Follow Techotopia on Twitter

On-line Guides
All Guides
eBook Store
iOS / Android
Linux for Beginners
Office Productivity
Linux Installation
Linux Security
Linux Utilities
Linux Virtualization
Linux Kernel
System/Network Admin
Scripting Languages
Development Tools
Web Development
GUI Toolkits/Desktop
Mail Systems
Eclipse Documentation

How To Guides
General System Admin
Linux Security
Linux Filesystems
Web Servers
Graphics & Desktop
PC Hardware
Problem Solutions




The sed FAQ
Prev Home Next

4.30. How do I read environment variables with sed?

4.30.1. - on Unix platforms

In Unix, environment variables begin with a dollar sign, such as $TERM, $PATH, $var or $i. In sed, the dollar sign is used to indicate the last line of the input file, the end of a line (in the LHS), or a literal symbol (in the RHS). Sed cannot access variables directly, so one must pay attention to shell quoting requirements to expand the variables properly.

To ALLOW the Unix shell to interpret the dollar sign, put the script in double quotes:

     sed "s/_terminal-type_/$TERM/g" input.file >output.file

To PREVENT the Unix shell from interpreting the dollar sign as a shell variable, put the script in single quotes:

     sed 's/.$//' infile >outfile

To use BOTH Unix $environment_vars and sed /end-of-line$/ pattern matching, there are two solutions. (1) The easiest is to enclose the script in "double quotes" so the shell can see the $variables, and to prefix the sed metacharacter ($) with a backslash. Thus, in

     sed "s/$user\$/root/" file

the shell interpolates $user and sed interprets \$ as the symbol for end-of-line.

(2) Another method--somewhat less readable--is to concatenate the script with 'single quotes' where the $ should not be interpolated and "double quotes" where variable interpolation should occur. To demonstrate using the preceding script:

     sed "s/$user"'$/root/' file

Solution #1 seems easier to remember. In either case, we search for the user's name (stored in a variable called $user) when it occurs at the end of the line ($), and substitute the word "root" in all matches.

For longer shell scripts, it is sometimes useful to begin with single quote marks ('), close them upon encountering the variable, enclose the variable name in double quotes ("), and resume with single quotes, closing them at the end of the sed script. Example:

     #! /bin/sh
     # sed script to illustrate 'quote'"matching"'usage'
     sed -e '
     y/'"$FROM"'/'"$TO"'/;    # note the quote pairing
     # some more commands go here . . .
     # last line is a single quote mark

Thus, each variable named $FROM is replaced by $TO, and the single quotes are used to glue the multiple lines together in the script. (See also section 4.10, "How do I handle shell quoting in sed?")

4.30.2. - on MS-DOS and 4DOS platforms

Under 4DOS and MS-DOS version 7.0 (Win95) or 7.10 (Win95 OSR2), environment variables can be accessed from the command prompt. Under MS-DOS v6.22 and below, environment variables can only be accessed from within batch files. Environment variables should be enclosed between percent signs and are case-insensitive; i.e., %USER% or %user% will display the USER variable. To generate a true percent sign, just enter it twice.

DOS versions of sed require that sed scripts be enclosed by double quote marks "..." (not single quotes!) if the script contains embedded tabs, spaces, redirection arrows or the vertical bar. In fact, if the input for sed comes from piping, a sed script should not contain a vertical bar, even if it is protected by double quotes (this seems to be bug in the normal MS-DOS syntax). Thus,

       echo blurk | sed "s/^/ |foo /"     # will cause an error
       sed "s/^/ |foo /" blurk.txt        # will work as expected

Using DOS environment variables which contain DOS path statements (such as a TMP variable set to "C:\TEMP") within sed scripts is discouraged because sed will interpret the backslash '\' as a metacharacter to "quote" the next character, not as a normal symbol. Thus,

       sed "s/^/%TMP% /" somefile.txt

will not prefix each line with (say) "C:\TEMP ", but will prefix each line with "C:TEMP "; sed will discard the backslash, which is probably not what you want. Other variables such as %PATH% and %COMSPEC% will also lose the backslash within sed scripts.

Environment variables which do not use backslashes are usually workable. Thus, all the following should work without difficulty, if they are invoked from within DOS batch files:

       sed "s/=username=/%USER%/g" somefile.txt
       echo %FILENAME% | sed "s/\.TXT/.BAK/"
       grep -Ei "%string%" somefile.txt | sed "s/^/  /"

while from either the DOS prompt or from within a batch file,

       sed "s/%%/ percent/g" input.fil >output.fil

will replace each percent symbol in a file with " percent" (adding the leading space for readability).

The sed FAQ
Prev Home Next

   Reprinted courtesy of Eric Pement. Also available at Design by Interspire