n    echo \"usage: $0 directory_1 directory_2\" 1>&2\n    exit 1\nfi\n\n# Make sure both arguments are directories\nif [ ! -d $1 ]; then\n    echo \"$1 is not a directory!\" 1>&2\n    exit 1\nfi\n\nif [ ! -d $2 ]; then\n    echo \"$2 is not a directory!\" 1>&2\n    exit 1\nfi\n\n# Process each file in directory_1, comparing it to directory_2\nmissing=0\nfor filename in $1\/*; do\n    fn=$(basename \"$filename\")\n    if [ -f \"$filename\" ]; then\n        if [ ! -f \"$2\/$fn\" ]; then\n            echo \"$fn is missing from $2\"\n            missing=$((missing + 1))\n        fi\n    fi\ndone\necho \"$missing files missing\"<\/pre>\n<\/div>\n
\nNow on to the real work. We are going to improve the home_space function in our script to output more information. You will recall that our previous version looked like this:<\/div>\n
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home_space()\n{\n    # Only the superuser can get this information\n\n    if [ \"$(id -u)\" = \"0\" ]; then\n    echo \"<h2>Home directory space by user<\/h2>\"\n    echo \"<pre>\"\n    echo \"Bytes Directory\"\n        du -s \/home\/* | sort -nr\n    echo \"<\/pre>\"\n    fi\n\n}   # end of home_space\n     \n<\/pre>\n<\/div>\n
\nHere is the new version:<\/div>\n
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home_space()\n{\n    echo \"<h2>Home directory space by user<\/h2>\"\n    echo \"<pre>\"\n    format=\"%8s%10s%10s   %-sn\"\n    printf \"$format\" \"Dirs\" \"Files\" \"Blocks\" \"Directory\"\n    printf \"$format\" \"----\" \"-----\" \"------\" \"---------\"\n    if [ $(id -u) = \"0\" ]; then\n        dir_list=\"\/home\/*\"\n    else\n        dir_list=$HOME\n    fi\n    for home_dir in $dir_list; do\n        total_dirs=$(find $home_dir -type d | wc -l)\n        total_files=$(find $home_dir -type f | wc -l)\n        total_blocks=$(du -s $home_dir)\n        printf \"$format\" $total_dirs $total_files $total_blocks\n    done\n    echo \"<\/pre>\"\n\n}   # end of home_space<\/pre>\n<\/div>\n
\nThis improved version introduces a new command printf<\/a>, which is used to produce formatted output according to the contents of a format string<\/i>. printfcomes from the C programming language and has been implemented in many other programming languages including C++, Perl, awk, java, PHP, and of course, bash. You can read more about printf format strings at:<\/div>\n