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If you do not change IFS, filenames that contain spaces will be interpreted as multiple files instead of as a single file. What command can I use to rename all the *.aaa files to *.bbb files? In DOS and Windows, you can rename all the *.aaa files in a directory to *.bbb by using the rename command as follows: rename *.aaa *.bbb In UNIX you can use the mv command to rename files, but you cannot use it to rename more than one file at the same time. To do this, you need to use a for loop: OLDSUFFIX=aaa NEWSUFFIX=bbb for FILE in *.”$OLDSUFFIX” do NEWNAME= echo “$FILE” | sed -e “s/${OLDSUFFIX}$/$NEWSUFFIX/” mv “$FILE” “$NEWNAME” done Here you generate a list of all the files in the current directory that end with the value of the variable OLDSUFFIX. Then you use sed to modify the name of each file by removing the value of OLDSUFFIX from the filename and replacing it with the value of NEWSUFFIX. You use the $ character in your sed expression to anchor the suffix in OLDSUFFIX to the end of the line. You do this to make sure the pattern that is replaced is really a filename suffix. After you have a new name, you rename the file from its original name, stored in FILE, to the new name stored, stored in NEWNAME. To prevent a potential loss of data, you might need to modify this loop to specify the -i option to the mv command. For example, if the files 1.aaa and 1.bbb exist prior to executing this loop, after the loops exits, the original version of 1.aaa will be overwritten when 1.bbb is renamed as 1.aaa. If mv -i is used, you will be prompted before 1.bbb is renamed: mv: overwrite 1.aaa (yes/no)? You can answer no to avoid losing the information in this file. The actual prompt produced by mv might be different on your version of UNIX. What command can I use to rename all the aaa* files to bbb* files? The technique used in the last question can be used to solve this problem as well. In this case you will use the variables OLDPREFIX to hold the prefix a file currently has and NEWPREFIX to hold the prefix you want the file to have. As an example, you can use the following for loop to rename all files that start with aaa to start with bbb instead:
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The structure of the UNIX directory tree sometimes makes locating files and commands difficult. To locate a file, often you need to search through a directory and all its subdirectories. The easiest way to do this is to use the find command: find dir -name file -print Here dir is the name of a directory where find should start its search, and file is the name of the file it should look for. The name option of the find command also works with the standard filename substitution operators covered in Chapter 8. For example, the command find dir -name “*txt” -print displays a list of all the files in the directory dir and all its subdirectories that end with the string txt. How can I grep for a string in every file in a directory? When you work on a large project involving many files, remembering the contents of the individual files becomes difficult. It is much easier to look through all the files for a particular piece of information. You can use the find command in conjunction with the xargs command to look for a particular string in every file contained within a directory and all its subdirectories: find dir -type f -print | xargs grep “string” Here dir is the name of a directory in which to start searching, and string is the string to look for. Here you specify the -type option to the find command so that only regular files are searched for the string. How do I remove all the files in a directory matching a particular name? Some editors and programs create large numbers of temporary files. Often you need to clean up after these programs, to prevent your hard drive from filling up. To generate the list of files to delete, you can use the find command. Most of the time you can combine the find command with the xargs command, but in this case the filenames can contain one or more spaces. Spaces can confuse the xargs command, so you need to use a for loop instead: OLDIFS=”$IFS” IFS=’ ‘ for FILE in find . -type f -name “*string*” -print do rm “$FILE” done IFS=”$OLDIFS” Here string is a string that should be part of the name of each file you want to delete. So that the for loop is set to the correct value of FILE in each iteration, IFS needs to be set to the newline character.
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Sams Teach Yourself Shell Programming in 24 Hours Contents Index Hour 24: Shell Programming FAQs Previous Chapter Next Chapter Sections in this Chapter: Shell and Command Questions File and Directory Questions Variable and Argument Questions Summary Previous Section Next Section File and Directory Questions In this section, I will look at some questions about files and directories. These questions include issues with specific commands and examples that illustrate the usage of commands to solve particular problems. How do I determine the full pathname of a directory? Shell scripts that work with directories often need to determine the full pathname of a directory to perform the correct operations on these directories. You can determine the full pathname of a directory by using the cd and pwd commands: FULLPATH= (cd dir ; pwd) Here dir is the name of a directory. This command changes directories to the specified directory, dir, and then displays the full pathname of the directory using the pwd command. Because command substitution is used, the full pathname is assigned to the variable FULLPATH. Because the cd command changes the working directory of the current shell, you execute it in a subshell. Thus the working directory of the shell script is unchanged. How do I determine the full pathname of a file? Determining the full pathname of a file is slightly harder than determining the full pathname of a directory. You need to use the dirname and basename commands in conjunction with the cd and pwd commands to determine the full pathname of a file: CURDIR= pwd cd dirname file FULLPATH=” pwd / basename file ” cd $CURDIR Here file is the name of a file whose full pathname you want to determine. First you save the current path of the current directory in the variable CURDIR. Next you change to the directory containing the specified file, file. Then you join the output of the pwd command and the name of the file determined using the basename command to get the full pathname. This value gets stored in the variable FULLPATH. Finally you change back to the original directory. How can locate a particular file?
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Sams Teach Yourself Shell Programming in 24 Hours Contents Index Hour 24: Shell Programming FAQs Previous Chapter Next Chapter Sections in this Chapter: Shell and Command Questions File and Directory Questions Variable and Argument Questions Summary Previous Section Next Section Copyright Macmillan Computer Publishing. All rights reserved.
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Here command is the name of the command you want to execute. The one problem with this is that if no arguments were specified to your script, some versions of the shell will expand “$@” to “”. If no arguments were specified, you want to execute command, not command “”.To avoid this problem, use the form: command ${@:+”$@”} Here you are using one of the forms of variable substitution discussed in Chapter 8, “Substitution.” In this case you check to see whether the variable $@ has a value. If it does, you substitute the value “$@” for it. If your script was not given any command line arguments, $@ will be null; thus no value will be substituted. How do I use the value of a shell variable in a sed command? The simplest method to use variables in a sed command is to enclose your sed command in double quotes ( “) instead of single quotes ( ‘). Because the shell performs variable substitution on double-quoted strings, the shell will substitute the value of any variables you specify before sed executes. For example, the command sed “/$DEL/d” file1 > file2 deletes all the lines in file1 that contain the value stored in the variable $DEL. How do I check to see whether a variable has a value? There are several methods for determining this. The simplest is the if statement: if [ -z “$VAR” ] ; then list ; fi Here VAR is the name of the variable, and list is the command to execute if VAR does not contain a value. Usually list initializes VAR to some default value. You can initialize variables more succinctly using variable substitution. For example, the previous if statement can be written as : ${VAR:=default} Here default is the default that should be assigned to VAR, if VAR does not have a value. If you need execute a set of commands to obtain a default value, use command substitution with the backquote ( ) operator to obtain the value that should be substituted: : ${VAR:= default } Here default is a list of commands to execute. If VAR does not have a value, the output of these commands will be assigned to it.
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Sams Teach Yourself Shell Programming in 24 Hours Contents Index Hour 24: Shell Programming FAQs Previous Chapter Next Chapter Sections in this Chapter: Shell and Command Questions File and Directory Questions Variable and Argument Questions Summary Previous Section Next Section Variable and Argument Questions In this section I will examine some questions related to variables and their use in shell scripts. I will also cover some questions related to command line arguments. How can I include functions and variable definitions from one file into another file? To include functions and variable definitions defined in one file into another file you need to use the . command as follows: . file Here file is the name of the file you want to include. I covered this topic in Chapter 22, “Problem Solving with Shell Scripts.” Is it possible to consider each argument to a shell script one at a time? You can do this using a for loop: for arg in “$@” do list done Here the variable arg will be set to each argument in turn. The specified list of commands, list, will be executed for each argument. You use $@ in this example for the arguments instead of $*, because $@ preserves the quoting used when the command was issued. The difference between $@ and $* was discussed in Chapter 12, “Parameters.” How can I forward all the arguments given to my script to another command? A common task for shell programmers is writing a wrapper script for command. A wrapper script might need to define a set of variables or change the environment in some way before a particular command starts executing. When writing wrapper scripts, you will need to forward all the arguments given to your script to a command. Usually the following is sufficient: command “$@”
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How do I determine whether the shell can find a particular command? You can check to make sure that the shell can find a command or shell function by using the type command covered in Chapter 18, “Miscellaneous Tools”: type name > /dev/null 2>&1 ; if [ $? -ne 0 ] ; then list ; fi Here name is the name of the command you want check for, and list is the list of commands to execute if the shell does not know about name. Usually list is used to determine a fallback command. The type command is a builtin in sh and bash. In ksh, it is usually an alias, whence -v. How do I determine whether job control is available in the shell? Job control, covered in Chapter 6, “Processes,” is the shell feature that enables you to control background processes based on a job ID. This feature is not available in the Bourne shell, sh. It is available in ksh and bash. A common method used to check whether job control is enabled is to check whether the jobs command is defined: if type jobs > /dev/null 2>&1 ; then echo “We have job control” fi This check is effective in most cases because the jobs command is not available in most versions of the Bourne shell. Unfortunately, some versions of UNIX such as Sun Solaris, include a version of the Bourne shell that has a built-in command called jobs. On these systems when the shell is invoked as /bin/sh, the jobs command exists but does nothing. If the shell is invoked as /bin/jsh (as in job control shell), the jobs command behaves normally. Sams Teach Yourself Shell Programming in 24 Hours Contents Index Hour 24: Shell Programming FAQs Previous Chapter Next Chapter Sections in this Chapter: Shell and Command Questions File and Directory Questions Variable and Argument Questions Summary Previous Section Next Section Copyright Macmillan Computer Publishing. All rights reserved.
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produce the same result if the current shell is interactive: interactive On modern versions of UNIX both forms work equally well. On some older versions of UNIX the test -t command was not available, so the tty -s command had to be used. How do I discard the output of a command? Sometimes you will need to execute a command, but you don’t want the output displayed to the screen. In these cases you can discard the output by redirecting it to the file /dev/null: command > /dev/null Here command is the name of the command you want to execute. The file is a special file (called the bit bucket) that automatically discards all its input. To discard both output of a command and its error output, use standard redirection to redirect STDERR to STDOUT: command > /dev/null 2>&1 How can I display messages on STDERR? You can display a message on to STDERR by redirecting STDIN into STDERR as follows: echo message 1>&2 Here message is the message you want to display. If you are interested in shell functions that perform additional formatting please consult Chapter 21, “Problem Solving with Functions,” which covers several shell functions that display messages on to STDERR. How can I check whether a command was successful? A command is successful if it exits with a status of zero. A nonzero exit code indicates that an error has occurred. To check the exit code of the most recent command you executed, use the variable $?. For example: grep root /etc/passwd > /dev/null 2>&1 if [ $? -ne 0 ] ; then echo “No one is in charge!” ; fi Here you execute a grep command and then check the exit status of this command using the value stored in $?.
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instead of executed directly. The rest of the line is treated as the name of the interpreter to use. Usually the interpreter is /bin/sh, but you can also specify options to the shell on this line. Sometimes options such as -x or -nv are specified to enable debugging. This also enables you to write scripts tuned for a particular shell such as ksh or bash by using /bin/ksh or /bin/bash instead of /bin/sh. How can I access the name of the current shell in my initialization scripts? In your shell initialization scripts, the name of the current shell is stored in the variable $0. Users who have a single .profile that is shared by sh, ksh, and bash use this variable in conjunction with a case statement near the end of this file to execute additional shell specific startup. For example, I use the following case statement near the end of my .profile to set up the prompt, PS1, differently depending on the shell I am using: case “$0″ in *bash) PS1=”t h #$ ” ;; *ksh) PS1=” uname -n !$ ” ;; *sh) PS1=” uname -n $ ” ;; esac export PS1 I have specified the shells as *bash, *ksh, and *sh, because some versions of UNIX place the - character in front of login shells, but not in front of other shells. How do I tell whether the current shell is interactive or noninteractive? Some scripts will need the capability to determine whether they are running in an interactive shell or noninteractive shell. Usually this is restricted to your shell initialization scripts because you don’t want to perform a full-blown initialization every time these scripts execute. Some other examples include scripts that can run from the at or cron commands. Two common methods can determine whether a shell is interactive: l test -t or [ -t ] l tty -s Both commands exit with zero status if STDIN is connected to a terminal. For example, the commands $ if [ -t ] ; then echo interactive ; fi and $ if tty -s ; then echo interactive ; fi
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