lwv@n8emr.UUCP (Larry W. Virden) (04/11/89)
Using sed as a simple example of a complex situation, how would one code a shell program to invoke sed so that the arguments passed would always be valid ? That is, lets say that I wanted to write a script which would read the arguments to sed in from the keyboard. Also, assume that I was dealing with users who wouldnt know sed from a hole in the ground ;-0. I want to use delimiters around the shell variable containing their input which wont clash: sed -e /$ans/ filename doesnt work - what if there are multiple /'s in the user's answer. Using ^, !, etc - basically a hard coded printable character - have the same problems. Do folks just ignore this possibility, or is there a standard trick to this? -- Larry W. Virden 674 Falls Place, Reynoldsburg, OH 43068 (614) 864-8817 75046,606 (CIS) ; LVirden (ALPE) ; osu-cis!n8emr!lwv (UUCP) osu-cis!n8emr!lwv@TUT.CIS.OHIO-STATE.EDU (INTERNET) The world's not inherited from our parents, but borrowed from our children.
mchinni@pica.army.mil (Michael J. Chinni, SMCAR-CCS-E) (04/11/89)
Larry,
I don't know if there is a "standard" trick for this, but I had to
write a shell script where this was going to be a problem. I got around this by
using "tr" to translate all the possible characters that could mess up sed into
characters that are ok to sed.
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/
Michael J. Chinni
US Army Armament Research, Development, and Engineering Center
User to skeleton sitting at cobweb () Picatinny Arsenal, New Jersey
and dust covered terminal and desk () ARPA: mchinni@pica.army.mil
"System been down long?" () UUCP: ...!uunet!pica.army.mil!mchinni
/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/\/dhesi@bsu-cs.bsu.edu (Rahul Dhesi) (04/12/89)
In article <993@n8emr.UUCP> lwv@n8emr.UUCP (Larry W. Virden) writes: > >Using sed as a simple example of a complex situation, how would one code >a shell program to invoke sed so that the arguments passed would always be >valid?... >sed -e /$ans/ filename First quote those slashes: # works from /bin/sh pattern="`echo "$ans" | sed -e 's/\//\\\\\\//g'`" sed -e "/$pattern/d" filename Count those backslashes carefully. What UNIX badly needs is a way of specifying out-of-band characters. The current quoting scheme causes problems because a level of quoting is removed at each level of interpretation. -- Rahul Dhesi <dhesi@bsu-cs.bsu.edu> UUCP: ...!{iuvax,pur-ee}!bsu-cs!dhesi
tale@pawl.rpi.edu (David C Lawrence) (04/12/89)
In article <993@n8emr.UUCP> lwv@n8emr.UUCP (Larry W. Virden) writes: Larry> Using sed as a simple example of a complex situation, how would Larry> one code a shell program to invoke sed so that the arguments Larry> passed would always be valid?... Larry> sed -e /$ans/ filename In article <6710@bsu-cs.bsu.edu> dhesi@bsu-cs.bsu.edu (Rahul Dhesi) writes: Rahul> First quote those slashes: Rahul> # works from /bin/sh Rahul> pattern="`echo "$ans" | sed -e 's/\//\\\\\\//g'`" Rahul> sed -e "/$pattern/d" filename Rahul> Count those backslashes carefully. pattern="`echo $ans | sed 'sX/X\\\/Xg'`" a) quoting $ans is only necessary if "-n" could lead it. If you want to allow for that you need to quote it as "`echo \"$ans\" ...`" b) it is generally better to not muddle sed script lines with characters that are appearing inside delimited strings; chosing such a character (as "X", above) means you don't need to add additional quoting levels inside the strings. The above example should make $pattern parseable by sed using / as a delimiter. -- tale@rpitsmts.bitnet, tale%mts@itsgw.rpi.edu, tale@pawl.rpi.edu
chris@mimsy.UUCP (Chris Torek) (04/12/89)
>In article <6710@bsu-cs.bsu.edu> dhesi@bsu-cs.bsu.edu (Rahul Dhesi) >suggests something like >> pattern="`echo "$ans" | sed -e 's/\//\\\\\\//g'`" In article <TALE.89Apr11192859@imagine.pawl.rpi.edu> tale@pawl.rpi.edu (David C Lawrence) writes: >pattern="`echo $ans | sed 'sX/X\\\/Xg'`" Actually, sed 'sX/X\\/Xg' suffices (two backslashes, not three). (I also prefer a comma as the separator: sed 's,/,\\/,g'.) >a) quoting $ans is only necessary if "-n" could lead it. If you want >to allow for that you need to quote it as "`echo \"$ans\" ...`" This makes little difference. Quoting $ans is necessary if and only if it contains special characters (space, tab, newline, and globbing). If the string is "-n", echo will still break; worse, if it contains backslashes, it will break on SysV, where echo does escape interpretation (echo should never have acquired a -n flag either; that job should have been left to printf(1).) It is true that if "$ans" is "-n foo", then echo $ans produces foo without a newline, while echo "$ans" produces -n foo (with a newline). But echo "-n" produces nothing. There is no workaround for this. -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) Domain: chris@mimsy.umd.edu Path: uunet!mimsy!chris
bernsten@phoenix.Princeton.EDU (Dan Bernstein) (04/13/89)
So far, nobody has given an answer to this question that handles more than a few special cases. Now Chris Torek gives up completely. Here are some solutions, all thoroughly tested, from the devious mind that brought you the aliases `quote' and `makealias'. Each example munges $ans into $pattern, so that sed "s/$pattern/whatever/g" acts as if it had a literal $ans in the first position. sh, if you have printenv: export ans ; pattern="`printenv ans | sed 's-\([\.\*\[\\\^\$\/]\)-\\\\\1-g'`" sh, if you have a working echo (whose only caveat is -n): pattern="`(echo -n \"$ans\";echo '') | sed 's-\([\.\*\[\\\^\$\/]\)-\\\\\1-g'`" NOTE: Chris, want to take back that ``no workaround''? csh, if you have a working /bin/echo (whose only caveat is -n): set pattern="`(echo -n "\"\$ans\"";echo '') | sed 's-\([\.\*\[\\\^\"\$"\/]\)-\\\1-g'`" csh, if you have a working builtin echo (whose only caveat is -n): set pattern="`echo "\"\$ans\"" | sed 's-\([\.\*\[\\\^\"\$"\/]\)-\\\1-g'`" NOTE: csh parses builtins strangely, so this works even if ans is "-n ...". sh, on any machine (put it all on one line): pattern="`sed \"$ans\" 2>&1 | sed 's/^Unrecognized command: //' | sed 's-\([\.\*\[\\\^\$\/]\)-\\\\\1-g'`" CAVEAT: Does not work if $ans contains newlines. csh, on any machine (put it all on one line): set pattern="`sed "\"\$ans\"" |& sed 's/^Unrecognized command: //' | sed 's-\([\.\*\[\\\^\$\/]\)-\\\1-g'`" ANTI-CAVEAT: Because csh is csh, this one works if $ans contains newlines. Some notes about the last two: The sequence ^A (appearing twice in each---but not the ^U) can be any (identical) string upon which sed will choke; I use the control character. The general idea of sed "^A$ans" 2>&1 | sed 's/^Unrecognized command: ^A//' is to somehow manage to get that environment variable into the input-output stream, which is difficult if both echo and printenv are screwed. Other similar replacements include using ls imaginatively and then stripping off the `file not found', etc. It wouldn't take much work to make a `literal' alias, by feeding the above ideas through `makealias', so that all you'd have to do for this problem is type sed "s/`literal ans`/whatever/g" Those who have seen my csh aliases `quote' and `makealias' know both that I have a masochistic enjoyment of these problems and that my solutions work. So unless I've screwed up, let's cut the discussion. ---Dan Bernstein, bernsten@phoenix.princeton.edu
bernsten@phoenix.Princeton.EDU (Dan Bernstein) (04/14/89)
In article <7702@phoenix.Princeton.EDU> I write: > It wouldn't take much work to make a `literal' alias, so I'll now put my money where my mouth is. alias literal 'sed '\''s-\([\.\*\[\\\^\$\/]\)-\\\1-g'\' alias show 'echo "$\!:1"' The acid test, of course, is show ans | sed "s/`show ans | literal`/xxx/g" which, no matter what $ans is, will output xxx. If your csh doesn't have a working builtin echo, you'll have to rewrite show along the lines of my previous article. ---Dan Bernstein, bernsten@phoenix.princeton.edu